Merge pull request #15342 from NixOS/backport-15341-to-2.32-maintenance

[Backport 2.32-maintenance] Fix the nix-community/patsh/0.2.1 flake regression test (again)

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,6 +15,10 @@ so you understand the process and the expectations.
 - volunteering contributions effectively
 - how to get help and our review process.
 
+PR stuck in review? We have two Nix team meetings per week online that are open for everyone in a jitsi conference:
+
+- https://calendar.google.com/calendar/u/0/embed?src=b9o52fobqjak8oq8lfkhg3t0qg@group.calendar.google.com
+
 -->
 
 ## Motivation

.github/actions/install-nix-action/action.yaml

@@ -4,15 +4,29 @@ inputs:
   dogfood:
     description: "Whether to use Nix installed from the latest artifact from master branch"
     required: true # Be explicit about the fact that we are using unreleased artifacts
+  experimental-installer:
+    description: "Whether to use the experimental installer to install Nix"
+    default: false
+  experimental-installer-version:
+    description: "Version of the experimental installer to use. If `latest`, the newest artifact from the default branch is used."
+    # TODO: This should probably be pinned to a release after https://github.com/NixOS/experimental-nix-installer/pull/49 lands in one
+    default: "latest"
   extra_nix_config:
     description: "Gets appended to `/etc/nix/nix.conf` if passed."
   install_url:
     description: "URL of the Nix installer"
     required: false
     default: "https://releases.nixos.org/nix/nix-2.30.2/install"
+  tarball_url:
+    description: "URL of the Nix tarball to use with the experimental installer"
+    required: false
   github_token:
     description: "Github token"
     required: true
+  use_cache:
+    description: "Whether to setup github actions cache (not implemented currently)"
+    default: false
+    required: false
 runs:
   using: "composite"
   steps:
@@ -37,14 +51,74 @@ runs:
 
         gh run download "$RUN_ID" --repo "$DOGFOOD_REPO" -n "$INSTALLER_ARTIFACT" -D "$INSTALLER_DOWNLOAD_DIR"
         echo "installer-path=file://$INSTALLER_DOWNLOAD_DIR" >> "$GITHUB_OUTPUT"
+        TARBALL_PATH="$(find "$INSTALLER_DOWNLOAD_DIR" -name 'nix*.tar.xz' -print | head -n 1)"
+        echo "tarball-path=file://$TARBALL_PATH" >> "$GITHUB_OUTPUT"
 
         echo "::notice ::Dogfooding Nix installer from master (https://github.com/$DOGFOOD_REPO/actions/runs/$RUN_ID)"
       env:
         GH_TOKEN: ${{ inputs.github_token }}
         DOGFOOD_REPO: "NixOS/nix"
+    - name: "Gather system info for experimental installer"
+      shell: bash
+      if: ${{ inputs.experimental-installer == 'true' }}
+      run: |
+        echo "::notice Using experimental installer from $EXPERIMENTAL_INSTALLER_REPO (https://github.com/$EXPERIMENTAL_INSTALLER_REPO)"
+
+        if [ "$RUNNER_OS" == "Linux" ]; then
+          EXPERIMENTAL_INSTALLER_SYSTEM="linux"
+          echo "EXPERIMENTAL_INSTALLER_SYSTEM=$EXPERIMENTAL_INSTALLER_SYSTEM" >> "$GITHUB_ENV"
+        elif [ "$RUNNER_OS" == "macOS" ]; then
+          EXPERIMENTAL_INSTALLER_SYSTEM="darwin"
+          echo "EXPERIMENTAL_INSTALLER_SYSTEM=$EXPERIMENTAL_INSTALLER_SYSTEM" >> "$GITHUB_ENV"
+        else
+          echo "::error ::Unsupported RUNNER_OS: $RUNNER_OS"
+          exit 1
+        fi
+
+        if [ "$RUNNER_ARCH" == "X64" ]; then
+          EXPERIMENTAL_INSTALLER_ARCH=x86_64
+          echo "EXPERIMENTAL_INSTALLER_ARCH=$EXPERIMENTAL_INSTALLER_ARCH" >> "$GITHUB_ENV"
+        elif [ "$RUNNER_ARCH" == "ARM64" ]; then
+          EXPERIMENTAL_INSTALLER_ARCH=aarch64
+          echo "EXPERIMENTAL_INSTALLER_ARCH=$EXPERIMENTAL_INSTALLER_ARCH" >> "$GITHUB_ENV"
+        else
+          echo "::error ::Unsupported RUNNER_ARCH: $RUNNER_ARCH"
+          exit 1
+        fi
+
+        echo "EXPERIMENTAL_INSTALLER_ARTIFACT=nix-installer-$EXPERIMENTAL_INSTALLER_ARCH-$EXPERIMENTAL_INSTALLER_SYSTEM" >> "$GITHUB_ENV"
+      env:
+        EXPERIMENTAL_INSTALLER_REPO: "NixOS/experimental-nix-installer"
+    - name: "Download latest experimental installer"
+      shell: bash
+      id: download-latest-experimental-installer
+      if: ${{ inputs.experimental-installer == 'true' && inputs.experimental-installer-version == 'latest' }}
+      run: |
+        RUN_ID=$(gh run list --repo "$EXPERIMENTAL_INSTALLER_REPO" --workflow ci.yml --branch main --status success --json databaseId --jq ".[0].databaseId")
+
+        EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR="$GITHUB_WORKSPACE/$EXPERIMENTAL_INSTALLER_ARTIFACT"
+        mkdir -p "$EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR"
+
+        gh run download "$RUN_ID" --repo "$EXPERIMENTAL_INSTALLER_REPO" -n "$EXPERIMENTAL_INSTALLER_ARTIFACT" -D "$EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR"
+        # Executable permissions are lost in artifacts
+        find $EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR -type f -exec chmod +x {} +
+        echo "installer-path=$EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR" >> "$GITHUB_OUTPUT"
+      env:
+        GH_TOKEN: ${{ inputs.github_token }}
+        EXPERIMENTAL_INSTALLER_REPO: "NixOS/experimental-nix-installer"
     - uses: cachix/install-nix-action@c134e4c9e34bac6cab09cf239815f9339aaaf84e # v31.5.1
+      if: ${{ inputs.experimental-installer != 'true' }}
       with:
         # Ternary operator in GHA: https://www.github.com/actions/runner/issues/409#issuecomment-752775072
         install_url: ${{ inputs.dogfood == 'true' && format('{0}/install', steps.download-nix-installer.outputs.installer-path) || inputs.install_url }}
         install_options: ${{ inputs.dogfood == 'true' && format('--tarball-url-prefix {0}', steps.download-nix-installer.outputs.installer-path) || '' }}
         extra_nix_config: ${{ inputs.extra_nix_config }}
+    - uses: DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196 # v20
+      if: ${{ inputs.experimental-installer == 'true' }}
+      with:
+        diagnostic-endpoint: ""
+        # TODO: It'd be nice to use `artifacts.nixos.org` for both of these, maybe through an `/experimental-installer/latest` endpoint? or `/commit/<hash>`?
+        local-root: ${{ inputs.experimental-installer-version == 'latest' && steps.download-latest-experimental-installer.outputs.installer-path || '' }}
+        source-url: ${{ inputs.experimental-installer-version != 'latest' && 'https://artifacts.nixos.org/experimental-installer/tag/${{ inputs.experimental-installer-version }}/${{ env.EXPERIMENTAL_INSTALLER_ARTIFACT }}' || '' }}
+        nix-package-url: ${{ inputs.dogfood == 'true' && steps.download-nix-installer.outputs.tarball-path || (inputs.tarball_url || '') }}
+        extra-conf: ${{ inputs.extra_nix_config }}

.github/workflows/ci.yml

@@ -27,9 +27,34 @@ jobs:
         extra_nix_config:
           experimental-features = nix-command flakes
         github_token: ${{ secrets.GITHUB_TOKEN }}
+        use_cache: false
     - run: nix flake show --all-systems --json
 
+  pre-commit-checks:
+    name: pre-commit checks
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v5
+      - uses: ./.github/actions/install-nix-action
+        with:
+          dogfood: ${{ github.event_name == 'workflow_dispatch' && inputs.dogfood || github.event_name != 'workflow_dispatch' }}
+          extra_nix_config: experimental-features = nix-command flakes
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+      - run: ./ci/gha/tests/pre-commit-checks
+
+  basic-checks:
+    name: aggregate basic checks
+    if: ${{ always() }}
+    runs-on: ubuntu-24.04
+    needs: [pre-commit-checks, eval]
+    steps:
+      - name: Exit with any errors
+        if: ${{ contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled') }}
+        run: |
+          exit 1
+
   tests:
+    needs: basic-checks
     strategy:
       fail-fast: false
       matrix:
@@ -65,7 +90,6 @@ jobs:
         dogfood: ${{ github.event_name == 'workflow_dispatch' && inputs.dogfood || github.event_name != 'workflow_dispatch' }}
         # The sandbox would otherwise be disabled by default on Darwin
         extra_nix_config: "sandbox = true"
-    - uses: DeterminateSystems/magic-nix-cache-action@main
     # Since ubuntu 22.30, unprivileged usernamespaces are no longer allowed to map to the root user:
     # https://ubuntu.com/blog/ubuntu-23-10-restricted-unprivileged-user-namespaces
     - run: sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0
@@ -140,80 +164,23 @@ jobs:
     - run: exec bash -c "nix-channel --add https://releases.nixos.org/nixos/unstable/nixos-23.05pre466020.60c1d71f2ba nixpkgs"
     - run: exec bash -c "nix-channel --update && nix-env -iA nixpkgs.hello && hello"
 
-  # Steps to test CI automation in your own fork.
-  # 1. Sign-up for https://hub.docker.com/
-  # 2. Store your dockerhub username as DOCKERHUB_USERNAME in "Repository secrets" of your fork repository settings (https://github.com/$githubuser/nix/settings/secrets/actions)
-  # 3. Create an access token in https://hub.docker.com/settings/security and store it as DOCKERHUB_TOKEN in "Repository secrets" of your fork
-  check_secrets:
-    permissions:
-      contents: none
-    name: Check presence of secrets
-    runs-on: ubuntu-24.04
-    outputs:
-      docker: ${{ steps.secret.outputs.docker }}
-    steps:
-      - name: Check for DockerHub secrets
-        id: secret
-        env:
-          _DOCKER_SECRETS: ${{ secrets.DOCKERHUB_USERNAME }}${{ secrets.DOCKERHUB_TOKEN }}
-        run: |
-          echo "docker=${{ env._DOCKER_SECRETS != '' }}" >> $GITHUB_OUTPUT
-
   docker_push_image:
-    needs: [tests, vm_tests, check_secrets]
+    name: Push docker image to DockerHub and GHCR
+    needs: [tests, vm_tests]
+    if: github.event_name == 'push' && github.ref_name == 'master'
+    uses: ./.github/workflows/docker-push.yml
+    with:
+      ref: ${{ github.sha }}
+      is_master: true
     permissions:
       contents: read
       packages: write
-    if: >-
-      needs.check_secrets.outputs.docker == 'true' &&
-      github.event_name == 'push' &&
-      github.ref_name == 'master'
-    runs-on: ubuntu-24.04
-    steps:
-    - uses: actions/checkout@v5
-      with:
-        fetch-depth: 0
-    - uses: cachix/install-nix-action@v31
-      with:
-        install_url: https://releases.nixos.org/nix/nix-2.20.3/install
-    - uses: DeterminateSystems/magic-nix-cache-action@main
-    - run: echo NIX_VERSION="$(nix --experimental-features 'nix-command flakes' eval .\#nix.version | tr -d \")" >> $GITHUB_ENV
-    - run: nix --experimental-features 'nix-command flakes' build .#dockerImage -L
-    - run: docker load -i ./result/image.tar.gz
-    - run: docker tag nix:$NIX_VERSION ${{ secrets.DOCKERHUB_USERNAME }}/nix:$NIX_VERSION
-    - run: docker tag nix:$NIX_VERSION ${{ secrets.DOCKERHUB_USERNAME }}/nix:master
-    # We'll deploy the newly built image to both Docker Hub and Github Container Registry.
-    #
-    # Push to Docker Hub first
-    - name: Login to Docker Hub
-      uses: docker/login-action@v3
-      with:
-        username: ${{ secrets.DOCKERHUB_USERNAME }}
-        password: ${{ secrets.DOCKERHUB_TOKEN }}
-    - run: docker push ${{ secrets.DOCKERHUB_USERNAME }}/nix:$NIX_VERSION
-    - run: docker push ${{ secrets.DOCKERHUB_USERNAME }}/nix:master
-    # Push to GitHub Container Registry as well
-    - name: Login to GitHub Container Registry
-      uses: docker/login-action@v3
-      with:
-        registry: ghcr.io
-        username: ${{ github.actor }}
-        password: ${{ secrets.GITHUB_TOKEN }}
-    - name: Push image
-      run: |
-        IMAGE_ID=ghcr.io/${{ github.repository_owner }}/nix
-        # Change all uppercase to lowercase
-        IMAGE_ID=$(echo $IMAGE_ID | tr '[A-Z]' '[a-z]')
-
-        docker tag nix:$NIX_VERSION $IMAGE_ID:$NIX_VERSION
-        docker tag nix:$NIX_VERSION $IMAGE_ID:latest
-        docker push $IMAGE_ID:$NIX_VERSION
-        docker push $IMAGE_ID:latest
-        # deprecated 2024-02-24
-        docker tag nix:$NIX_VERSION $IMAGE_ID:master
-        docker push $IMAGE_ID:master
+    secrets:
+      DOCKERHUB_USERNAME: ${{ secrets.DOCKERHUB_USERNAME }}
+      DOCKERHUB_TOKEN: ${{ secrets.DOCKERHUB_TOKEN }}
 
   vm_tests:
+    needs: basic-checks
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v5
@@ -254,7 +221,6 @@ jobs:
           extra_nix_config:
             experimental-features = nix-command flakes
           github_token: ${{ secrets.GITHUB_TOKEN }}
-      - uses: DeterminateSystems/magic-nix-cache-action@main
       - run: nix build -L --out-link ./new-nix && PATH=$(pwd)/new-nix/bin:$PATH MAX_FLAKES=25 flake-regressions/eval-all.sh
 
   profile_build:
@@ -275,7 +241,6 @@ jobs:
         extra_nix_config: |
           experimental-features = flakes nix-command ca-derivations impure-derivations
           max-jobs = 1
-    - uses: DeterminateSystems/magic-nix-cache-action@main
     - run: |
         nix build -L --file ./ci/gha/profile-build buildTimeReport --out-link build-time-report.md
         cat build-time-report.md >> $GITHUB_STEP_SUMMARY

.github/workflows/docker-push.yml

@@ -0,0 +1,101 @@
+name: "Push Docker Image"
+
+on:
+  workflow_call:
+    inputs:
+      ref:
+        description: "Git ref to build the docker image from"
+        required: true
+        type: string
+      is_master:
+        description: "Whether run from master branch"
+        required: true
+        type: boolean
+    secrets:
+      DOCKERHUB_USERNAME:
+        required: true
+      DOCKERHUB_TOKEN:
+        required: true
+
+permissions: {}
+
+jobs:
+  # Steps to test CI automation in your own fork.
+  # 1. Sign-up for https://hub.docker.com/
+  # 2. Store your dockerhub username as DOCKERHUB_USERNAME in "Repository secrets" of your fork repository settings (https://github.com/$githubuser/nix/settings/secrets/actions)
+  # 3. Create an access token in https://hub.docker.com/settings/security and store it as DOCKERHUB_TOKEN in "Repository secrets" of your fork
+  check_secrets:
+    permissions:
+      contents: none
+    name: Check presence of secrets
+    runs-on: ubuntu-24.04
+    outputs:
+      docker: ${{ steps.secret.outputs.docker }}
+    steps:
+      - name: Check for DockerHub secrets
+        id: secret
+        env:
+          _DOCKER_SECRETS: ${{ secrets.DOCKERHUB_USERNAME }}${{ secrets.DOCKERHUB_TOKEN }}
+        run: |
+          echo "docker=${{ env._DOCKER_SECRETS != '' }}" >> $GITHUB_OUTPUT
+
+  push:
+    name: Push docker image to DockerHub and GHCR
+    needs: [check_secrets]
+    permissions:
+      contents: read
+      packages: write
+    if: needs.check_secrets.outputs.docker == 'true'
+    runs-on: ubuntu-24.04
+    steps:
+    - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      with:
+        fetch-depth: 0
+        ref: ${{ inputs.ref }}
+    - uses: ./.github/actions/install-nix-action
+      with:
+        dogfood: false
+        extra_nix_config: |
+          experimental-features = flakes nix-command
+    - run: echo NIX_VERSION="$(nix eval .\#nix.version | tr -d \")" >> $GITHUB_ENV
+    - run: nix build .#dockerImage -L
+    - run: docker load -i ./result/image.tar.gz
+    # We'll deploy the newly built image to both Docker Hub and Github Container Registry.
+    #
+    # Push to Docker Hub first
+    - name: Login to Docker Hub
+      uses: docker/login-action@5e57cd118135c172c3672efd75eb46360885c0ef # v3.6.0
+      with:
+        username: ${{ secrets.DOCKERHUB_USERNAME }}
+        password: ${{ secrets.DOCKERHUB_TOKEN }}
+    - name: Push to Docker Hub
+      env:
+        IS_MASTER: ${{ inputs.is_master }}
+        DOCKERHUB_REPO: ${{ secrets.DOCKERHUB_USERNAME }}/nix
+      run: |
+        docker tag nix:$NIX_VERSION $DOCKERHUB_REPO:$NIX_VERSION
+        docker push $DOCKERHUB_REPO:$NIX_VERSION
+        if [ "$IS_MASTER" = "true" ]; then
+          docker tag nix:$NIX_VERSION $DOCKERHUB_REPO:master
+          docker push $DOCKERHUB_REPO:master
+        fi
+    # Push to GitHub Container Registry as well
+    - name: Login to GitHub Container Registry
+      uses: docker/login-action@5e57cd118135c172c3672efd75eb46360885c0ef # v3.6.0
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+    - name: Push to GHCR
+      env:
+        IS_MASTER: ${{ inputs.is_master }}
+      run: |
+        IMAGE_ID=ghcr.io/${{ github.repository_owner }}/nix
+        IMAGE_ID=$(echo $IMAGE_ID | tr '[A-Z]' '[a-z]')
+
+        docker tag nix:$NIX_VERSION $IMAGE_ID:$NIX_VERSION
+        docker push $IMAGE_ID:$NIX_VERSION
+        if [ "$IS_MASTER" = "true" ]; then
+          docker tag nix:$NIX_VERSION $IMAGE_ID:master
+          docker push $IMAGE_ID:master
+        fi

.github/workflows/labels.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-24.04
     if: github.repository_owner == 'NixOS'
     steps:
-    - uses: actions/labeler@v5
+    - uses: actions/labeler@v6
       with:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         sync-labels: false

.github/workflows/upload-release.yml

@@ -0,0 +1,80 @@
+name: Upload Release
+on:
+  workflow_dispatch:
+    inputs:
+      eval_id:
+        description: "Hydra evaluation ID"
+        required: true
+        type: number
+      is_latest:
+        description: "Mark as latest release"
+        required: false
+        type: boolean
+        default: false
+permissions:
+  contents: read
+  id-token: write
+  packages: write
+jobs:
+  release:
+    runs-on: ubuntu-24.04
+    environment: releases
+    steps:
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: ./.github/actions/install-nix-action
+        with:
+          dogfood: false # Use stable version
+          use_cache: false # Don't want any cache injection shenanigans
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+      - name: Set NIX_PATH from flake input
+        run: |
+          NIXPKGS_PATH=$(nix build --inputs-from .# nixpkgs#path --print-out-paths --no-link)
+          # Shebangs with perl have issues. Pin nixpkgs this way. nix shell should maybe
+          # get the same uberhack that nix-shell has to support it.
+          echo "NIX_PATH=nixpkgs=$NIXPKGS_PATH" >> "$GITHUB_ENV"
+      - name: Configure AWS credentials
+        uses: aws-actions/configure-aws-credentials@61815dcd50bd041e203e49132bacad1fd04d2708 # v5.1.1
+        with:
+          role-to-assume: "arn:aws:iam::080433136561:role/nix-release"
+          role-session-name: nix-release-oidc-${{ github.run_id }}
+          aws-region: eu-west-1
+      - name: Disable containerd image store
+        run: |
+          # Docker 28+ defaults to the containerd image store, which
+          # pushes layers uncompressed instead of gzip. OCI clients
+          # that only support gzip (e.g. go-containerregistry) fail
+          # with "gzip: invalid header". Disabling the containerd
+          # snapshotter restores the classic storage driver, which
+          # preserves gzip-compressed layers through the
+          # `docker load` / `docker push` pipeline.
+          echo '{"features":{"containerd-snapshotter":false}}' | sudo tee /etc/docker/daemon.json > /dev/null
+          sudo systemctl restart docker
+      - name: Login to Docker Hub
+        uses: docker/login-action@5e57cd118135c172c3672efd75eb46360885c0ef # v3.6.0
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@5e57cd118135c172c3672efd75eb46360885c0ef # v3.6.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Upload release
+        run: |
+          ./maintainers/upload-release.pl \
+            ${{ inputs.eval_id }} \
+            --skip-git
+        env:
+          IS_LATEST: ${{ inputs.is_latest && '1' || '' }}
+      - name: Push to GHCR
+        run: |
+          DOCKER_OWNER="ghcr.io/$(echo '${{ github.repository_owner }}' | tr '[A-Z]' '[a-z]')/nix"
+          ./maintainers/upload-release.pl \
+            ${{ inputs.eval_id }} \
+            --skip-git \
+            --skip-s3 \
+            --docker-owner "$DOCKER_OWNER"
+        env:
+          IS_LATEST: ${{ inputs.is_latest && '1' || '' }}

.mergify.yml

@@ -161,3 +161,14 @@ pull_request_rules:
         labels:
           - automatic backport
           - merge-queue
+
+  - name: backport patches to 2.31
+    conditions:
+      - label=backport 2.31-maintenance
+    actions:
+      backport:
+        branches:
+          - "2.31-maintenance"
+        labels:
+          - automatic backport
+          - merge-queue

.version

@@ -1 +1 @@
-2.31.0
+2.32.7

COPYING

@@ -1,8 +1,8 @@
-		  GNU LESSER GENERAL PUBLIC LICENSE
-		       Version 2.1, February 1999
+                  GNU LESSER GENERAL PUBLIC LICENSE
+                       Version 2.1, February 1999
 
  Copyright (C) 1991, 1999 Free Software Foundation, Inc.
- 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ <https://fsf.org/>
  Everyone is permitted to copy and distribute verbatim copies
  of this license document, but changing it is not allowed.
 
@@ -10,7 +10,7 @@
  as the successor of the GNU Library Public License, version 2, hence
  the version number 2.1.]
 
-			    Preamble
+                            Preamble
 
   The licenses for most software are designed to take away your
 freedom to share and change it.  By contrast, the GNU General Public
@@ -112,7 +112,7 @@ modification follow.  Pay close attention to the difference between a
 former contains code derived from the library, whereas the latter must
 be combined with the library in order to run.
 
-		  GNU LESSER GENERAL PUBLIC LICENSE
+                  GNU LESSER GENERAL PUBLIC LICENSE
    TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
 
   0. This License Agreement applies to any software library or other
@@ -146,7 +146,7 @@ such a program is covered only if its contents constitute a work based
 on the Library (independent of the use of the Library in a tool for
 writing it).  Whether that is true depends on what the Library does
 and what the program that uses the Library does.
-  
+
   1. You may copy and distribute verbatim copies of the Library's
 complete source code as you receive it, in any medium, provided that
 you conspicuously and appropriately publish on each copy an
@@ -432,7 +432,7 @@ decision will be guided by the two goals of preserving the free status
 of all derivatives of our free software and of promoting the sharing
 and reuse of software generally.
 
-			    NO WARRANTY
+                            NO WARRANTY
 
   15. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO
 WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW.
@@ -455,7 +455,7 @@ FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
 SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
 DAMAGES.
 
-		     END OF TERMS AND CONDITIONS
+                     END OF TERMS AND CONDITIONS
 
            How to Apply These Terms to Your New Libraries
 
@@ -484,8 +484,7 @@ convey the exclusion of warranty; and each file should have at least the
     Lesser General Public License for more details.
 
     You should have received a copy of the GNU Lesser General Public
-    License along with this library; if not, write to the Free Software
-    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+    License along with this library; if not, see <https://www.gnu.org/licenses/>.
 
 Also add information on how to contact you by electronic and paper mail.
 
@@ -496,9 +495,7 @@ necessary.  Here is a sample; alter the names:
   Yoyodyne, Inc., hereby disclaims all copyright interest in the
   library `Frob' (a library for tweaking knobs) written by James Random Hacker.
 
-  <signature of Ty Coon>, 1 April 1990
-  Ty Coon, President of Vice
+  <signature of Moe Ghoul>, 1 April 1990
+  Moe Ghoul, President of Vice
 
 That's all there is to it!
-
-

ci/gha/tests/default.nix

@@ -24,16 +24,7 @@ let
   enableSanitizersLayer = finalAttrs: prevAttrs: {
     mesonFlags =
       (prevAttrs.mesonFlags or [ ])
-      ++ [
-        # Run all tests with UBSAN enabled. Running both with ubsan and
-        # without doesn't seem to have much immediate benefit for doubling
-        # the GHA CI workaround.
-        #
-        # TODO: Work toward enabling "address,undefined" if it seems feasible.
-        # This would maybe require dropping Boost coroutines and ignoring intentional
-        # memory leaks with detect_leaks=0.
-        (lib.mesonOption "b_sanitize" "undefined")
-      ]
+      ++ [ (lib.mesonOption "b_sanitize" "address,undefined") ]
       ++ (lib.optionals stdenv.cc.isClang [
         # https://www.github.com/mesonbuild/meson/issues/764
         (lib.mesonBool "b_lundef" false)
@@ -71,8 +62,12 @@ rec {
   nixComponentsInstrumented = nixComponents.overrideScope (
     final: prev: {
       nix-store-tests = prev.nix-store-tests.override { withBenchmarks = true; };
+      # Boehm is incompatible with ASAN.
+      nix-expr = prev.nix-expr.override { enableGC = !withSanitizers; };
 
       mesonComponentOverrides = lib.composeManyExtensions componentOverrides;
+      # Unclear how to make Perl bindings work with a dynamically linked ASAN.
+      nix-perl-bindings = if withSanitizers then null else prev.nix-perl-bindings;
     }
   );
 

ci/gha/tests/pre-commit-checks

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+system=$(nix eval --raw --impure --expr builtins.currentSystem)
+
+echo "::group::Running pre-commit checks"
+
+if nix build ".#checks.$system.pre-commit" -L; then
+    echo "::endgroup::"
+    exit 0
+fi
+
+echo "::error ::Changes do not pass pre-commit checks"
+
+cat <<EOF
+The code isn't formatted or doesn't pass lints. You can run pre-commit locally with:
+
+    nix develop -c ./maintainers/format.sh
+EOF
+
+echo "::endgroup::"
+
+exit 1

doc/manual/anchors.jq

@@ -24,8 +24,15 @@ def map_contents_recursively(transformer):
 def process_command:
     .[0] as $context |
     .[1] as $body |
-    $body + {
-        sections: $body.sections | map(map_contents_recursively(if $context.renderer == "html" then transform_anchors_html else transform_anchors_strip end)),
-    };
+    # mdbook 0.5.x uses 'items' instead of 'sections'
+    if $body.items then
+        $body + {
+            items: $body.items | map(map_contents_recursively(if $context.renderer == "html" then transform_anchors_html else transform_anchors_strip end)),
+        }
+    else
+        $body + {
+            sections: $body.sections | map(map_contents_recursively(if $context.renderer == "html" then transform_anchors_html else transform_anchors_strip end)),
+        }
+    end;
 
 process_command

doc/manual/book.toml.in

@@ -23,12 +23,3 @@ renderers = ["html"]
 command = "jq --from-file ./anchors.jq"
 
 [output.markdown]
-
-[output.linkcheck]
-# no Internet during the build (in the sandbox)
-follow-web-links = false
-
-# mdbook-linkcheck does not understand [foo]{#bar} style links, resulting in
-# excessive "Potential incomplete link" warnings. No other kind of warning was
-# produced at the time of writing.
-warning-policy = "ignore"

doc/manual/generate-store-types.nix

@@ -24,9 +24,9 @@ let
     in
     concatStringsSep "\n" (map showEntry storesList);
 
-  "index.md" =
-    replaceStrings [ "@store-types@" ] [ index ]
-      (readFile ./source/store/types/index.md.in);
+  "index.md" = replaceStrings [ "@store-types@" ] [ index ] (
+    readFile ./source/store/types/index.md.in
+  );
 
   tableOfContents =
     let

doc/manual/meson.build

@@ -15,6 +15,7 @@ pymod = import('python')
 python = pymod.find_installation('python3')
 
 nix_env_for_docs = {
+  'ASAN_OPTIONS' : 'abort_on_error=1:print_summary=1:detect_leaks=0',
   'HOME' : '/dummy',
   'NIX_CONF_DIR' : '/dummy',
   'NIX_SSL_CERT_FILE' : '/dummy/no-ca-bundle.crt',

doc/manual/package.nix

@@ -6,7 +6,6 @@
   ninja,
   lowdown-unsandboxed,
   mdbook,
-  mdbook-linkcheck,
   jq,
   python3,
   rsync,
@@ -51,7 +50,6 @@ mkMesonDerivation (finalAttrs: {
     ninja
     (lib.getBin lowdown-unsandboxed)
     mdbook
-    mdbook-linkcheck
     jq
     python3
     rsync

doc/manual/source/SUMMARY.md.in

@@ -138,6 +138,7 @@
   - [Contributing](development/contributing.md)
 - [Releases](release-notes/index.md)
 {{#include ./SUMMARY-rl-next.md}}
+  - [Release 2.32 (2025-10-06)](release-notes/rl-2.32.md)
   - [Release 2.31 (2025-08-21)](release-notes/rl-2.31.md)
   - [Release 2.30 (2025-07-07)](release-notes/rl-2.30.md)
   - [Release 2.29 (2025-05-14)](release-notes/rl-2.29.md)

doc/manual/source/command-ref/meson.build

@@ -2,6 +2,7 @@ xp_features_json = custom_target(
   command : [ nix, '__dump-xp-features' ],
   capture : true,
   output : 'xp-features.json',
+  env : nix_env_for_docs,
 )
 
 experimental_features_shortlist_md = custom_target(

doc/manual/source/development/building.md

@@ -23,7 +23,7 @@ $ nix-shell
 To get a shell with one of the other [supported compilation environments](#compilation-environments):
 
 ```console
-$ nix-shell --attr devShells.x86_64-linux.native-clangStdenvPackages
+$ nix-shell --attr devShells.x86_64-linux.native-clangStdenv
 ```
 
 > **Note**
@@ -34,7 +34,7 @@ $ nix-shell --attr devShells.x86_64-linux.native-clangStdenvPackages
 To build Nix itself in this shell:
 
 ```console
-[nix-shell]$ mesonFlags+=" --prefix=$(pwd)/outputs/out"
+[nix-shell]$ out="$(pwd)/outputs/out" dev=$out debug=$out mesonFlags+=" --prefix=${out}"
 [nix-shell]$ dontAddPrefix=1 configurePhase
 [nix-shell]$ buildPhase
 ```

doc/manual/source/development/debugging.md

@@ -24,6 +24,19 @@ It is also possible to build without debugging for faster build:
 
 (The first line is needed because `fortify` hardening requires at least some optimization.)
 
+## Building Nix with sanitizers
+
+Nix can be built with [Address](https://clang.llvm.org/docs/AddressSanitizer.html) and
+[UB](https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html) sanitizers using LLVM
+or GCC. This is useful when debugging memory corruption issues.
+
+```console
+[nix-shell]$ export mesonBuildType=debugoptimized
+[nix-shell]$ appendToVar mesonFlags "-Dlibexpr:gc=disabled" # Disable Boehm
+[nix-shell]$ appendToVar mesonFlags "-Dbindings=false" # Disable nix-perl
+[nix-shell]$ appendToVar mesonFlags "-Db_sanitize=address,undefined"
+```
+
 ## Debugging the Nix Binary
 
 Obtain your preferred debugger within the development shell:

doc/manual/source/development/meson.build

@@ -7,5 +7,6 @@ experimental_feature_descriptions_md = custom_target(
     xp_features_json,
   ],
   capture : true,
+  env : nix_env_for_docs,
   output : 'experimental-feature-descriptions.md',
 )

doc/manual/source/language/builtins-prefix.md

@@ -5,12 +5,28 @@ All built-ins are available through the global [`builtins`](#builtins-builtins)
 
 Some built-ins are also exposed directly in the global scope:
 
-<!-- TODO(@rhendric, #10970): this list is incomplete -->
-
 - [`derivation`](#builtins-derivation)
-- [`import`](#builtins-import)
+- `derivationStrict`
 - [`abort`](#builtins-abort)
+- [`baseNameOf`](#builtins-baseNameOf)
+- [`break`](#builtins-break)
+- [`dirOf`](#builtins-dirOf)
+- [`false`](#builtins-false)
+- [`fetchGit`](#builtins-fetchGit)
+- `fetchMercurial`
+- [`fetchTarball`](#builtins-fetchTarball)
+- [`fetchTree`](#builtins-fetchTree)
+- [`fromTOML`](#builtins-fromTOML)
+- [`import`](#builtins-import)
+- [`isNull`](#builtins-isNull)
+- [`map`](#builtins-map)
+- [`null`](#builtins-null)
+- [`placeholder`](#builtins-placeholder)
+- [`removeAttrs`](#builtins-removeAttrs)
+- `scopedImport`
 - [`throw`](#builtins-throw)
+- [`toString`](#builtins-toString)
+- [`true`](#builtins-true)
 
 <dl>
   <dt id="builtins-derivation"><a href="#builtins-derivation"><code>derivation <var>attrs</var></code></a></dt>

doc/manual/source/protocols/json/derivation.md

@@ -14,6 +14,21 @@ is a JSON object with the following fields:
   The name of the derivation.
   This is used when calculating the store paths of the derivation's outputs.
 
+* `version`:
+  Must be `3`.
+  This is a guard that allows us to continue evolving this format.
+  The choice of `3` is fairly arbitrary, but corresponds to this informal version:
+
+  - Version 0: A-Term format
+
+  - Version 1: Original JSON format, with ugly `"r:sha256"` inherited from A-Term format.
+
+  - Version 2: Separate `method` and `hashAlgo` fields in output specs
+
+  - Verison 3: Drop store dir from store paths, just include base name.
+
+  Note that while this format is experimental, the maintenance of versions is best-effort, and not promised to identify every change.
+
 * `outputs`:
   Information about the output paths of the derivation.
   This is a JSON object with one member per output, where the key is the output name and the value is a JSON object with these fields:
@@ -52,7 +67,6 @@ is a JSON object with the following fields:
   > ```json
   > "outputs": {
   >   "out": {
-  >     "path": "/nix/store/2543j7c6jn75blc3drf4g5vhb1rhdq29-source",
   >     "method": "nar",
   >     "hashAlgo": "sha256",
   >     "hash": "6fc80dcc62179dbc12fc0b5881275898f93444833d21b89dfe5f7fbcbb1d0d62"
@@ -63,6 +77,15 @@ is a JSON object with the following fields:
 * `inputSrcs`:
   A list of store paths on which this derivation depends.
 
+  > **Example**
+  >
+  > ```json
+  > "inputSrcs": [
+  >   "47y241wqdhac3jm5l7nv0x4975mb1975-separate-debug-info.sh",
+  >   "56d0w71pjj9bdr363ym3wj1zkwyqq97j-fix-pop-var-context-error.patch"
+  > ]
+  > ```
+
 * `inputDrvs`:
   A JSON object specifying the derivations on which this derivation depends, and what outputs of those derivations.
 
@@ -70,8 +93,8 @@ is a JSON object with the following fields:
   >
   > ```json
   > "inputDrvs": {
-  >   "/nix/store/6lkh5yi7nlb7l6dr8fljlli5zfd9hq58-curl-7.73.0.drv": ["dev"],
-  >   "/nix/store/fn3kgnfzl5dzym26j8g907gq3kbm8bfh-unzip-6.0.drv": ["out"]
+  >   "6lkh5yi7nlb7l6dr8fljlli5zfd9hq58-curl-7.73.0.drv": ["dev"],
+  >   "fn3kgnfzl5dzym26j8g907gq3kbm8bfh-unzip-6.0.drv": ["out"]
   > }
   > ```
 

doc/manual/source/release-notes/rl-2.32.md

@@ -0,0 +1,165 @@
+# Release 2.32.0 (2025-10-06)
+
+## Incompatible changes
+
+- Removed support for daemons and clients older than Nix 2.0 [#13951](https://github.com/NixOS/nix/pull/13951)
+
+  We have dropped support in the daemon worker protocol for daemons and clients that don't speak at least version 18 of the protocol. This first Nix release that supports this version is Nix 2.0, released in February 2018.
+
+- Derivation JSON format now uses store path basenames only [#13570](https://github.com/NixOS/nix/issues/13570) [#13980](https://github.com/NixOS/nix/pull/13980)
+
+  Experience with many JSON frameworks (e.g. nlohmann/json in C++, Serde in Rust, and Aeson in Haskell) has shown that the use of the store directory in JSON formats is an impediment to systematic JSON formats, because it requires the serializer/deserializer to take an extra parameter (the store directory).
+
+  We ultimately want to rectify this issue with all JSON formats to the extent allowed by our stability promises. To start with, we are changing the JSON format for derivations because the `nix derivation` commands are — in addition to being formally unstable — less widely used than other unstable commands.
+
+  See the documentation on the [JSON format for derivations](@docroot@/protocols/json/derivation.md) for further details.
+
+- C API: `nix_get_attr_name_byidx`, `nix_get_attr_byidx` take a `nix_value *` instead of `const nix_value *` [#13987](https://github.com/NixOS/nix/pull/13987)
+
+  In order to accommodate a more optimized internal representation of attribute set merges these functions require
+  a mutable `nix_value *` that might be modified on access. This does *not* break the ABI of these functions.
+
+## New features
+
+- C API: Add lazy attribute and list item accessors [#14030](https://github.com/NixOS/nix/pull/14030)
+
+  The C API now includes lazy accessor functions for retrieving values from lists and attribute sets without forcing evaluation:
+
+  - `nix_get_list_byidx_lazy()` - Get a list element without forcing its evaluation
+  - `nix_get_attr_byname_lazy()` - Get an attribute value by name without forcing evaluation
+  - `nix_get_attr_byidx_lazy()` - Get an attribute by index without forcing evaluation
+
+  These functions are useful when forwarding unevaluated sub-values to other lists, attribute sets, or function calls. They allow more efficient handling of Nix values by deferring evaluation until actually needed.
+
+  Additionally, bounds checking has been improved for all `_byidx` functions to properly validate indices before access, preventing potential out-of-bounds errors.
+
+  The documentation for `NIX_ERR_KEY` error handling has also been clarified to specify when this error code is returned.
+
+- HTTP binary caches now support transparent compression for metadata
+
+  HTTP binary cache stores can now compress `.narinfo`, `.ls`, and build log files before uploading them,
+  reducing bandwidth usage and storage requirements. The compression is applied transparently using the
+  `Content-Encoding` header, allowing compatible clients to automatically decompress the files.
+
+  Three new configuration options control this behavior:
+  - `narinfo-compression`: Compression method for `.narinfo` files
+  - `ls-compression`: Compression method for `.ls` files
+  - `log-compression`: Compression method for build logs in `log/` directory
+
+  Example usage:
+  ```
+  nix copy --to 'http://cache.example.com?narinfo-compression=gzip&ls-compression=gzip' /nix/store/...
+  nix store copy-log --to 'http://cache.example.com?log-compression=br' /nix/store/...
+  ```
+
+- Temporary build directories no longer include derivation names [#13839](https://github.com/NixOS/nix/pull/13839)
+
+  Temporary build directories created during derivation builds no longer include the derivation name in their path to avoid build failures when the derivation name is too long. This change ensures predictable prefix lengths for build directories under `/nix/var/nix/builds`.
+
+- External derivation builders [#14145](https://github.com/NixOS/nix/pull/14145)
+
+  These are helper programs that Nix calls to perform derivations for specified system types, e.g. by using QEMU to emulate a different type of platform. For more information, see the [`external-builders` setting](../command-ref/conf-file.md#conf-external-builders).
+
+  This is currently an experimental feature.
+
+## Performance improvements
+
+- Optimize memory usage of attribute set merges [#13987](https://github.com/NixOS/nix/pull/13987)
+
+  [Attribute set update operations](@docroot@/language/operators.md#update) have been optimized to
+  reduce reallocations in cases when the second operand is small.
+
+  For typical evaluations of nixpkgs this optimization leads to ~20% less memory allocated in total
+  without significantly affecting evaluation performance.
+
+  See [eval-attrset-update-layer-rhs-threshold](@docroot@/command-ref/conf-file.md#conf-eval-attrset-update-layer-rhs-threshold)
+
+- Substituted flake inputs are no longer re-copied to the store [#14041](https://github.com/NixOS/nix/pull/14041)
+
+  Since 2.25, Nix would fail to store a cache entry for substituted flake inputs, which in turn would cause them to be re-copied to the store on initial evaluation. Caching these inputs results in a near doubling of performance in some cases — especially on I/O-bound machines and when using commands that fetch many inputs, like `nix flake [archive|prefetch-inputs]`.
+
+- `nix flake check` now skips derivations that can be substituted [#13574](https://github.com/NixOS/nix/pull/13574)
+
+  Previously, `nix flake check` would evaluate and build/substitute all
+  derivations. Now, it will skip downloading derivations that can be substituted.
+  This can drastically decrease the time invocations take in environments where
+  checks may already be cached (like in CI).
+
+- `fetchTarball` and `fetchurl` now correctly substitute (#14138)
+
+  At some point we stopped substituting calls to `fetchTarball` and `fetchurl` with a set `narHash` to avoid incorrectly substituting things in `fetchTree`, even though it would be safe to substitute when calling the legacy `fetch{Tarball,url}`. This fixes that regression where it is safe.
+- Started moving AST allocations into a bump allocator [#14088](https://github.com/NixOS/nix/issues/14088)
+
+  This leaves smaller, immutable structures in the AST. So far this saves about 2% memory on a NixOS config evaluation.
+## Contributors
+
+This release was made possible by the following 32 contributors:
+
+- Farid Zakaria [**(@fzakaria)**](https://github.com/fzakaria)
+- dram [**(@dramforever)**](https://github.com/dramforever)
+- Ephraim Siegfried [**(@EphraimSiegfried)**](https://github.com/EphraimSiegfried)
+- Robert Hensing [**(@roberth)**](https://github.com/roberth)
+- Taeer Bar-Yam [**(@Radvendii)**](https://github.com/Radvendii)
+- Emily [**(@emilazy)**](https://github.com/emilazy)
+- Jens Petersen [**(@juhp)**](https://github.com/juhp)
+- Bernardo Meurer [**(@lovesegfault)**](https://github.com/lovesegfault)
+- Jörg Thalheim [**(@Mic92)**](https://github.com/Mic92)
+- Leandro Emmanuel Reina Kiperman [**(@kip93)**](https://github.com/kip93)
+- Marie [**(@NyCodeGHG)**](https://github.com/NyCodeGHG)
+- Ethan Evans [**(@ethanavatar)**](https://github.com/ethanavatar)
+- Yaroslav Bolyukin [**(@CertainLach)**](https://github.com/CertainLach)
+- Matej Urbas [**(@urbas)**](https://github.com/urbas)
+- Jami Kettunen [**(@JamiKettunen)**](https://github.com/JamiKettunen)
+- Clayton [**(@netadr)**](https://github.com/netadr)
+- Grégory Marti [**(@gmarti)**](https://github.com/gmarti)
+- Eelco Dolstra [**(@edolstra)**](https://github.com/edolstra)
+- rszyma [**(@rszyma)**](https://github.com/rszyma)
+- Philip Wilk [**(@philipwilk)**](https://github.com/philipwilk)
+- John Ericson [**(@Ericson2314)**](https://github.com/Ericson2314)
+- Tom Westerhout [**(@twesterhout)**](https://github.com/twesterhout)
+- Tristan Ross [**(@RossComputerGuy)**](https://github.com/RossComputerGuy)
+- Sergei Zimmerman [**(@xokdvium)**](https://github.com/xokdvium)
+- Jean-François Roche [**(@jfroche)**](https://github.com/jfroche)
+- Seth Flynn [**(@getchoo)**](https://github.com/getchoo)
+- éclairevoyant [**(@eclairevoyant)**](https://github.com/eclairevoyant)
+- Glen Huang [**(@hgl)**](https://github.com/hgl)
+- osman - オスマン [**(@osbm)**](https://github.com/osbm)
+- David McFarland [**(@corngood)**](https://github.com/corngood)
+- Cole Helbling [**(@cole-h)**](https://github.com/cole-h)
+- Sinan Mohd [**(@sinanmohd)**](https://github.com/sinanmohd)
+- Philipp Otterbein
+# Release 2.32.5 (2026-01-02)
+
+## Bug fixes
+
+- Fix `builtins.fetchGit` with `ref = "HEAD"` [#13948](https://github.com/NixOS/nix/issues/13948) [#14673](https://github.com/NixOS/nix/pull/14673)
+
+- Fix dynamic attributes that are simple string expressions [#14642](https://github.com/NixOS/nix/issues/14642) [#14646](https://github.com/NixOS/nix/pull/14646)
+
+  Dynamic attributes are typically not allowed in [`let` expressions](https://nix.dev/manual/nix/2.32/language/syntax.html#let-expressions), but simple [string literals](https://nix.dev/manual/nix/2.32/language/string-literals.html) are allowed. This special-case was broken in prior releases of 2.32.
+
+- Fix null pointer dereference on reading non-existent derivations [#14571](https://github.com/NixOS/nix/issues/14571) [#14891](https://github.com/NixOS/nix/issues/14891) [#14572](https://github.com/NixOS/nix/pull/14572)
+
+- Fix use-after-free in derivation build scheduler [#14782](https://github.com/NixOS/nix/pull/14782)
+
+- Avoid querying remaining substituters after first success [#14836](https://github.com/NixOS/nix/issues/14836) [#14839](https://github.com/NixOS/nix/pull/14839)
+
+- Improve temporary path creation in hard link store optimisation [#7273](https://github.com/NixOS/nix/issues/7273) [#14680](https://github.com/NixOS/nix/pull/14680)
+
+  `auto-optimise-store` has been known to be flaky on Darwin due the usage of `rand()` when constructing the path of a temporary hardlink.
+
+- Move singletons out of headers [#14558](https://github.com/NixOS/nix/issues/14558)
+
+  Such variables have ["vague linkage"](https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Vague-Linkage.html) and require special support from the dynamic linker to deduplicate across different shared libraries. Platforms such as Cygwin do not support this and duplicate the symbols instead.
+
+- Fix `curl` with `c-ares` failing to resolve DNS inside sandbox on macOS [#14859](https://github.com/NixOS/nix/pull/14859)
+
+- Fix `recursive-nix` [#14730](https://github.com/NixOS/nix/pull/14730)
+
+## Improvements
+
+- Support building manual with mdbook 0.5 [#14628](https://github.com/NixOS/nix/issues/14628) [#14693](https://github.com/NixOS/nix/pull/14693)
+
+- Improve `parent directory is world-writable or a symlink` error message to include the offending path component [#13701](https://github.com/NixOS/nix/issues/13701) [#14849](https://github.com/NixOS/nix/pull/14849)
+
+- Correct `build-dir` documentation in manual [#14748](https://github.com/NixOS/nix/pull/14748)

doc/manual/source/store/building.md

@@ -12,10 +12,11 @@
 
 The [`builder`](./derivation/index.md#builder) is executed as follows:
 
-- A temporary directory is created under the directory specified by
-  `TMPDIR` (default `/tmp`) where the build will take place. The
+- A temporary directory is created where the build will take place. The
   current directory is changed to this directory.
 
+  See the per-store [`build-dir`](@docroot@/store/types/local-store.md#store-local-store-build-dir) setting for more information.
+
 - The environment is cleared and set to the derivation attributes, as
   specified above.
 

doc/manual/source/store/store-object.md

@@ -20,7 +20,8 @@ The graph of references excluding self-references thus forms a [directed acyclic
 
 [directed acyclic graph]: @docroot@/glossary.md#gloss-directed-acyclic-graph
 
-We can take the [transitive closure] of the references graph, which any pair of store objects have an edge not if there is a single reference from the first to the second, but a path of one or more references from the first to the second.
+We can take the [transitive closure] of the references graph, in which any pair of store objects have an edge if a *path* of one or more references exists from the first to the second object.
+(A single reference always forms a path which is one reference long, but longer paths may connect objects which have no direct reference between them.)
 The *requisites* of a store object are all store objects reachable by paths of references which start with given store object's references.
 
 [transitive closure]: https://en.wikipedia.org/wiki/Transitive_closure

doc/manual/substitute.py

@@ -41,6 +41,10 @@ def recursive_replace(data: dict[str, t.Any], book_root: Path, search_path: Path
             return data | dict(
                 sections = [recursive_replace(section, book_root, search_path) for section in sections],
             )
+        case {'items': items}:
+            return data | dict(
+                items = [recursive_replace(item, book_root, search_path) for item in items],
+            )
         case {'Chapter': chapter}:
             path_to_chapter = Path(chapter['path'])
             chapter_content = chapter['content']

docker.nix

@@ -281,7 +281,10 @@ let
 
           # may get replaced by pkgs.dockerTools.caCertificates
           mkdir -p $out/etc/ssl/certs
+          # Old NixOS compatibility.
           ln -s /nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt $out/etc/ssl/certs
+          # NixOS canonical location
+          ln -s /nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt $out/etc/ssl/certs/ca-certificates.crt
 
           cat $passwdContentsPath > $out/etc/passwd
           echo "" >> $out/etc/passwd

flake.lock

@@ -63,16 +63,16 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1755442223,
-        "narHash": "sha256-VtMQg02B3kt1oejwwrGn50U9Xbjgzfbb5TV5Wtx8dKI=",
+        "lastModified": 1761597516,
+        "narHash": "sha256-wxX7u6D2rpkJLWkZ2E932SIvDJW8+ON/0Yy8+a5vsDU=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "cd32a774ac52caaa03bcfc9e7591ac8c18617ced",
+        "rev": "daf6dc47aa4b44791372d6139ab7b25269184d55",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "nixos-25.05-small",
+        "ref": "nixos-25.05",
         "repo": "nixpkgs",
         "type": "github"
       }

flake.nix

@@ -1,7 +1,7 @@
 {
   description = "The purely functional package manager";
 
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05-small";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
 
   inputs.nixpkgs-regression.url = "github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
   inputs.nixpkgs-23-11.url = "github:NixOS/nixpkgs/a62e6edd6d5e1fa0329b8653c801147986f8d446";
@@ -32,7 +32,7 @@
     let
       inherit (nixpkgs) lib;
 
-      officialRelease = false;
+      officialRelease = true;
 
       linux32BitSystems = [ "i686-linux" ];
       linux64BitSystems = [

maintainers/README.md

@@ -46,7 +46,7 @@ The team meets twice a week (times are denoted in the [Europe/Amsterdam](https:/
        - mark it as draft if it is blocked on the contributor
        - escalate it back to the team by moving it to To discuss, and leaving a comment as to why the issue needs to be discussed again.
 
-- Work meeting: Mondays 14:00-16:00 Europe/Amsterdam see [calendar](https://calendar.google.com/calendar/u/0/embed?src=b9o52fobqjak8oq8lfkhg3t0qg@group.calendar.google.com).
+- Work meeting: Mondays 18:00-20:00 Europe/Amsterdam; see [calendar](https://calendar.google.com/calendar/u/0/embed?src=b9o52fobqjak8oq8lfkhg3t0qg@group.calendar.google.com).
 
   1. Code review on pull requests from [In review](#in-review).
   2. Other chores and tasks.

maintainers/data/release-credits-email-to-handle.json

@@ -203,5 +203,26 @@
     "ConnorBaker01@Gmail.com": "ConnorBaker",
     "jsoo1@asu.edu": "jsoo1",
     "hsngrmpf+github@gmail.com": "DavHau",
-    "matthew@floxdev.com": "mkenigs"
+    "matthew@floxdev.com": "mkenigs",
+    "taeer@bar-yam.me": "Radvendii",
+    "beme@anthropic.com": "lovesegfault",
+    "osbm@osbm.dev": "osbm",
+    "jami.kettunen@protonmail.com": "JamiKettunen",
+    "ephraim.siegfried@hotmail.com": "EphraimSiegfried",
+    "rszyma.dev@gmail.com": "rszyma",
+    "tristan.ross@determinate.systems": "RossComputerGuy",
+    "corngood@gmail.com": "corngood",
+    "jfroche@pyxel.be": "jfroche",
+    "848000+eclairevoyant@users.noreply.github.com": "eclairevoyant",
+    "petersen@redhat.com": "juhp",
+    "dramforever@live.com": "dramforever",
+    "me@glenhuang.com": "hgl",
+    "philip.wilk@fivium.co.uk": "philipwilk",
+    "me@nycode.dev": "NyCodeGHG",
+    "14264576+twesterhout@users.noreply.github.com": "twesterhout",
+    "sinan@sinanmohd.com": "sinanmohd",
+    "42688647+netadr@users.noreply.github.com": "netadr",
+    "matej.urbas@gmail.com": "urbas",
+    "ethanalexevans@gmail.com": "ethanavatar",
+    "greg.marti@gmail.com": "gmarti"
 }

maintainers/data/release-credits-handle-to-name.json

@@ -177,5 +177,24 @@
     "avnik": "Alexander V. Nikolaev",
     "DavHau": null,
     "aln730": "AGawas",
-    "vog": "Volker Diels-Grabsch"
+    "vog": "Volker Diels-Grabsch",
+    "corngood": "David McFarland",
+    "twesterhout": "Tom Westerhout",
+    "JamiKettunen": "Jami Kettunen",
+    "dramforever": "dram",
+    "philipwilk": "Philip Wilk",
+    "netadr": "Clayton",
+    "NyCodeGHG": "Marie",
+    "jfroche": "Jean-Fran\u00e7ois Roche",
+    "urbas": "Matej Urbas",
+    "osbm": "osman - \u30aa\u30b9\u30de\u30f3",
+    "rszyma": null,
+    "eclairevoyant": "\u00e9clairevoyant",
+    "Radvendii": "Taeer Bar-Yam",
+    "sinanmohd": "Sinan Mohd",
+    "ethanavatar": "Ethan Evans",
+    "gmarti": "Gr\u00e9gory Marti",
+    "lovesegfault": "Bernardo Meurer",
+    "EphraimSiegfried": "Ephraim Siegfried",
+    "hgl": "Glen Huang"
 }

maintainers/flake-module.nix

@@ -79,6 +79,8 @@
               # Not supported by nixfmt
               ''^tests/functional/lang/eval-okay-deprecate-cursed-or\.nix$''
               ''^tests/functional/lang/eval-okay-attrs5\.nix$''
+              ''^tests/functional/lang/eval-fail-dynamic-attrs-inherit\.nix$''
+              ''^tests/functional/lang/eval-fail-dynamic-attrs-inherit-2\.nix$''
 
               # More syntax tests
               # These tests, or parts of them, should have been parse-* test cases.
@@ -104,151 +106,6 @@
           };
           shellcheck = {
             enable = true;
-            excludes = [
-              # We haven't linted these files yet
-              ''^config/install-sh$''
-              ''^misc/bash/completion\.sh$''
-              ''^misc/fish/completion\.fish$''
-              ''^misc/zsh/completion\.zsh$''
-              ''^scripts/create-darwin-volume\.sh$''
-              ''^scripts/install-darwin-multi-user\.sh$''
-              ''^scripts/install-multi-user\.sh$''
-              ''^scripts/install-systemd-multi-user\.sh$''
-              ''^src/nix/get-env\.sh$''
-              ''^tests/functional/ca/build-dry\.sh$''
-              ''^tests/functional/ca/build-with-garbage-path\.sh$''
-              ''^tests/functional/ca/common\.sh$''
-              ''^tests/functional/ca/concurrent-builds\.sh$''
-              ''^tests/functional/ca/eval-store\.sh$''
-              ''^tests/functional/ca/gc\.sh$''
-              ''^tests/functional/ca/import-from-derivation\.sh$''
-              ''^tests/functional/ca/new-build-cmd\.sh$''
-              ''^tests/functional/ca/nix-shell\.sh$''
-              ''^tests/functional/ca/post-hook\.sh$''
-              ''^tests/functional/ca/recursive\.sh$''
-              ''^tests/functional/ca/repl\.sh$''
-              ''^tests/functional/ca/selfref-gc\.sh$''
-              ''^tests/functional/ca/why-depends\.sh$''
-              ''^tests/functional/characterisation-test-infra\.sh$''
-              ''^tests/functional/common/vars-and-functions\.sh$''
-              ''^tests/functional/completions\.sh$''
-              ''^tests/functional/compute-levels\.sh$''
-              ''^tests/functional/config\.sh$''
-              ''^tests/functional/db-migration\.sh$''
-              ''^tests/functional/debugger\.sh$''
-              ''^tests/functional/dependencies\.builder0\.sh$''
-              ''^tests/functional/dependencies\.sh$''
-              ''^tests/functional/dump-db\.sh$''
-              ''^tests/functional/dyn-drv/build-built-drv\.sh$''
-              ''^tests/functional/dyn-drv/common\.sh$''
-              ''^tests/functional/dyn-drv/dep-built-drv\.sh$''
-              ''^tests/functional/dyn-drv/eval-outputOf\.sh$''
-              ''^tests/functional/dyn-drv/old-daemon-error-hack\.sh$''
-              ''^tests/functional/dyn-drv/recursive-mod-json\.sh$''
-              ''^tests/functional/eval-store\.sh$''
-              ''^tests/functional/export-graph\.sh$''
-              ''^tests/functional/export\.sh$''
-              ''^tests/functional/extra-sandbox-profile\.sh$''
-              ''^tests/functional/fetchClosure\.sh$''
-              ''^tests/functional/fetchGit\.sh$''
-              ''^tests/functional/fetchGitRefs\.sh$''
-              ''^tests/functional/fetchGitSubmodules\.sh$''
-              ''^tests/functional/fetchGitVerification\.sh$''
-              ''^tests/functional/fetchMercurial\.sh$''
-              ''^tests/functional/fixed\.builder1\.sh$''
-              ''^tests/functional/fixed\.builder2\.sh$''
-              ''^tests/functional/fixed\.sh$''
-              ''^tests/functional/flakes/absolute-paths\.sh$''
-              ''^tests/functional/flakes/check\.sh$''
-              ''^tests/functional/flakes/config\.sh$''
-              ''^tests/functional/flakes/flakes\.sh$''
-              ''^tests/functional/flakes/follow-paths\.sh$''
-              ''^tests/functional/flakes/prefetch\.sh$''
-              ''^tests/functional/flakes/run\.sh$''
-              ''^tests/functional/flakes/show\.sh$''
-              ''^tests/functional/formatter\.sh$''
-              ''^tests/functional/formatter\.simple\.sh$''
-              ''^tests/functional/gc-auto\.sh$''
-              ''^tests/functional/gc-concurrent\.builder\.sh$''
-              ''^tests/functional/gc-concurrent\.sh$''
-              ''^tests/functional/gc-concurrent2\.builder\.sh$''
-              ''^tests/functional/gc-non-blocking\.sh$''
-              ''^tests/functional/hash-convert\.sh$''
-              ''^tests/functional/impure-derivations\.sh$''
-              ''^tests/functional/impure-eval\.sh$''
-              ''^tests/functional/install-darwin\.sh$''
-              ''^tests/functional/legacy-ssh-store\.sh$''
-              ''^tests/functional/linux-sandbox\.sh$''
-              ''^tests/functional/local-overlay-store/add-lower-inner\.sh$''
-              ''^tests/functional/local-overlay-store/add-lower\.sh$''
-              ''^tests/functional/local-overlay-store/bad-uris\.sh$''
-              ''^tests/functional/local-overlay-store/build-inner\.sh$''
-              ''^tests/functional/local-overlay-store/build\.sh$''
-              ''^tests/functional/local-overlay-store/check-post-init-inner\.sh$''
-              ''^tests/functional/local-overlay-store/check-post-init\.sh$''
-              ''^tests/functional/local-overlay-store/common\.sh$''
-              ''^tests/functional/local-overlay-store/delete-duplicate-inner\.sh$''
-              ''^tests/functional/local-overlay-store/delete-duplicate\.sh$''
-              ''^tests/functional/local-overlay-store/delete-refs-inner\.sh$''
-              ''^tests/functional/local-overlay-store/delete-refs\.sh$''
-              ''^tests/functional/local-overlay-store/gc-inner\.sh$''
-              ''^tests/functional/local-overlay-store/gc\.sh$''
-              ''^tests/functional/local-overlay-store/optimise-inner\.sh$''
-              ''^tests/functional/local-overlay-store/optimise\.sh$''
-              ''^tests/functional/local-overlay-store/redundant-add-inner\.sh$''
-              ''^tests/functional/local-overlay-store/redundant-add\.sh$''
-              ''^tests/functional/local-overlay-store/remount\.sh$''
-              ''^tests/functional/local-overlay-store/stale-file-handle-inner\.sh$''
-              ''^tests/functional/local-overlay-store/stale-file-handle\.sh$''
-              ''^tests/functional/local-overlay-store/verify-inner\.sh$''
-              ''^tests/functional/local-overlay-store/verify\.sh$''
-              ''^tests/functional/logging\.sh$''
-              ''^tests/functional/misc\.sh$''
-              ''^tests/functional/multiple-outputs\.sh$''
-              ''^tests/functional/nested-sandboxing\.sh$''
-              ''^tests/functional/nested-sandboxing/command\.sh$''
-              ''^tests/functional/nix-build\.sh$''
-              ''^tests/functional/nix-channel\.sh$''
-              ''^tests/functional/nix-collect-garbage-d\.sh$''
-              ''^tests/functional/nix-copy-ssh-common\.sh$''
-              ''^tests/functional/nix-copy-ssh-ng\.sh$''
-              ''^tests/functional/nix-copy-ssh\.sh$''
-              ''^tests/functional/nix-daemon-untrusting\.sh$''
-              ''^tests/functional/nix-profile\.sh$''
-              ''^tests/functional/nix-shell\.sh$''
-              ''^tests/functional/nix_path\.sh$''
-              ''^tests/functional/optimise-store\.sh$''
-              ''^tests/functional/output-normalization\.sh$''
-              ''^tests/functional/parallel\.builder\.sh$''
-              ''^tests/functional/parallel\.sh$''
-              ''^tests/functional/pass-as-file\.sh$''
-              ''^tests/functional/path-from-hash-part\.sh$''
-              ''^tests/functional/path-info\.sh$''
-              ''^tests/functional/placeholders\.sh$''
-              ''^tests/functional/post-hook\.sh$''
-              ''^tests/functional/pure-eval\.sh$''
-              ''^tests/functional/push-to-store-old\.sh$''
-              ''^tests/functional/push-to-store\.sh$''
-              ''^tests/functional/read-only-store\.sh$''
-              ''^tests/functional/readfile-context\.sh$''
-              ''^tests/functional/recursive\.sh$''
-              ''^tests/functional/referrers\.sh$''
-              ''^tests/functional/remote-store\.sh$''
-              ''^tests/functional/repair\.sh$''
-              ''^tests/functional/restricted\.sh$''
-              ''^tests/functional/search\.sh$''
-              ''^tests/functional/secure-drv-outputs\.sh$''
-              ''^tests/functional/selfref-gc\.sh$''
-              ''^tests/functional/shell\.shebang\.sh$''
-              ''^tests/functional/simple\.builder\.sh$''
-              ''^tests/functional/supplementary-groups\.sh$''
-              ''^tests/functional/toString-path\.sh$''
-              ''^tests/functional/user-envs-migration\.sh$''
-              ''^tests/functional/user-envs-test-case\.sh$''
-              ''^tests/functional/user-envs\.builder\.sh$''
-              ''^tests/functional/user-envs\.sh$''
-              ''^tests/functional/why-depends\.sh$''
-            ];
           };
         };
       };

maintainers/keys/158A6F530EA202E5F651611314FAEA63448E1DF9.asc

@@ -0,0 +1,110 @@
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+=mXYP
+-----END PGP PUBLIC KEY BLOCK-----

maintainers/keys/B541D55301270E0BCF15CA5D8170B4726D7198DE.asc

@@ -0,0 +1,51 @@
+-----BEGIN PGP PUBLIC KEY BLOCK-----
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+-----END PGP PUBLIC KEY BLOCK-----

maintainers/keys/README.md

@@ -0,0 +1,13 @@
+# Maintainer GPG Keys
+
+Release tags are signed by members of the [Nix maintainer team](https://nixos.org/community/teams/nix/) as part of the [release process](../release-process.md). This directory contains the public GPG keys used for signing.
+
+## Keys
+
+- **Eelco Dolstra**
+  GPG Fingerprint: `B541 D553 0127 0E0B CF15 CA5D 8170 B472 6D71 98DE`
+
+- **Sergei Zimmerman**
+  GPG Fingerprint: [`158A 6F53 0EA2 02E5 F651 6113 14FA EA63 448E 1DF9`](https://keys.openpgp.org/vks/v1/by-fingerprint/158A6F530EA202E5F651611314FAEA63448E1DF9)
+
+<!-- TODO: Add keys for other Nix team members -->

maintainers/release-notes-todo

@@ -0,0 +1,58 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+# debug:
+# set -x
+
+START_REF="${1}"
+END_REF="${2:-upstream/master}"
+
+# Get the merge base
+MERGE_BASE=$(git merge-base "$START_REF" "$END_REF")
+unset START_REF
+
+# Get date range
+START_DATE=$(git show -s --format=%cI "$MERGE_BASE")
+END_DATE=$(git show -s --format=%cI "$END_REF")
+
+echo "Checking PRs merged between $START_DATE and $END_DATE" >&2
+
+# Get all commits between merge base and HEAD
+COMMITS=$(git rev-list "$MERGE_BASE..$END_REF")
+
+# Convert to set for fast lookup
+declare -A commit_set
+for commit in $COMMITS; do
+    commit_set["$commit"]=1
+done
+
+# Get the current changelog
+LOG_DONE="$(changelog-d doc/manual/rl-next)"
+is_done(){
+    local nr="$1"
+    echo "$LOG_DONE" | grep -E "^- .*/pull/$nr)"
+}
+
+# Query merged PRs in date range
+gh pr list \
+    --repo NixOS/nix \
+    --state merged \
+    --limit 1000 \
+    --json number,title,author,mergeCommit \
+    --search "merged:$START_DATE..$END_DATE" | \
+jq -r '.[] | [.number, .mergeCommit.oid, .title, .author.login] | @tsv' | \
+while IFS=$'\t' read -r pr_num merge_commit _title author; do
+    # Check if this PR's merge commit is in our branch
+    if [[ -n "${commit_set[$merge_commit]:-}" ]]; then
+        # Full detail, not suitable for comment due to mass ping and duplicate title
+        # echo "- #$pr_num $_title (@$author)"
+        echo "- #$pr_num ($author)"
+        if is_done "$pr_num"
+        then
+            echo "  - [x] has note"
+        else
+            echo "  - [ ] has note"
+        fi
+        echo "  - [ ] skip"
+    fi
+done

maintainers/release-process.md

@@ -5,11 +5,11 @@
 The release process is intended to create the following for each
 release:
 
-* A Git tag
+* A signed Git tag (public keys in `maintainers/keys/`)
 
 * Binary tarballs in https://releases.nixos.org/?prefix=nix/
 
-* Docker images
+* Docker images (arm64 and amd64 variants, uploaded to DockerHub and GHCR)
 
 * Closures in https://cache.nixos.org
 
@@ -24,6 +24,12 @@ release:
 * In a checkout of the Nix repo, make sure you're on `master` and run
   `git pull`.
 
+* Compile a release notes to-do list by running
+
+  ```console
+  $ ./maintainers/release-notes-todo PREV_RELEASE HEAD
+  ```
+
 * Compile the release notes by running
 
   ```console
@@ -98,21 +104,17 @@ release:
   evaluation ID (e.g. `1780832` in
   `https://hydra.nixos.org/eval/1780832`).
 
-* Tag the release and upload the release artifacts to
-  [`releases.nixos.org`](https://releases.nixos.org/) and [Docker Hub](https://hub.docker.com/):
+* Tag the release:
 
   ```console
-  $ IS_LATEST=1 ./maintainers/upload-release.pl <EVAL-ID>
+  $ IS_LATEST=1 ./maintainers/upload-release.pl --skip-docker --skip-s3 --project-root $PWD <EVAL-ID>
   ```
 
   Note: `IS_LATEST=1` causes the `latest-release` branch to be
   force-updated. This is used by the `nixos.org` website to get the
   [latest Nix manual](https://nixos.org/manual/nixpkgs/unstable/).
 
-  TODO: This script requires the right AWS credentials. Document.
-
-  TODO: This script currently requires a
-  `/home/eelco/Dev/nix-pristine`.
+* Trigger the [`upload-release.yml` workflow](https://github.com/NixOS/nix/actions/workflows/upload-release.yml) via `workflow_dispatch` trigger. At the top click `Run workflow` -> select the current release branch from `Use workflow from` -> fill in `Hydra evaluation ID` with `<EVAL-ID>` value from previous steps -> click `Run workflow`. Wait for the run to be approved by `NixOS/nix-team` (or bypass checks if warranted). Wait for the workflow to succeed.
 
   TODO: trigger nixos.org netlify: https://docs.netlify.com/configure-builds/build-hooks/
 
@@ -127,6 +129,8 @@ release:
 
   Commit and push this to the maintenance branch.
 
+* Create a backport label.
+
 * Bump the version of `master`:
 
   ```console
@@ -134,6 +138,7 @@ release:
   $ git pull
   $ NEW_VERSION=2.13.0
   $ echo $NEW_VERSION > .version
+  $ ... edit .mergify.yml to add the previous version ...
   $ git checkout -b bump-$NEW_VERSION
   $ git commit -a -m 'Bump version'
   $ git push --set-upstream origin bump-$NEW_VERSION
@@ -141,10 +146,6 @@ release:
 
   Make a pull request and auto-merge it.
 
-* Create a backport label.
-
-* Add the new backport label to `.mergify.yml`.
-
 * Post an [announcement on Discourse](https://discourse.nixos.org/c/announcements/8), including the contents of
   `rl-$VERSION.md`.
 
@@ -177,16 +178,18 @@ release:
 * Wait for the desired evaluation of the maintenance jobset to finish
   building.
 
-* Run
+* Tag the release
 
   ```console
-  $ IS_LATEST=1 ./maintainers/upload-release.pl <EVAL-ID>
+  $ IS_LATEST=1 ./maintainers/upload-release.pl --skip-docker --skip-s3 --project-root $PWD <EVAL-ID>
   ```
 
   Omit `IS_LATEST=1` when creating a point release that is not on the
   most recent stable branch. This prevents `nixos.org` to going back
   to an older release.
 
+* Trigger the [`upload-release.yml` workflow](https://github.com/NixOS/nix/actions/workflows/upload-release.yml) via `workflow_dispatch` trigger. At the top click `Run workflow` -> select the current release branch from `Use workflow from` -> fill in `Hydra evaluation ID` with `<EVAL-ID>` value from previous steps -> click `Run workflow`. Wait for the run to be approved by `NixOS/nix-team` (or bypass checks if warranted). Wait for the workflow to succeed.
+
 * Bump the version number of the release branch as above (e.g. to
   `2.12.2`).
 

maintainers/upload-release.pl

@@ -1,7 +1,8 @@
 #! /usr/bin/env nix-shell
-#! nix-shell -i perl -p perl perlPackages.LWPUserAgent perlPackages.LWPProtocolHttps perlPackages.FileSlurp perlPackages.NetAmazonS3 gnupg1
+#! nix-shell -i perl -p awscli2 perl perlPackages.LWPUserAgent perlPackages.LWPProtocolHttps perlPackages.FileSlurp perlPackages.NetAmazonS3 perlPackages.GetoptLongDescriptive gnupg1
 
 use strict;
+use Getopt::Long::Descriptive;
 use Data::Dumper;
 use File::Basename;
 use File::Path;
@@ -13,7 +14,30 @@ use Net::Amazon::S3;
 
 delete $ENV{'shell'}; # shut up a LWP::UserAgent.pm warning
 
-my $evalId = $ARGV[0] or die "Usage: $0 EVAL-ID\n";
+my ($opt, $usage) = describe_options(
+    '%c %o <eval-id>',
+    [ 'skip-docker',      'Skip Docker image upload' ],
+    [ 'skip-git',         'Skip Git tagging' ],
+    [ 'skip-s3',          'Skip S3 upload' ],
+    [ 'docker-owner=s',   'Docker image owner', { default => 'nixos/nix' } ],
+    [ 'project-root=s',   'Pristine git repository path' ],
+    [ 's3-endpoint=s',    'Custom S3 endpoint' ],
+    [ 's3-host=s',        'S3 host', { default => 's3-eu-west-1.amazonaws.com' } ],
+    [],
+    [ 'help|h',           'Show this help message', { shortcircuit => 1 } ],
+    [],
+    [ 'Environment variables:' ],
+    [ 'AWS_ACCESS_KEY_ID' ],
+    [ 'AWS_SECRET_ACCESS_KEY' ],
+    [ 'AWS_SESSION_TOKEN   For OIDC' ],
+    [ 'IS_LATEST           Set to "1" to mark as latest release' ],
+);
+
+print($usage->text), exit if $opt->help;
+
+my $evalId = $ARGV[0] or do { print STDERR $usage->text; exit 1 };
+
+die "--project-root is required unless --skip-git is specified\n" unless $opt->skip_git || $opt->project_root;
 
 my $releasesBucketName = "nix-releases";
 my $channelsBucketName = "nix-channels";
@@ -62,25 +86,38 @@ File::Path::make_path($narCache);
 my $binaryCache = "https://cache.nixos.org/?local-nar-cache=$narCache";
 
 # S3 setup.
-my $aws_access_key_id = $ENV{'AWS_ACCESS_KEY_ID'} or die "No AWS_ACCESS_KEY_ID given.";
-my $aws_secret_access_key = $ENV{'AWS_SECRET_ACCESS_KEY'} or die "No AWS_SECRET_ACCESS_KEY given.";
+my $aws_access_key_id = $ENV{'AWS_ACCESS_KEY_ID'};
+my $aws_secret_access_key = $ENV{'AWS_SECRET_ACCESS_KEY'};
+my $aws_session_token = $ENV{'AWS_SESSION_TOKEN'};
 
-my $s3 = Net::Amazon::S3->new(
-    { aws_access_key_id     => $aws_access_key_id,
-      aws_secret_access_key => $aws_secret_access_key,
-      retry                 => 1,
-      host                  => "s3-eu-west-1.amazonaws.com",
-    });
+my ($s3, $releasesBucket, $s3_channels, $channelsBucket);
 
-my $releasesBucket = $s3->bucket($releasesBucketName) or die;
+unless ($opt->skip_s3) {
+    $aws_access_key_id or die "No AWS_ACCESS_KEY_ID given.";
+    $aws_secret_access_key or die "No AWS_SECRET_ACCESS_KEY given.";
 
-my $s3_us = Net::Amazon::S3->new(
-    { aws_access_key_id     => $aws_access_key_id,
-      aws_secret_access_key => $aws_secret_access_key,
-      retry                 => 1,
-    });
+    $s3 = Net::Amazon::S3->new(
+        { aws_access_key_id     => $aws_access_key_id,
+          aws_secret_access_key => $aws_secret_access_key,
+          $aws_session_token ? (aws_session_token => $aws_session_token) : (),
+          retry                 => 1,
+          host                  => $opt->s3_host,
+          secure                => ($opt->s3_endpoint && $opt->s3_endpoint =~ /^http:/) ? 0 : 1,
+        });
 
-my $channelsBucket = $s3_us->bucket($channelsBucketName) or die;
+    $releasesBucket = $s3->bucket($releasesBucketName) or die;
+
+    $s3_channels = Net::Amazon::S3->new(
+        { aws_access_key_id     => $aws_access_key_id,
+          aws_secret_access_key => $aws_secret_access_key,
+          $aws_session_token ? (aws_session_token => $aws_session_token) : (),
+          retry                 => 1,
+          $opt->s3_endpoint ? (host => $opt->s3_host) : (),
+          $opt->s3_endpoint ? (secure => ($opt->s3_endpoint =~ /^http:/) ? 0 : 1) : (),
+        });
+
+    $channelsBucket = $s3_channels->bucket($channelsBucketName) or die;
+}
 
 sub getStorePath {
     my ($jobName, $output) = @_;
@@ -115,11 +152,12 @@ sub copyManual {
         File::Path::remove_tree("$tmpDir/manual.tmp", {safe => 1});
     }
 
-    system("aws s3 sync '$tmpDir/manual' s3://$releasesBucketName/$releaseDir/manual") == 0
+    my $awsEndpoint = $opt->s3_endpoint ? "--endpoint-url " . $opt->s3_endpoint : "";
+    system("aws $awsEndpoint s3 sync '$tmpDir/manual' s3://$releasesBucketName/$releaseDir/manual") == 0
         or die "syncing manual to S3\n";
 }
 
-copyManual;
+copyManual unless $opt->skip_s3;
 
 sub downloadFile {
     my ($jobName, $productNr, $dstName) = @_;
@@ -158,30 +196,12 @@ sub downloadFile {
     return $sha256_expected;
 }
 
-downloadFile("binaryTarball.i686-linux", "1");
-downloadFile("binaryTarball.x86_64-linux", "1");
-downloadFile("binaryTarball.aarch64-linux", "1");
-downloadFile("binaryTarball.x86_64-darwin", "1");
-downloadFile("binaryTarball.aarch64-darwin", "1");
-eval {
-    downloadFile("binaryTarballCross.x86_64-linux.armv6l-unknown-linux-gnueabihf", "1");
-};
-warn "$@" if $@;
-eval {
-    downloadFile("binaryTarballCross.x86_64-linux.armv7l-unknown-linux-gnueabihf", "1");
-};
-warn "$@" if $@;
-eval {
-    downloadFile("binaryTarballCross.x86_64-linux.riscv64-unknown-linux-gnu", "1");
-};
-warn "$@" if $@;
-downloadFile("installerScript", "1");
-
-# Upload docker images to dockerhub.
+# Upload docker images.
 my $dockerManifest = "";
 my $dockerManifestLatest = "";
 my $haveDocker = 0;
 
+unless ($opt->skip_docker) {
 for my $platforms (["x86_64-linux", "amd64"], ["aarch64-linux", "arm64"]) {
     my $system = $platforms->[0];
     my $dockerPlatform = $platforms->[1];
@@ -195,8 +215,8 @@ for my $platforms (["x86_64-linux", "amd64"], ["aarch64-linux", "arm64"]) {
     print STDERR "loading docker image for $dockerPlatform...\n";
     system("docker load -i $tmpDir/$fn") == 0 or die;
 
-    my $tag = "nixos/nix:$version-$dockerPlatform";
-    my $latestTag = "nixos/nix:latest-$dockerPlatform";
+    my $tag = $opt->docker_owner . ":$version-$dockerPlatform";
+    my $latestTag = $opt->docker_owner . ":latest-$dockerPlatform";
 
     print STDERR "tagging $version docker image for $dockerPlatform...\n";
     system("docker tag nix:$version $tag") == 0 or die;
@@ -219,68 +239,94 @@ for my $platforms (["x86_64-linux", "amd64"], ["aarch64-linux", "arm64"]) {
 }
 
 if ($haveDocker) {
+    my $dockerOwner = $opt->docker_owner;
     print STDERR "creating multi-platform docker manifest...\n";
-    system("docker manifest rm nixos/nix:$version");
-    system("docker manifest create nixos/nix:$version $dockerManifest") == 0 or die;
+    system("docker manifest rm $dockerOwner:$version");
+    system("docker manifest create $dockerOwner:$version $dockerManifest") == 0 or die;
     if ($isLatest) {
         print STDERR "creating latest multi-platform docker manifest...\n";
-        system("docker manifest rm nixos/nix:latest");
-        system("docker manifest create nixos/nix:latest $dockerManifestLatest") == 0 or die;
+        system("docker manifest rm $dockerOwner:latest");
+        system("docker manifest create $dockerOwner:latest $dockerManifestLatest") == 0 or die;
     }
 
     print STDERR "pushing multi-platform docker manifest...\n";
-    system("docker manifest push nixos/nix:$version") == 0 or die;
+    system("docker manifest push $dockerOwner:$version") == 0 or die;
 
     if ($isLatest) {
         print STDERR "pushing latest multi-platform docker manifest...\n";
-        system("docker manifest push nixos/nix:latest") == 0 or die;
+        system("docker manifest push $dockerOwner:latest") == 0 or die;
     }
 }
+}
 
-# Upload nix-fallback-paths.nix.
-write_file("$tmpDir/fallback-paths.nix",
-    "{\n" .
-    "  x86_64-linux = \"" . getStorePath("build.nix-everything.x86_64-linux") . "\";\n" .
-    "  i686-linux = \"" . getStorePath("build.nix-everything.i686-linux") . "\";\n" .
-    "  aarch64-linux = \"" . getStorePath("build.nix-everything.aarch64-linux") . "\";\n" .
-    "  riscv64-linux = \"" . getStorePath("buildCross.nix-everything.riscv64-unknown-linux-gnu.x86_64-linux") . "\";\n" .
-    "  x86_64-darwin = \"" . getStorePath("build.nix-everything.x86_64-darwin") . "\";\n" .
-    "  aarch64-darwin = \"" . getStorePath("build.nix-everything.aarch64-darwin") . "\";\n" .
-    "}\n");
 
 # Upload release files to S3.
-for my $fn (glob "$tmpDir/*") {
-    my $name = basename($fn);
-    next if $name eq "manual";
-    my $dstKey = "$releaseDir/" . $name;
-    unless (defined $releasesBucket->head_key($dstKey)) {
-        print STDERR "uploading $fn to s3://$releasesBucketName/$dstKey...\n";
+unless ($opt->skip_s3) {
+    downloadFile("binaryTarball.i686-linux", "1");
+    downloadFile("binaryTarball.x86_64-linux", "1");
+    downloadFile("binaryTarball.aarch64-linux", "1");
+    downloadFile("binaryTarball.x86_64-darwin", "1");
+    downloadFile("binaryTarball.aarch64-darwin", "1");
+    eval {
+        downloadFile("binaryTarballCross.x86_64-linux.armv6l-unknown-linux-gnueabihf", "1");
+    };
+    warn "$@" if $@;
+    eval {
+        downloadFile("binaryTarballCross.x86_64-linux.armv7l-unknown-linux-gnueabihf", "1");
+    };
+    warn "$@" if $@;
+    eval {
+        downloadFile("binaryTarballCross.x86_64-linux.riscv64-unknown-linux-gnu", "1");
+    };
+    warn "$@" if $@;
+    downloadFile("installerScript", "1");
 
-        my $configuration = ();
-        $configuration->{content_type} = "application/octet-stream";
+    # Upload nix-fallback-paths.nix.
+    write_file("$tmpDir/fallback-paths.nix",
+        "{\n" .
+        "  x86_64-linux = \"" . getStorePath("build.nix-everything.x86_64-linux") . "\";\n" .
+        "  i686-linux = \"" . getStorePath("build.nix-everything.i686-linux") . "\";\n" .
+        "  aarch64-linux = \"" . getStorePath("build.nix-everything.aarch64-linux") . "\";\n" .
+        "  riscv64-linux = \"" . getStorePath("buildCross.nix-everything.riscv64-unknown-linux-gnu.x86_64-linux") . "\";\n" .
+        "  x86_64-darwin = \"" . getStorePath("build.nix-everything.x86_64-darwin") . "\";\n" .
+        "  aarch64-darwin = \"" . getStorePath("build.nix-everything.aarch64-darwin") . "\";\n" .
+        "}\n");
 
-        if ($fn =~ /.sha256|install|\.nix$/) {
-            $configuration->{content_type} = "text/plain";
+    for my $fn (glob "$tmpDir/*") {
+        my $name = basename($fn);
+        next if $name eq "manual";
+        my $dstKey = "$releaseDir/" . $name;
+        unless (defined $releasesBucket->head_key($dstKey)) {
+            print STDERR "uploading $fn to s3://$releasesBucketName/$dstKey...\n";
+
+            my $configuration = ();
+            $configuration->{content_type} = "application/octet-stream";
+
+            if ($fn =~ /.sha256|install|\.nix$/) {
+                $configuration->{content_type} = "text/plain";
+            }
+
+            $releasesBucket->add_key_filename($dstKey, $fn, $configuration)
+                or die $releasesBucket->err . ": " . $releasesBucket->errstr;
         }
-
-        $releasesBucket->add_key_filename($dstKey, $fn, $configuration)
-            or die $releasesBucket->err . ": " . $releasesBucket->errstr;
     }
+
+    # Update the "latest" symlink.
+    $channelsBucket->add_key(
+        "nix-latest/install", "",
+        { "x-amz-website-redirect-location" => "https://releases.nixos.org/$releaseDir/install" })
+        or die $channelsBucket->err . ": " . $channelsBucket->errstr
+        if $isLatest;
 }
 
-# Update the "latest" symlink.
-$channelsBucket->add_key(
-    "nix-latest/install", "",
-    { "x-amz-website-redirect-location" => "https://releases.nixos.org/$releaseDir/install" })
-    or die $channelsBucket->err . ": " . $channelsBucket->errstr
-    if $isLatest;
-
 # Tag the release in Git.
-chdir("/home/eelco/Dev/nix-pristine") or die;
-system("git remote update origin") == 0 or die;
-system("git tag --force --sign $version $nixRev -m 'Tagging release $version'") == 0 or die;
-system("git push --tags") == 0 or die;
-system("git push --force-with-lease origin $nixRev:refs/heads/latest-release") == 0 or die if $isLatest;
+unless ($opt->skip_git) {
+    chdir($opt->project_root) or die "Cannot chdir to " . $opt->project_root . ": $!";
+    system("git remote update origin") == 0 or die;
+    system("git tag --force --sign $version $nixRev -m 'Tagging release $version'") == 0 or die;
+    system("git push origin refs/tags/$version") == 0 or die;
+    system("git push --force-with-lease origin $nixRev:refs/heads/latest-release") == 0 or die if $isLatest;
+}
 
 File::Path::remove_tree($narCache, {safe => 1});
 File::Path::remove_tree($tmpDir, {safe => 1});

meson.build

@@ -41,8 +41,10 @@ subproject('libexpr-c')
 subproject('libflake-c')
 subproject('libmain-c')
 
+asan_enabled = 'address' in get_option('b_sanitize')
+
 # Language Bindings
-if get_option('bindings') and not meson.is_cross_build()
+if get_option('bindings') and not meson.is_cross_build() and not asan_enabled
   subproject('perl')
 endif
 

misc/bash/completion.sh

@@ -1,3 +1,4 @@
+# shellcheck shell=bash
 function _complete_nix {
     local -a words
     local cword cur

misc/fish/completion.fish

@@ -1,3 +1,4 @@
+# shellcheck disable=all
 function _nix_complete
   # Get the current command up to a cursor.
   # - Behaves correctly even with pipes and nested in commands like env.

misc/zsh/completion.zsh

@@ -1,4 +1,5 @@
 #compdef nix
+# shellcheck disable=all
 
 function _nix() {
   local ifs_bk="$IFS"

nix-meson-build-support/asan-options/meson.build

@@ -0,0 +1,12 @@
+asan_test_options_env = {
+  'ASAN_OPTIONS' : 'abort_on_error=1:print_summary=1:detect_leaks=0',
+}
+
+# Clang gets grumpy about missing libasan symbols if -shared-libasan is not
+# passed when building shared libs, at least on Linux
+if cxx.get_id() == 'clang' and ('address' in get_option('b_sanitize') or 'undefined' in get_option(
+  'b_sanitize',
+))
+  add_project_link_arguments('-shared-libasan', language : 'cpp')
+endif
+

nix-meson-build-support/common/assert-fail/meson.build

@@ -0,0 +1,32 @@
+can_wrap_assert_fail_test_code = '''
+#include <cstdlib>
+#include <cassert>
+
+int main()
+{
+    assert(0);
+}
+
+extern "C" void * __real___assert_fail(const char *, const char *, unsigned int, const char *);
+
+extern "C" void *
+__wrap___assert_fail(const char *, const char *, unsigned int, const char *)
+{
+    return __real___assert_fail(nullptr, nullptr, 0, nullptr);
+}
+'''
+
+wrap_assert_fail_args = [ '-Wl,--wrap=__assert_fail' ]
+
+can_wrap_assert_fail = cxx.links(
+  can_wrap_assert_fail_test_code,
+  args : wrap_assert_fail_args,
+  name : 'linker can wrap __assert_fail',
+)
+
+if can_wrap_assert_fail
+  deps_other += declare_dependency(
+    sources : 'wrap-assert-fail.cc',
+    link_args : wrap_assert_fail_args,
+  )
+endif

nix-meson-build-support/common/assert-fail/wrap-assert-fail.cc

@@ -0,0 +1,17 @@
+#include "nix/util/error.hh"
+
+#include <cstdio>
+#include <cstdlib>
+#include <cinttypes>
+#include <string_view>
+
+extern "C" [[noreturn]] void __attribute__((weak))
+__wrap___assert_fail(const char * assertion, const char * file, unsigned int line, const char * function)
+{
+    char buf[512];
+    int n =
+        snprintf(buf, sizeof(buf), "Assertion '%s' failed in %s at %s:%" PRIuLEAST32, assertion, function, file, line);
+    if (n < 0)
+        nix::panic("Assertion failed and could not format error message");
+    nix::panic(std::string_view(buf, std::min(static_cast<int>(sizeof(buf)), n)));
+}

nix-meson-build-support/common/meson.build

@@ -5,6 +5,15 @@ if not (host_machine.system() == 'windows' and cxx.get_id() == 'gcc')
   deps_private += dependency('threads')
 endif
 
+if host_machine.system() == 'cygwin'
+  # -std=gnu on cygwin defines 'unix', which conflicts with the namespace
+  add_project_arguments(
+    '-D_POSIX_C_SOURCE=200809L',
+    '-D_GNU_SOURCE',
+    language : 'cpp',
+  )
+endif
+
 add_project_arguments(
   '-Wdeprecated-copy',
   '-Werror=suggest-override',
@@ -33,10 +42,27 @@ if cxx.get_id() == 'clang'
   add_project_arguments('-fpch-instantiate-templates', language : 'cpp')
 endif
 
-# Clang gets grumpy about missing libasan symbols if -shared-libasan is not
-# passed when building shared libs, at least on Linux
-if cxx.get_id() == 'clang' and ('address' in get_option('b_sanitize') or 'undefined' in get_option(
-  'b_sanitize',
-))
-  add_project_link_arguments('-shared-libasan', language : 'cpp')
+# Detect if we're using libstdc++ (GCC's standard library)
+# libstdc++ uses Intel TBB as backend for C++17 parallel algorithms when <execution> is included.
+# boost::concurrent_flat_map includes <execution>, which would require linking against TBB.
+# Since we don't actually use parallel algorithms, disable the TBB backend to avoid the dependency.
+# TBB is a dependency of blake3 and leaking into our build environment.
+is_using_libstdcxx = cxx.compiles(
+  '''
+  #include <ciso646>
+  #ifndef __GLIBCXX__
+  #error "not libstdc++"
+  #endif
+  int main() { return 0; }
+''',
+  name : 'using libstdc++',
+)
+
+if is_using_libstdcxx
+  add_project_arguments('-D_GLIBCXX_USE_TBB_PAR_BACKEND=0', language : 'cpp')
 endif
+
+# Darwin ld doesn't like "X.Y.ZpreABCD+W"
+nix_soversion = meson.project_version().split('+')[0].split('pre')[0]
+
+subdir('assert-fail')

nix-meson-build-support/libatomic/meson.build

@@ -3,6 +3,6 @@
 # This is needed for std::atomic on some platforms
 # We did not manage to test this reliably on all platforms, so we hardcode
 # it for now.
-if host_machine.cpu_family() == 'arm'
+if host_machine.cpu_family() in [ 'arm', 'ppc' ]
   deps_other += cxx.find_library('atomic')
 endif

packaging/components.nix

@@ -164,6 +164,24 @@ let
   };
 
   mesonLibraryLayer = finalAttrs: prevAttrs: {
+    preConfigure =
+      let
+        interpositionFlags = [
+          "-fno-semantic-interposition"
+          "-Wl,-Bsymbolic-functions"
+        ];
+      in
+      # NOTE: By default GCC disables interprocedular optimizations (in particular inlining) for
+      # position-independent code and thus shared libraries.
+      # Since LD_PRELOAD tricks aren't worth losing out on optimizations, we disable it for good.
+      # This is not the case for Clang, where inlining is done by default even without -fno-semantic-interposition.
+      # https://reviews.llvm.org/D102453
+      # https://fedoraproject.org/wiki/Changes/PythonNoSemanticInterpositionSpeedup
+      prevAttrs.preConfigure or ""
+      + lib.optionalString stdenv.cc.isGNU ''
+        export CFLAGS="''${CFLAGS:-} ${toString interpositionFlags}"
+        export CXXFLAGS="''${CXXFLAGS:-} ${toString interpositionFlags}"
+      '';
     outputs = prevAttrs.outputs or [ "out" ] ++ [ "dev" ];
   };
 

packaging/dependencies.nix

@@ -10,27 +10,8 @@
   stdenv,
 }:
 
-let
-  prevStdenv = stdenv;
-in
-
 let
   inherit (pkgs) lib;
-
-  stdenv = if prevStdenv.isDarwin && prevStdenv.isx86_64 then darwinStdenv else prevStdenv;
-
-  # Fix the following error with the default x86_64-darwin SDK:
-  #
-  #     error: aligned allocation function of type 'void *(std::size_t, std::align_val_t)' is only available on macOS 10.13 or newer
-  #
-  # Despite the use of the 10.13 deployment target here, the aligned
-  # allocation function Clang uses with this setting actually works
-  # all the way back to 10.6.
-  # NOTE: this is not just a version constraint, but a request to make Darwin
-  #       provide this version level of support. Removing this minimum version
-  #       request will regress the above error.
-  darwinStdenv = pkgs.overrideSDK prevStdenv { darwinMinVersion = "10.13"; };
-
 in
 scope: {
   inherit stdenv;
@@ -73,9 +54,24 @@ scope: {
     nativeBuildInputs = prevAttrs.nativeBuildInputs ++ [ pkgs.buildPackages.bmake ];
     postInstall =
       lib.replaceStrings [ "lowdown.so.1" "lowdown.1.dylib" ] [ "lowdown.so.2" "lowdown.2.dylib" ]
-        prevAttrs.postInstall;
+        (prevAttrs.postInstall or "");
   });
 
+  # TODO: Remove this when https://github.com/NixOS/nixpkgs/pull/442682 is included in a stable release
+  toml11 =
+    if lib.versionAtLeast pkgs.toml11.version "4.4.0" then
+      pkgs.toml11
+    else
+      pkgs.toml11.overrideAttrs rec {
+        version = "4.4.0";
+        src = pkgs.fetchFromGitHub {
+          owner = "ToruNiina";
+          repo = "toml11";
+          tag = "v${version}";
+          hash = "sha256-sgWKYxNT22nw376ttGsTdg0AMzOwp8QH3E8mx0BZJTQ=";
+        };
+      };
+
   # TODO Hack until https://github.com/NixOS/nixpkgs/issues/45462 is fixed.
   boost =
     (pkgs.boost.override {

packaging/dev-shell.nix

@@ -118,6 +118,7 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
         modular.pre-commit.settings.package
         (pkgs.writeScriptBin "pre-commit-hooks-install" modular.pre-commit.settings.installationScript)
         pkgs.buildPackages.nixfmt-rfc-style
+        pkgs.buildPackages.shellcheck
         pkgs.buildPackages.gdb
       ]
       ++ lib.optional (stdenv.cc.isClang && stdenv.hostPlatform == stdenv.buildPlatform) (

scripts/install-multi-user.sh

@@ -55,18 +55,22 @@ readonly NIX_INSTALLED_NIX="@nix@"
 readonly NIX_INSTALLED_CACERT="@cacert@"
 #readonly NIX_INSTALLED_NIX="/nix/store/j8dbv5w6jl34caywh2ygdy88knx1mdf7-nix-2.3.6"
 #readonly NIX_INSTALLED_CACERT="/nix/store/7dxhzymvy330i28ii676fl1pqwcahv2f-nss-cacert-3.49.2"
-readonly EXTRACTED_NIX_PATH="$(dirname "$0")"
+EXTRACTED_NIX_PATH="$(dirname "$0")"
+readonly EXTRACTED_NIX_PATH
 
 # allow to override identity change command
-readonly NIX_BECOME=${NIX_BECOME:-sudo}
+NIX_BECOME=${NIX_BECOME:-sudo}
+readonly NIX_BECOME
 
-readonly ROOT_HOME=~root
+ROOT_HOME=~root
+readonly ROOT_HOME
 
 if [ -t 0 ] && [ -z "${NIX_INSTALLER_YES:-}" ]; then
-    readonly IS_HEADLESS='no'
+    IS_HEADLESS='no'
 else
-    readonly IS_HEADLESS='yes'
+    IS_HEADLESS='yes'
 fi
+readonly IS_HEADLESS
 
 headless() {
     if [ "$IS_HEADLESS" = "yes" ]; then
@@ -156,6 +160,7 @@ EOF
 }
 
 nix_user_for_core() {
+    # shellcheck disable=SC2059
     printf "$NIX_BUILD_USER_NAME_TEMPLATE" "$1"
 }
 
@@ -381,10 +386,12 @@ _sudo() {
 
 # Ensure that $TMPDIR exists if defined.
 if [[ -n "${TMPDIR:-}" ]] && [[ ! -d "${TMPDIR:-}" ]]; then
+    # shellcheck disable=SC2174
     mkdir -m 0700 -p "${TMPDIR:-}"
 fi
 
-readonly SCRATCH=$(mktemp -d)
+SCRATCH=$(mktemp -d)
+readonly SCRATCH
 finish_cleanup() {
     rm -rf "$SCRATCH"
 }
@@ -677,7 +684,8 @@ create_directories() {
         # hiding behind || true, and the general state
         # should be one the user can repair once they
         # figure out where chown is...
-        local get_chr_own="$(PATH="$(getconf PATH 2>/dev/null)" command -vp chown)"
+        local get_chr_own
+        get_chr_own="$(PATH="$(getconf PATH 2>/dev/null)" command -vp chown)"
         if [[ -z "$get_chr_own" ]]; then
             get_chr_own="$(command -v chown)"
         fi
@@ -915,9 +923,11 @@ configure_shell_profile() {
         fi
 
         if [ -e "$profile_target" ]; then
-            shell_source_lines \
-                | _sudo "extend your $profile_target with nix-daemon settings" \
-                        tee -a "$profile_target"
+            {
+                shell_source_lines
+                cat "$profile_target"
+            } | _sudo "extend your $profile_target with nix-daemon settings" \
+                      tee "$profile_target"
         fi
     done
 
@@ -1013,6 +1023,7 @@ main() {
 
     # Set profile targets after OS-specific scripts are loaded
     if command -v poly_configure_default_profile_targets > /dev/null 2>&1; then
+        # shellcheck disable=SC2207
         PROFILE_TARGETS=($(poly_configure_default_profile_targets))
     else
         PROFILE_TARGETS=("/etc/bashrc" "/etc/profile.d/nix.sh" "/etc/zshrc" "/etc/bash.bashrc" "/etc/zsh/zshrc")

scripts/install-systemd-multi-user.sh

@@ -39,7 +39,7 @@ create_systemd_proxy_env() {
     vars="http_proxy https_proxy ftp_proxy all_proxy no_proxy HTTP_PROXY HTTPS_PROXY FTP_PROXY ALL_PROXY NO_PROXY"
     for v in $vars; do
         if [ "x${!v:-}" != "x" ]; then
-            echo "Environment=${v}=$(escape_systemd_env ${!v})"
+            echo "Environment=${v}=$(escape_systemd_env "${!v}")"
         fi
     done
 }

src/libcmd/built-path.cc

@@ -83,12 +83,22 @@ nlohmann::json SingleBuiltPath::Built::toJSON(const StoreDirConfig & store) cons
 
 nlohmann::json SingleBuiltPath::toJSON(const StoreDirConfig & store) const
 {
-    return std::visit([&](const auto & buildable) { return buildable.toJSON(store); }, raw());
+    return std::visit(
+        overloaded{
+            [&](const SingleBuiltPath::Opaque & o) -> nlohmann::json { return store.printStorePath(o.path); },
+            [&](const SingleBuiltPath::Built & b) { return b.toJSON(store); },
+        },
+        raw());
 }
 
 nlohmann::json BuiltPath::toJSON(const StoreDirConfig & store) const
 {
-    return std::visit([&](const auto & buildable) { return buildable.toJSON(store); }, raw());
+    return std::visit(
+        overloaded{
+            [&](const BuiltPath::Opaque & o) -> nlohmann::json { return store.printStorePath(o.path); },
+            [&](const BuiltPath::Built & b) { return b.toJSON(store); },
+        },
+        raw());
 }
 
 RealisedPath::Set BuiltPath::toRealisedPaths(Store & store) const

src/libcmd/command.cc

@@ -2,6 +2,7 @@
 #include <nlohmann/json.hpp>
 
 #include "nix/cmd/command.hh"
+#include "nix/cmd/legacy.hh"
 #include "nix/cmd/markdown.hh"
 #include "nix/store/store-open.hh"
 #include "nix/store/local-fs-store.hh"
@@ -14,6 +15,18 @@
 
 namespace nix {
 
+RegisterCommand::Commands & RegisterCommand::commands()
+{
+    static RegisterCommand::Commands commands;
+    return commands;
+}
+
+RegisterLegacyCommand::Commands & RegisterLegacyCommand::commands()
+{
+    static RegisterLegacyCommand::Commands commands;
+    return commands;
+}
+
 nix::Commands RegisterCommand::getCommandsFor(const std::vector<std::string> & prefix)
 {
     nix::Commands res;

src/libcmd/include/nix/cmd/command.hh

@@ -286,11 +286,7 @@ struct RegisterCommand
 {
     typedef std::map<std::vector<std::string>, std::function<ref<Command>()>> Commands;
 
-    static Commands & commands()
-    {
-        static Commands commands;
-        return commands;
-    }
+    static Commands & commands();
 
     RegisterCommand(std::vector<std::string> && name, std::function<ref<Command>()> command)
     {

src/libcmd/include/nix/cmd/legacy.hh

@@ -13,11 +13,7 @@ struct RegisterLegacyCommand
 {
     typedef std::map<std::string, MainFunction> Commands;
 
-    static Commands & commands()
-    {
-        static Commands commands;
-        return commands;
-    }
+    static Commands & commands();
 
     RegisterLegacyCommand(const std::string & name, MainFunction fun)
     {

src/libcmd/installable-flake.cc

@@ -105,8 +105,8 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
 
     std::optional<NixInt::Inner> priority;
 
-    if (attr->maybeGetAttr(state->sOutputSpecified)) {
-    } else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
+    if (attr->maybeGetAttr(state->s.outputSpecified)) {
+    } else if (auto aMeta = attr->maybeGetAttr(state->s.meta)) {
         if (auto aPriority = aMeta->maybeGetAttr("priority"))
             priority = aPriority->getInt().value;
     }
@@ -119,12 +119,12 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
                     overloaded{
                         [&](const ExtendedOutputsSpec::Default & d) -> OutputsSpec {
                             StringSet outputsToInstall;
-                            if (auto aOutputSpecified = attr->maybeGetAttr(state->sOutputSpecified)) {
+                            if (auto aOutputSpecified = attr->maybeGetAttr(state->s.outputSpecified)) {
                                 if (aOutputSpecified->getBool()) {
                                     if (auto aOutputName = attr->maybeGetAttr("outputName"))
                                         outputsToInstall = {aOutputName->getString()};
                                 }
-                            } else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
+                            } else if (auto aMeta = attr->maybeGetAttr(state->s.meta)) {
                                 if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
                                     for (auto & s : aOutputsToInstall->getListOfStrings())
                                         outputsToInstall.insert(s);

src/libcmd/installables.cc

@@ -178,10 +178,16 @@ MixFlakeOptions::MixFlakeOptions()
             for (auto & [inputName, input] : flake.lockFile.root->inputs) {
                 auto input2 = flake.lockFile.findInput({inputName}); // resolve 'follows' nodes
                 if (auto input3 = std::dynamic_pointer_cast<const flake::LockedNode>(input2)) {
+                    fetchers::Attrs extraAttrs;
+
+                    if (!input3->lockedRef.subdir.empty()) {
+                        extraAttrs["dir"] = input3->lockedRef.subdir;
+                    }
+
                     overrideRegistry(
                         fetchers::Input::fromAttrs(fetchSettings, {{"type", "indirect"}, {"id", inputName}}),
                         input3->lockedRef.input,
-                        {});
+                        extraAttrs);
                 }
             }
         }},
@@ -598,28 +604,28 @@ std::vector<BuiltPathWithResult> Installable::build(
 
 static void throwBuildErrors(std::vector<KeyedBuildResult> & buildResults, const Store & store)
 {
-    std::vector<KeyedBuildResult> failed;
+    std::vector<std::pair<const KeyedBuildResult *, const KeyedBuildResult::Failure *>> failed;
     for (auto & buildResult : buildResults) {
-        if (!buildResult.success()) {
-            failed.push_back(buildResult);
+        if (auto * failure = buildResult.tryGetFailure()) {
+            failed.push_back({&buildResult, failure});
         }
     }
 
     auto failedResult = failed.begin();
     if (failedResult != failed.end()) {
         if (failed.size() == 1) {
-            failedResult->rethrow();
+            failedResult->second->rethrow();
         } else {
             StringSet failedPaths;
             for (; failedResult != failed.end(); failedResult++) {
-                if (!failedResult->errorMsg.empty()) {
+                if (!failedResult->second->errorMsg.empty()) {
                     logError(
                         ErrorInfo{
                             .level = lvlError,
-                            .msg = failedResult->errorMsg,
+                            .msg = failedResult->second->errorMsg,
                         });
                 }
-                failedPaths.insert(failedResult->path.to_string(store));
+                failedPaths.insert(failedResult->first->path.to_string(store));
             }
             throw Error("build of %s failed", concatStringsSep(", ", quoteStrings(failedPaths)));
         }
@@ -689,12 +695,14 @@ std::vector<std::pair<ref<Installable>, BuiltPathWithResult>> Installable::build
         auto buildResults = store->buildPathsWithResults(pathsToBuild, bMode, evalStore);
         throwBuildErrors(buildResults, *store);
         for (auto & buildResult : buildResults) {
+            // If we didn't throw, they must all be sucesses
+            auto & success = std::get<nix::BuildResult::Success>(buildResult.inner);
             for (auto & aux : backmap[buildResult.path]) {
                 std::visit(
                     overloaded{
                         [&](const DerivedPath::Built & bfd) {
                             std::map<std::string, StorePath> outputs;
-                            for (auto & [outputName, realisation] : buildResult.builtOutputs)
+                            for (auto & [outputName, realisation] : success.builtOutputs)
                                 outputs.emplace(outputName, realisation.outPath);
                             res.push_back(
                                 {aux.installable,

src/libcmd/meson.build

@@ -67,6 +67,7 @@ config_priv_h = configure_file(
 )
 
 subdir('nix-meson-build-support/common')
+subdir('nix-meson-build-support/asan-options')
 
 sources = files(
   'built-path.cc',
@@ -95,6 +96,7 @@ this_library = library(
   'nixcmd',
   sources,
   config_priv_h,
+  soversion : nix_soversion,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   link_args : linker_export_flags,

src/libcmd/repl.cc

@@ -669,7 +669,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
                 ss << "No documentation found.\n\n";
             }
 
-            auto markdown = toView(ss);
+            auto markdown = ss.view();
             logger->cout(trim(renderMarkdownToTerminal(markdown)));
 
         } else
@@ -760,7 +760,7 @@ void NixRepl::loadFlake(const std::string & flakeRefS)
 
 void NixRepl::initEnv()
 {
-    env = &state->allocEnv(envSize);
+    env = &state->mem.allocEnv(envSize);
     env->up = &state->baseEnv;
     displ = 0;
     staticEnv->vars.clear();
@@ -869,14 +869,8 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
 
 Expr * NixRepl::parseString(std::string s)
 {
-    return state->parseExprFromString(std::move(s), state->rootPath("."), staticEnv);
-}
-
-void NixRepl::evalString(std::string s, Value & v)
-{
-    Expr * e;
     try {
-        e = parseString(s);
+        return state->parseExprFromString(std::move(s), state->rootPath("."), staticEnv);
     } catch (ParseError & e) {
         if (e.msg().find("unexpected end of file") != std::string::npos)
             // For parse errors on incomplete input, we continue waiting for the next line of
@@ -885,6 +879,11 @@ void NixRepl::evalString(std::string s, Value & v)
         else
             throw;
     }
+}
+
+void NixRepl::evalString(std::string s, Value & v)
+{
+    Expr * e = parseString(s);
     e->eval(*state, *env, v);
     state->forceValue(v, v.determinePos(noPos));
 }

src/libexpr-c/meson.build

@@ -28,6 +28,7 @@ deps_public_maybe_subproject = [
 subdir('nix-meson-build-support/subprojects')
 
 subdir('nix-meson-build-support/common')
+subdir('nix-meson-build-support/asan-options')
 
 sources = files(
   'nix_api_expr.cc',
@@ -50,6 +51,7 @@ subdir('nix-meson-build-support/windows-version')
 this_library = library(
   'nixexprc',
   sources,
+  soversion : nix_soversion,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   link_args : linker_export_flags,

src/libexpr-c/nix_api_expr.cc

@@ -16,7 +16,7 @@
 #include "nix_api_util_internal.h"
 
 #if NIX_USE_BOEHMGC
-#  include <mutex>
+#  include <boost/unordered/concurrent_flat_map.hpp>
 #endif
 
 /**
@@ -40,6 +40,8 @@ static T * unsafe_new_with_self(F && init)
     return new (p) T(init(static_cast<T *>(p)));
 }
 
+extern "C" {
+
 nix_err nix_libexpr_init(nix_c_context * context)
 {
     if (context)
@@ -135,7 +137,7 @@ nix_eval_state_builder * nix_eval_state_builder_new(nix_c_context * context, Sto
 
 void nix_eval_state_builder_free(nix_eval_state_builder * builder)
 {
-    delete builder;
+    operator delete(builder, static_cast<std::align_val_t>(alignof(nix_eval_state_builder)));
 }
 
 nix_err nix_eval_state_builder_load(nix_c_context * context, nix_eval_state_builder * builder)
@@ -201,32 +203,24 @@ EvalState * nix_state_create(nix_c_context * context, const char ** lookupPath_c
 
 void nix_state_free(EvalState * state)
 {
-    delete state;
+    operator delete(state, static_cast<std::align_val_t>(alignof(EvalState)));
 }
 
 #if NIX_USE_BOEHMGC
-std::unordered_map<
+boost::concurrent_flat_map<
     const void *,
     unsigned int,
     std::hash<const void *>,
     std::equal_to<const void *>,
     traceable_allocator<std::pair<const void * const, unsigned int>>>
-    nix_refcounts;
-
-std::mutex nix_refcount_lock;
+    nix_refcounts{};
 
 nix_err nix_gc_incref(nix_c_context * context, const void * p)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        std::scoped_lock lock(nix_refcount_lock);
-        auto f = nix_refcounts.find(p);
-        if (f != nix_refcounts.end()) {
-            f->second++;
-        } else {
-            nix_refcounts[p] = 1;
-        }
+        nix_refcounts.insert_or_visit({p, 1}, [](auto & kv) { kv.second++; });
     }
     NIXC_CATCH_ERRS
 }
@@ -237,12 +231,12 @@ nix_err nix_gc_decref(nix_c_context * context, const void * p)
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        std::scoped_lock lock(nix_refcount_lock);
-        auto f = nix_refcounts.find(p);
-        if (f != nix_refcounts.end()) {
-            if (--f->second == 0)
-                nix_refcounts.erase(f);
-        } else
+        bool fail = true;
+        nix_refcounts.erase_if(p, [&](auto & kv) {
+            fail = false;
+            return !--kv.second;
+        });
+        if (fail)
             throw std::runtime_error("nix_gc_decref: object was not referenced");
     }
     NIXC_CATCH_ERRS
@@ -287,3 +281,5 @@ void nix_gc_register_finalizer(void * obj, void * cd, void (*finalizer)(void * o
     GC_REGISTER_FINALIZER(obj, finalizer, cd, 0, 0);
 #endif
 }
+
+} // extern "C"

src/libexpr-c/nix_api_expr_internal.h

@@ -8,6 +8,8 @@
 #include "nix_api_value.h"
 #include "nix/expr/search-path.hh"
 
+extern "C" {
+
 struct nix_eval_state_builder
 {
     nix::ref<nix::Store> store;
@@ -61,4 +63,6 @@ struct nix_realised_string
     std::vector<StorePath> storePaths;
 };
 
+} // extern "C"
+
 #endif // NIX_API_EXPR_INTERNAL_H

src/libexpr-c/nix_api_external.cc

@@ -14,6 +14,8 @@
 
 #include <nlohmann/json.hpp>
 
+extern "C" {
+
 void nix_set_string_return(nix_string_return * str, const char * c)
 {
     str->str = c;
@@ -40,6 +42,8 @@ nix_err nix_external_add_string_context(nix_c_context * context, nix_string_cont
     NIXC_CATCH_ERRS
 }
 
+} // extern "C"
+
 class NixCExternalValue : public nix::ExternalValueBase
 {
     NixCExternalValueDesc & desc;
@@ -170,6 +174,8 @@ public:
     virtual ~NixCExternalValue() override {};
 };
 
+extern "C" {
+
 ExternalValue * nix_create_external_value(nix_c_context * context, NixCExternalValueDesc * desc, void * v)
 {
     if (context)
@@ -198,3 +204,5 @@ void * nix_get_external_value_content(nix_c_context * context, ExternalValue * b
     }
     NIXC_CATCH_ERRS_NULL
 }
+
+} // extern "C"

src/libexpr-c/nix_api_value.cc

@@ -111,6 +111,8 @@ static void nix_c_primop_wrapper(
     v = vTmp;
 }
 
+extern "C" {
+
 PrimOp * nix_alloc_primop(
     nix_c_context * context,
     PrimOpFun fun,
@@ -324,6 +326,10 @@ nix_value * nix_get_list_byidx(nix_c_context * context, const nix_value * value,
     try {
         auto & v = check_value_in(value);
         assert(v.type() == nix::nList);
+        if (ix >= v.listSize()) {
+            nix_set_err_msg(context, NIX_ERR_KEY, "list index out of bounds");
+            return nullptr;
+        }
         auto * p = v.listView()[ix];
         nix_gc_incref(nullptr, p);
         if (p != nullptr)
@@ -333,6 +339,26 @@ nix_value * nix_get_list_byidx(nix_c_context * context, const nix_value * value,
     NIXC_CATCH_ERRS_NULL
 }
 
+nix_value *
+nix_get_list_byidx_lazy(nix_c_context * context, const nix_value * value, EvalState * state, unsigned int ix)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_in(value);
+        assert(v.type() == nix::nList);
+        if (ix >= v.listSize()) {
+            nix_set_err_msg(context, NIX_ERR_KEY, "list index out of bounds");
+            return nullptr;
+        }
+        auto * p = v.listView()[ix];
+        nix_gc_incref(nullptr, p);
+        // Note: intentionally NOT calling forceValue() to keep the element lazy
+        return as_nix_value_ptr(p);
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
 nix_value * nix_get_attr_byname(nix_c_context * context, const nix_value * value, EvalState * state, const char * name)
 {
     if (context)
@@ -353,6 +379,27 @@ nix_value * nix_get_attr_byname(nix_c_context * context, const nix_value * value
     NIXC_CATCH_ERRS_NULL
 }
 
+nix_value *
+nix_get_attr_byname_lazy(nix_c_context * context, const nix_value * value, EvalState * state, const char * name)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_in(value);
+        assert(v.type() == nix::nAttrs);
+        nix::Symbol s = state->state.symbols.create(name);
+        auto attr = v.attrs()->get(s);
+        if (attr) {
+            nix_gc_incref(nullptr, attr->value);
+            // Note: intentionally NOT calling forceValue() to keep the attribute lazy
+            return as_nix_value_ptr(attr->value);
+        }
+        nix_set_err_msg(context, NIX_ERR_KEY, "missing attribute");
+        return nullptr;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
 bool nix_has_attr_byname(nix_c_context * context, const nix_value * value, EvalState * state, const char * name)
 {
     if (context)
@@ -369,13 +416,28 @@ bool nix_has_attr_byname(nix_c_context * context, const nix_value * value, EvalS
     NIXC_CATCH_ERRS_RES(false);
 }
 
-nix_value * nix_get_attr_byidx(
-    nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i, const char ** name)
+static void collapse_attrset_layer_chain_if_needed(nix::Value & v, EvalState * state)
+{
+    auto & attrs = *v.attrs();
+    if (attrs.isLayered()) {
+        auto bindings = state->state.buildBindings(attrs.size());
+        std::ranges::copy(attrs, std::back_inserter(bindings));
+        v.mkAttrs(bindings);
+    }
+}
+
+nix_value *
+nix_get_attr_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i, const char ** name)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_in(value);
+        collapse_attrset_layer_chain_if_needed(v, state);
+        if (i >= v.attrs()->size()) {
+            nix_set_err_msg(context, NIX_ERR_KEY, "attribute index out of bounds");
+            return nullptr;
+        }
         const nix::Attr & a = (*v.attrs())[i];
         *name = state->state.symbols[a.name].c_str();
         nix_gc_incref(nullptr, a.value);
@@ -385,13 +447,38 @@ nix_value * nix_get_attr_byidx(
     NIXC_CATCH_ERRS_NULL
 }
 
-const char *
-nix_get_attr_name_byidx(nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i)
+nix_value * nix_get_attr_byidx_lazy(
+    nix_c_context * context, nix_value * value, EvalState * state, unsigned int i, const char ** name)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_in(value);
+        collapse_attrset_layer_chain_if_needed(v, state);
+        if (i >= v.attrs()->size()) {
+            nix_set_err_msg(context, NIX_ERR_KEY, "attribute index out of bounds (Nix C API contract violation)");
+            return nullptr;
+        }
+        const nix::Attr & a = (*v.attrs())[i];
+        *name = state->state.symbols[a.name].c_str();
+        nix_gc_incref(nullptr, a.value);
+        // Note: intentionally NOT calling forceValue() to keep the attribute lazy
+        return as_nix_value_ptr(a.value);
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+const char * nix_get_attr_name_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_in(value);
+        collapse_attrset_layer_chain_if_needed(v, state);
+        if (i >= v.attrs()->size()) {
+            nix_set_err_msg(context, NIX_ERR_KEY, "attribute index out of bounds (Nix C API contract violation)");
+            return nullptr;
+        }
         const nix::Attr & a = (*v.attrs())[i];
         return state->state.symbols[a.name].c_str();
     }
@@ -592,7 +679,7 @@ nix_err nix_bindings_builder_insert(nix_c_context * context, BindingsBuilder * b
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_not_null(value);
-        nix::Symbol s = bb->builder.state.symbols.create(name);
+        nix::Symbol s = bb->builder.symbols.get().create(name);
         bb->builder.insert(s, &v);
     }
     NIXC_CATCH_ERRS
@@ -651,3 +738,5 @@ const StorePath * nix_realised_string_get_store_path(nix_realised_string * s, si
 {
     return &s->storePaths[i];
 }
+
+} // extern "C"

src/libexpr-c/nix_api_value.h

@@ -265,10 +265,25 @@ ExternalValue * nix_get_external(nix_c_context * context, nix_value * value);
  */
 nix_value * nix_get_list_byidx(nix_c_context * context, const nix_value * value, EvalState * state, unsigned int ix);
 
-/** @brief Get an attr by name
+/** @brief Get the ix'th element of a list without forcing evaluation of the element
+ *
+ * Returns the list element without forcing its evaluation, allowing access to lazy values.
+ * The list value itself must already be evaluated.
  *
  * Owned by the GC. Use nix_gc_decref when you're done with the pointer
  * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect (must be an evaluated list)
+ * @param[in] state nix evaluator state
+ * @param[in] ix list element to get
+ * @return value, NULL in case of errors
+ */
+nix_value *
+nix_get_list_byidx_lazy(nix_c_context * context, const nix_value * value, EvalState * state, unsigned int ix);
+
+/** @brief Get an attr by name
+ *
+ * Use nix_gc_decref when you're done with the pointer
+ * @param[out] context Optional, stores error information
  * @param[in] value Nix value to inspect
  * @param[in] state nix evaluator state
  * @param[in] name attribute name
@@ -276,6 +291,21 @@ nix_value * nix_get_list_byidx(nix_c_context * context, const nix_value * value,
  */
 nix_value * nix_get_attr_byname(nix_c_context * context, const nix_value * value, EvalState * state, const char * name);
 
+/** @brief Get an attribute value by attribute name, without forcing evaluation of the attribute's value
+ *
+ * Returns the attribute value without forcing its evaluation, allowing access to lazy values.
+ * The attribute set value itself must already be evaluated.
+ *
+ * Use nix_gc_decref when you're done with the pointer
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect (must be an evaluated attribute set)
+ * @param[in] state nix evaluator state
+ * @param[in] name attribute name
+ * @return value, NULL in case of errors
+ */
+nix_value *
+nix_get_attr_byname_lazy(nix_c_context * context, const nix_value * value, EvalState * state, const char * name);
+
 /** @brief Check if an attribute name exists on a value
  * @param[out] context Optional, stores error information
  * @param[in] value Nix value to inspect
@@ -285,11 +315,21 @@ nix_value * nix_get_attr_byname(nix_c_context * context, const nix_value * value
  */
 bool nix_has_attr_byname(nix_c_context * context, const nix_value * value, EvalState * state, const char * name);
 
-/** @brief Get an attribute by index in the sorted bindings
+/** @brief Get an attribute by index
  *
  * Also gives you the name.
  *
- * Owned by the GC. Use nix_gc_decref when you're done with the pointer
+ * Attributes are returned in an unspecified order which is NOT suitable for
+ * reproducible operations. In Nix's domain, reproducibility is paramount. The caller
+ * is responsible for sorting the attributes or storing them in an ordered map to
+ * ensure deterministic behavior in your application.
+ *
+ * @note When Nix does sort attributes, which it does for virtually all intermediate
+ * operations and outputs, it uses byte-wise lexicographic order (equivalent to
+ * lexicographic order by Unicode scalar value for valid UTF-8). We recommend
+ * applying this same ordering for consistency.
+ *
+ * Use nix_gc_decref when you're done with the pointer
  * @param[out] context Optional, stores error information
  * @param[in] value Nix value to inspect
  * @param[in] state nix evaluator state
@@ -297,12 +337,50 @@ bool nix_has_attr_byname(nix_c_context * context, const nix_value * value, EvalS
  * @param[out] name will store a pointer to the attribute name
  * @return value, NULL in case of errors
  */
-nix_value * nix_get_attr_byidx(
-    nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i, const char ** name);
+nix_value *
+nix_get_attr_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i, const char ** name);
 
-/** @brief Get an attribute name by index in the sorted bindings
+/** @brief Get an attribute by index, without forcing evaluation of the attribute's value
  *
- * Useful when you want the name but want to avoid evaluation.
+ * Also gives you the name.
+ *
+ * Returns the attribute value without forcing its evaluation, allowing access to lazy values.
+ * The attribute set value itself must already have been evaluated.
+ *
+ * Attributes are returned in an unspecified order which is NOT suitable for
+ * reproducible operations. In Nix's domain, reproducibility is paramount. The caller
+ * is responsible for sorting the attributes or storing them in an ordered map to
+ * ensure deterministic behavior in your application.
+ *
+ * @note When Nix does sort attributes, which it does for virtually all intermediate
+ * operations and outputs, it uses byte-wise lexicographic order (equivalent to
+ * lexicographic order by Unicode scalar value for valid UTF-8). We recommend
+ * applying this same ordering for consistency.
+ *
+ * Use nix_gc_decref when you're done with the pointer
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect (must be an evaluated attribute set)
+ * @param[in] state nix evaluator state
+ * @param[in] i attribute index
+ * @param[out] name will store a pointer to the attribute name
+ * @return value, NULL in case of errors
+ */
+nix_value * nix_get_attr_byidx_lazy(
+    nix_c_context * context, nix_value * value, EvalState * state, unsigned int i, const char ** name);
+
+/** @brief Get an attribute name by index
+ *
+ * Returns the attribute name without forcing evaluation of the attribute's value.
+ *
+ * Attributes are returned in an unspecified order which is NOT suitable for
+ * reproducible operations. In Nix's domain, reproducibility is paramount. The caller
+ * is responsible for sorting the attributes or storing them in an ordered map to
+ * ensure deterministic behavior in your application.
+ *
+ * @note When Nix does sort attributes, which it does for virtually all intermediate
+ * operations and outputs, it uses byte-wise lexicographic order (equivalent to
+ * lexicographic order by Unicode scalar value for valid UTF-8). We recommend
+ * applying this same ordering for consistency.
  *
  * Owned by the nix EvalState
  * @param[out] context Optional, stores error information
@@ -311,8 +389,7 @@ nix_value * nix_get_attr_byidx(
  * @param[in] i attribute index
  * @return name, NULL in case of errors
  */
-const char *
-nix_get_attr_name_byidx(nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i);
+const char * nix_get_attr_name_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i);
 
 /**@}*/
 /** @name Initializers

src/libexpr-test-support/include/nix/expr/tests/libexpr.hh

@@ -26,11 +26,20 @@ public:
     }
 
 protected:
-    LibExprTest()
+    LibExprTest(ref<Store> store, auto && makeEvalSettings)
         : LibStoreTest()
+        , evalSettings(makeEvalSettings(readOnlyMode))
         , state({}, store, fetchSettings, evalSettings, nullptr)
     {
-        evalSettings.nixPath = {};
+    }
+
+    LibExprTest()
+        : LibExprTest(openStore("dummy://"), [](bool & readOnlyMode) {
+            EvalSettings settings{readOnlyMode};
+            settings.nixPath = {};
+            return settings;
+        })
+    {
     }
 
     Value eval(std::string input, bool forceValue = true)

src/libexpr-test-support/meson.build

@@ -31,6 +31,7 @@ rapidcheck = dependency('rapidcheck')
 deps_public += rapidcheck
 
 subdir('nix-meson-build-support/common')
+subdir('nix-meson-build-support/asan-options')
 
 sources = files(
   'tests/value/context.cc',
@@ -44,6 +45,7 @@ subdir('nix-meson-build-support/windows-version')
 this_library = library(
   'nix-expr-test-support',
   sources,
+  soversion : nix_soversion,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   # TODO: Remove `-lrapidcheck` when https://github.com/emil-e/rapidcheck/pull/326

src/libexpr-tests/eval.cc

@@ -3,6 +3,7 @@
 
 #include "nix/expr/eval.hh"
 #include "nix/expr/tests/libexpr.hh"
+#include "nix/util/memory-source-accessor.hh"
 
 namespace nix {
 
@@ -174,4 +175,41 @@ TEST_F(EvalStateTest, getBuiltin_fail)
     ASSERT_THROW(state.getBuiltin("nonexistent"), EvalError);
 }
 
+class PureEvalTest : public LibExprTest
+{
+public:
+    PureEvalTest()
+        : LibExprTest(openStore("dummy://", {{"read-only", "false"}}), [](bool & readOnlyMode) {
+            EvalSettings settings{readOnlyMode};
+            settings.pureEval = true;
+            settings.restrictEval = true;
+            return settings;
+        })
+    {
+    }
+};
+
+TEST_F(PureEvalTest, pathExists)
+{
+    ASSERT_THAT(eval("builtins.pathExists /."), IsFalse());
+    ASSERT_THAT(eval("builtins.pathExists /nix"), IsFalse());
+    ASSERT_THAT(eval("builtins.pathExists /nix/store"), IsFalse());
+
+    {
+        std::string contents = "Lorem ipsum";
+
+        StringSource s{contents};
+        auto path = state.store->addToStoreFromDump(
+            s, "source", FileSerialisationMethod::Flat, ContentAddressMethod::Raw::Text, HashAlgorithm::SHA256);
+        auto printed = store->printStorePath(path);
+
+        ASSERT_THROW(eval(fmt("builtins.readFile %s", printed)), RestrictedPathError);
+        ASSERT_THAT(eval(fmt("builtins.pathExists %s", printed)), IsFalse());
+
+        ASSERT_THROW(eval("builtins.readDir /."), RestrictedPathError);
+        state.allowPath(path); // FIXME: This shouldn't behave this way.
+        ASSERT_THAT(eval("builtins.readDir /."), IsAttrsOfSize(0));
+    }
+}
+
 } // namespace nix

src/libexpr-tests/json.cc

@@ -54,7 +54,7 @@ TEST_F(JSONValueTest, IntNegative)
 TEST_F(JSONValueTest, String)
 {
     Value v;
-    v.mkString("test");
+    v.mkStringNoCopy("test");
     ASSERT_EQ(getJSONValue(v), "\"test\"");
 }
 
@@ -62,7 +62,7 @@ TEST_F(JSONValueTest, StringQuotes)
 {
     Value v;
 
-    v.mkString("test\"");
+    v.mkStringNoCopy("test\"");
     ASSERT_EQ(getJSONValue(v), "\"test\\\"\"");
 }
 

src/libexpr-tests/main.cc

@@ -1,40 +1,15 @@
 #include <gtest/gtest.h>
-#include <cstdlib>
-#include "nix/store/globals.hh"
-#include "nix/util/logging.hh"
+
+#include "nix/store/tests/test-main.hh"
+#include "nix/util/config-global.hh"
 
 using namespace nix;
 
 int main(int argc, char ** argv)
 {
-    if (argc > 1 && std::string_view(argv[1]) == "__build-remote") {
-        printError("test-build-remote: not supported in libexpr unit tests");
-        return 1;
-    }
-
-    // Disable build hook. We won't be testing remote builds in these unit tests. If we do, fix the above build hook.
-    settings.buildHook = {};
-
-#ifdef __linux__ // should match the conditional around sandboxBuildDir declaration.
-
-    // When building and testing nix within the host's Nix sandbox, our store dir will be located in the host's
-    // sandboxBuildDir, e.g.: Host
-    //   storeDir = /nix/store
-    //   sandboxBuildDir = /build
-    // This process
-    //   storeDir = /build/foo/bar/store
-    //   sandboxBuildDir = /build
-    // However, we have a rule that the store dir must not be inside the storeDir, so we need to pick a different
-    // sandboxBuildDir.
-    settings.sandboxBuildDir = "/test-build-dir-instead-of-usual-build-dir";
-#endif
-
-#ifdef __APPLE__
-    // Avoid this error, when already running in a sandbox:
-    // sandbox-exec: sandbox_apply: Operation not permitted
-    settings.sandboxMode = smDisabled;
-    setEnv("_NIX_TEST_NO_SANDBOX", "1");
-#endif
+    auto res = testMainForBuidingPre(argc, argv);
+    if (res)
+        return res;
 
     // For pipe operator tests in trivial.cc
     experimentalFeatureSettings.set("experimental-features", "pipe-operators");

src/libexpr-tests/meson.build

@@ -45,6 +45,7 @@ config_priv_h = configure_file(
 )
 
 subdir('nix-meson-build-support/common')
+subdir('nix-meson-build-support/asan-options')
 
 sources = files(
   'derived-path.cc',
@@ -55,6 +56,7 @@ sources = files(
   'nix_api_expr.cc',
   'nix_api_external.cc',
   'nix_api_value.cc',
+  'nix_api_value_internal.cc',
   'primops.cc',
   'search-path.cc',
   'trivial.cc',
@@ -81,7 +83,7 @@ this_exe = executable(
 test(
   meson.project_name(),
   this_exe,
-  env : {
+  env : asan_test_options_env + {
     '_NIX_TEST_UNIT_DATA' : meson.current_source_dir() / 'data',
   },
   protocol : 'gtest',

src/libexpr-tests/nix_api_expr.cc

@@ -1,7 +1,5 @@
 #include "nix_api_store.h"
-#include "nix_api_store_internal.h"
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix_api_expr.h"
 #include "nix_api_value.h"
 
@@ -151,8 +149,8 @@ TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_value)
     assert_ctx_ok();
     auto r = nix_string_realise(ctx, state, value, false);
     ASSERT_EQ(nullptr, r);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("cannot coerce")));
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("cannot coerce"));
 }
 
 TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_build)
@@ -168,8 +166,8 @@ TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_build)
     assert_ctx_ok();
     auto r = nix_string_realise(ctx, state, value, false);
     ASSERT_EQ(nullptr, r);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("failed with exit code 1")));
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("failed with exit code 1"));
 }
 
 TEST_F(nix_api_expr_test, nix_expr_realise_context)
@@ -381,12 +379,11 @@ TEST_F(nix_api_expr_test, nix_expr_primop_bad_no_return)
     nix_value * result = nix_alloc_value(ctx, state);
     assert_ctx_ok();
     nix_value_call(ctx, state, primopValue, three, result);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
     ASSERT_THAT(
-        ctx->last_err,
-        testing::Optional(
-            testing::HasSubstr("Implementation error in custom function: return value was not initialized")));
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("badNoReturn")));
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("Implementation error in custom function: return value was not initialized"));
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("badNoReturn"));
 }
 
 static void primop_bad_return_thunk(
@@ -419,12 +416,60 @@ TEST_F(nix_api_expr_test, nix_expr_primop_bad_return_thunk)
     assert_ctx_ok();
     NIX_VALUE_CALL(ctx, state, result, primopValue, toString, four);
 
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
     ASSERT_THAT(
-        ctx->last_err,
-        testing::Optional(
-            testing::HasSubstr("Implementation error in custom function: return value must not be a thunk")));
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("badReturnThunk")));
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("Implementation error in custom function: return value must not be a thunk"));
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("badReturnThunk"));
+}
+
+static void primop_with_nix_err_key(
+    void * user_data, nix_c_context * context, EvalState * state, nix_value ** args, nix_value * ret)
+{
+    nix_set_err_msg(context, NIX_ERR_KEY, "Test error from primop");
+}
+
+TEST_F(nix_api_expr_test, nix_expr_primop_nix_err_key_conversion)
+{
+    // Test that NIX_ERR_KEY from a custom primop gets converted to a generic EvalError
+    //
+    // RATIONALE: NIX_ERR_KEY must not be propagated from custom primops because it would
+    // create semantic confusion. NIX_ERR_KEY indicates missing keys/indices in C API functions
+    // (like nix_get_attr_byname, nix_get_list_byidx). If custom primops could return NIX_ERR_KEY,
+    // an evaluation error would be indistinguishable from an actual missing attribute.
+    //
+    // For example, if nix_get_attr_byname returned NIX_ERR_KEY when the attribute is present
+    // but the value evaluation fails, callers expecting NIX_ERR_KEY to mean "missing attribute"
+    // would incorrectly handle evaluation failures as missing attributes. In places where
+    // missing attributes are tolerated (like optional attributes), this would cause the
+    // program to continue after swallowing the error, leading to silent failures.
+    PrimOp * primop = nix_alloc_primop(
+        ctx, primop_with_nix_err_key, 1, "testErrorPrimop", nullptr, "a test primop that sets NIX_ERR_KEY", nullptr);
+    assert_ctx_ok();
+    nix_value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    nix_value * arg = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_int(ctx, arg, 42);
+    assert_ctx_ok();
+
+    nix_value * result = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_value_call(ctx, state, primopValue, arg, result);
+
+    // Verify that NIX_ERR_KEY gets converted to NIX_ERR_NIX_ERROR (generic evaluation error)
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("Error from custom function"));
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("Test error from primop"));
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("testErrorPrimop"));
+
+    // Clean up
+    nix_gc_decref(ctx, primopValue);
+    nix_gc_decref(ctx, arg);
+    nix_gc_decref(ctx, result);
 }
 
 TEST_F(nix_api_expr_test, nix_value_call_multi_no_args)
@@ -441,4 +486,31 @@ TEST_F(nix_api_expr_test, nix_value_call_multi_no_args)
     assert_ctx_ok();
     ASSERT_EQ(3, rInt);
 }
+
+TEST_F(nix_api_expr_test, nix_expr_attrset_update)
+{
+    nix_expr_eval_from_string(ctx, state, "{ a = 0; b = 2; } // { a = 1; b = 3; } // { a = 2; }", ".", value);
+    assert_ctx_ok();
+
+    ASSERT_EQ(nix_get_attrs_size(ctx, value), 2);
+    assert_ctx_ok();
+    std::array<std::pair<std::string_view, nix_value *>, 2> values;
+    for (unsigned int i = 0; i < 2; ++i) {
+        const char * name;
+        values[i].second = nix_get_attr_byidx(ctx, value, state, i, &name);
+        assert_ctx_ok();
+        values[i].first = name;
+    }
+    std::sort(values.begin(), values.end(), [](const auto & lhs, const auto & rhs) { return lhs.first < rhs.first; });
+
+    nix_value * a = values[0].second;
+    ASSERT_EQ("a", values[0].first);
+    ASSERT_EQ(nix_get_int(ctx, a), 2);
+    assert_ctx_ok();
+    nix_value * b = values[1].second;
+    ASSERT_EQ("b", values[1].first);
+    ASSERT_EQ(nix_get_int(ctx, b), 3);
+    assert_ctx_ok();
+}
+
 } // namespace nixC

src/libexpr-tests/nix_api_external.cc

@@ -1,9 +1,6 @@
 #include "nix_api_store.h"
-#include "nix_api_store_internal.h"
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix_api_expr.h"
-#include "nix_api_expr_internal.h"
 #include "nix_api_value.h"
 #include "nix_api_external.h"
 
@@ -39,7 +36,7 @@ private:
         std::string type_string = "nix-external<MyExternalValueDesc( ";
         type_string += std::to_string(obj->_x);
         type_string += " )>";
-        res->str = &*type_string.begin();
+        nix_set_string_return(res, &*type_string.begin());
     }
 };
 

src/libexpr-tests/nix_api_value.cc

@@ -1,10 +1,7 @@
 #include "nix_api_store.h"
-#include "nix_api_store_internal.h"
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix_api_expr.h"
 #include "nix_api_value.h"
-#include "nix_api_expr_internal.h"
 
 #include "nix/expr/tests/nix_api_expr.hh"
 #include "nix/util/tests/string_callback.hh"
@@ -16,14 +13,6 @@
 
 namespace nixC {
 
-TEST_F(nix_api_expr_test, as_nix_value_ptr)
-{
-    // nix_alloc_value casts nix::Value to nix_value
-    // It should be obvious from the decl that that works, but if it doesn't,
-    // the whole implementation would be utterly broken.
-    ASSERT_EQ(sizeof(nix::Value), sizeof(nix_value));
-}
-
 TEST_F(nix_api_expr_test, nix_value_get_int_invalid)
 {
     ASSERT_EQ(0, nix_get_int(ctx, nullptr));
@@ -173,6 +162,114 @@ TEST_F(nix_api_expr_test, nix_build_and_init_list)
     nix_gc_decref(ctx, intValue);
 }
 
+TEST_F(nix_api_expr_test, nix_get_list_byidx_large_indices)
+{
+    // Create a small list to test extremely large out-of-bounds access
+    ListBuilder * builder = nix_make_list_builder(ctx, state, 2);
+    nix_value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 42);
+    nix_list_builder_insert(ctx, builder, 0, intValue);
+    nix_list_builder_insert(ctx, builder, 1, intValue);
+    nix_make_list(ctx, builder, value);
+    nix_list_builder_free(builder);
+
+    // Test extremely large indices that would definitely crash without bounds checking
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, value, state, 1000000));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, value, state, UINT_MAX / 2));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, value, state, UINT_MAX / 2 + 1000000));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+
+    // Clean up
+    nix_gc_decref(ctx, intValue);
+}
+
+TEST_F(nix_api_expr_test, nix_get_list_byidx_lazy)
+{
+    // Create a list with a throwing lazy element, an already-evaluated int, and a lazy function call
+
+    // 1. Throwing lazy element - create a function application thunk that will throw when forced
+    nix_value * throwingFn = nix_alloc_value(ctx, state);
+    nix_value * throwingValue = nix_alloc_value(ctx, state);
+
+    nix_expr_eval_from_string(
+        ctx,
+        state,
+        R"(
+        _: throw "This should not be evaluated by the lazy accessor"
+    )",
+        "<test>",
+        throwingFn);
+    assert_ctx_ok();
+
+    nix_init_apply(ctx, throwingValue, throwingFn, throwingFn);
+    assert_ctx_ok();
+
+    // 2. Already evaluated int (not lazy)
+    nix_value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 42);
+    assert_ctx_ok();
+
+    // 3. Lazy function application that would compute increment 5 = 6
+    nix_value * lazyApply = nix_alloc_value(ctx, state);
+    nix_value * incrementFn = nix_alloc_value(ctx, state);
+    nix_value * argFive = nix_alloc_value(ctx, state);
+
+    nix_expr_eval_from_string(ctx, state, "x: x + 1", "<test>", incrementFn);
+    assert_ctx_ok();
+    nix_init_int(ctx, argFive, 5);
+
+    // Create a lazy application: (x: x + 1) 5
+    nix_init_apply(ctx, lazyApply, incrementFn, argFive);
+    assert_ctx_ok();
+
+    ListBuilder * builder = nix_make_list_builder(ctx, state, 3);
+    nix_list_builder_insert(ctx, builder, 0, throwingValue);
+    nix_list_builder_insert(ctx, builder, 1, intValue);
+    nix_list_builder_insert(ctx, builder, 2, lazyApply);
+    nix_make_list(ctx, builder, value);
+    nix_list_builder_free(builder);
+
+    // Test 1: Lazy accessor should return the throwing element without forcing evaluation
+    nix_value * lazyThrowingElement = nix_get_list_byidx_lazy(ctx, value, state, 0);
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, lazyThrowingElement);
+
+    // Verify the element is still lazy by checking that forcing it throws
+    nix_value_force(ctx, state, lazyThrowingElement);
+    assert_ctx_err();
+    ASSERT_THAT(
+        nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("This should not be evaluated by the lazy accessor"));
+
+    // Test 2: Lazy accessor should return the already-evaluated int
+    nix_value * intElement = nix_get_list_byidx_lazy(ctx, value, state, 1);
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, intElement);
+    ASSERT_EQ(42, nix_get_int(ctx, intElement));
+
+    // Test 3: Lazy accessor should return the lazy function application without forcing
+    nix_value * lazyFunctionElement = nix_get_list_byidx_lazy(ctx, value, state, 2);
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, lazyFunctionElement);
+
+    // Force the lazy function application - should compute 5 + 1 = 6
+    nix_value_force(ctx, state, lazyFunctionElement);
+    assert_ctx_ok();
+    ASSERT_EQ(6, nix_get_int(ctx, lazyFunctionElement));
+
+    // Clean up
+    nix_gc_decref(ctx, throwingFn);
+    nix_gc_decref(ctx, throwingValue);
+    nix_gc_decref(ctx, intValue);
+    nix_gc_decref(ctx, lazyApply);
+    nix_gc_decref(ctx, incrementFn);
+    nix_gc_decref(ctx, argFive);
+    nix_gc_decref(ctx, lazyThrowingElement);
+    nix_gc_decref(ctx, intElement);
+    nix_gc_decref(ctx, lazyFunctionElement);
+}
+
 TEST_F(nix_api_expr_test, nix_build_and_init_attr_invalid)
 {
     ASSERT_EQ(nullptr, nix_get_attr_byname(ctx, nullptr, state, 0));
@@ -255,6 +352,225 @@ TEST_F(nix_api_expr_test, nix_build_and_init_attr)
     free(out_name);
 }
 
+TEST_F(nix_api_expr_test, nix_get_attr_byidx_large_indices)
+{
+    // Create a small attribute set to test extremely large out-of-bounds access
+    const char ** out_name = (const char **) malloc(sizeof(char *));
+    BindingsBuilder * builder = nix_make_bindings_builder(ctx, state, 2);
+    nix_value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 42);
+    nix_bindings_builder_insert(ctx, builder, "test", intValue);
+    nix_make_attrs(ctx, value, builder);
+    nix_bindings_builder_free(builder);
+
+    // Test extremely large indices that would definitely crash without bounds checking
+    ASSERT_EQ(nullptr, nix_get_attr_byidx(ctx, value, state, 1000000, out_name));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+    ASSERT_EQ(nullptr, nix_get_attr_byidx(ctx, value, state, UINT_MAX / 2, out_name));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+    ASSERT_EQ(nullptr, nix_get_attr_byidx(ctx, value, state, UINT_MAX / 2 + 1000000, out_name));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+
+    // Test nix_get_attr_name_byidx with large indices too
+    ASSERT_EQ(nullptr, nix_get_attr_name_byidx(ctx, value, state, 1000000));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+    ASSERT_EQ(nullptr, nix_get_attr_name_byidx(ctx, value, state, UINT_MAX / 2));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+    ASSERT_EQ(nullptr, nix_get_attr_name_byidx(ctx, value, state, UINT_MAX / 2 + 1000000));
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+
+    // Clean up
+    nix_gc_decref(ctx, intValue);
+    free(out_name);
+}
+
+TEST_F(nix_api_expr_test, nix_get_attr_byname_lazy)
+{
+    // Create an attribute set with a throwing lazy attribute, an already-evaluated int, and a lazy function call
+
+    // 1. Throwing lazy element - create a function application thunk that will throw when forced
+    nix_value * throwingFn = nix_alloc_value(ctx, state);
+    nix_value * throwingValue = nix_alloc_value(ctx, state);
+
+    nix_expr_eval_from_string(
+        ctx,
+        state,
+        R"(
+        _: throw "This should not be evaluated by the lazy accessor"
+    )",
+        "<test>",
+        throwingFn);
+    assert_ctx_ok();
+
+    nix_init_apply(ctx, throwingValue, throwingFn, throwingFn);
+    assert_ctx_ok();
+
+    // 2. Already evaluated int (not lazy)
+    nix_value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 42);
+    assert_ctx_ok();
+
+    // 3. Lazy function application that would compute increment 7 = 8
+    nix_value * lazyApply = nix_alloc_value(ctx, state);
+    nix_value * incrementFn = nix_alloc_value(ctx, state);
+    nix_value * argSeven = nix_alloc_value(ctx, state);
+
+    nix_expr_eval_from_string(ctx, state, "x: x + 1", "<test>", incrementFn);
+    assert_ctx_ok();
+    nix_init_int(ctx, argSeven, 7);
+
+    // Create a lazy application: (x: x + 1) 7
+    nix_init_apply(ctx, lazyApply, incrementFn, argSeven);
+    assert_ctx_ok();
+
+    BindingsBuilder * builder = nix_make_bindings_builder(ctx, state, 3);
+    nix_bindings_builder_insert(ctx, builder, "throwing", throwingValue);
+    nix_bindings_builder_insert(ctx, builder, "normal", intValue);
+    nix_bindings_builder_insert(ctx, builder, "lazy", lazyApply);
+    nix_make_attrs(ctx, value, builder);
+    nix_bindings_builder_free(builder);
+
+    // Test 1: Lazy accessor should return the throwing attribute without forcing evaluation
+    nix_value * lazyThrowingAttr = nix_get_attr_byname_lazy(ctx, value, state, "throwing");
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, lazyThrowingAttr);
+
+    // Verify the attribute is still lazy by checking that forcing it throws
+    nix_value_force(ctx, state, lazyThrowingAttr);
+    assert_ctx_err();
+    ASSERT_THAT(
+        nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("This should not be evaluated by the lazy accessor"));
+
+    // Test 2: Lazy accessor should return the already-evaluated int
+    nix_value * intAttr = nix_get_attr_byname_lazy(ctx, value, state, "normal");
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, intAttr);
+    ASSERT_EQ(42, nix_get_int(ctx, intAttr));
+
+    // Test 3: Lazy accessor should return the lazy function application without forcing
+    nix_value * lazyFunctionAttr = nix_get_attr_byname_lazy(ctx, value, state, "lazy");
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, lazyFunctionAttr);
+
+    // Force the lazy function application - should compute 7 + 1 = 8
+    nix_value_force(ctx, state, lazyFunctionAttr);
+    assert_ctx_ok();
+    ASSERT_EQ(8, nix_get_int(ctx, lazyFunctionAttr));
+
+    // Test 4: Missing attribute should return NULL with NIX_ERR_KEY
+    nix_value * missingAttr = nix_get_attr_byname_lazy(ctx, value, state, "nonexistent");
+    ASSERT_EQ(nullptr, missingAttr);
+    ASSERT_EQ(NIX_ERR_KEY, nix_err_code(ctx));
+
+    // Clean up
+    nix_gc_decref(ctx, throwingFn);
+    nix_gc_decref(ctx, throwingValue);
+    nix_gc_decref(ctx, intValue);
+    nix_gc_decref(ctx, lazyApply);
+    nix_gc_decref(ctx, incrementFn);
+    nix_gc_decref(ctx, argSeven);
+    nix_gc_decref(ctx, lazyThrowingAttr);
+    nix_gc_decref(ctx, intAttr);
+    nix_gc_decref(ctx, lazyFunctionAttr);
+}
+
+TEST_F(nix_api_expr_test, nix_get_attr_byidx_lazy)
+{
+    // Create an attribute set with a throwing lazy attribute, an already-evaluated int, and a lazy function call
+
+    // 1. Throwing lazy element - create a function application thunk that will throw when forced
+    nix_value * throwingFn = nix_alloc_value(ctx, state);
+    nix_value * throwingValue = nix_alloc_value(ctx, state);
+
+    nix_expr_eval_from_string(
+        ctx,
+        state,
+        R"(
+        _: throw "This should not be evaluated by the lazy accessor"
+    )",
+        "<test>",
+        throwingFn);
+    assert_ctx_ok();
+
+    nix_init_apply(ctx, throwingValue, throwingFn, throwingFn);
+    assert_ctx_ok();
+
+    // 2. Already evaluated int (not lazy)
+    nix_value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 99);
+    assert_ctx_ok();
+
+    // 3. Lazy function application that would compute increment 10 = 11
+    nix_value * lazyApply = nix_alloc_value(ctx, state);
+    nix_value * incrementFn = nix_alloc_value(ctx, state);
+    nix_value * argTen = nix_alloc_value(ctx, state);
+
+    nix_expr_eval_from_string(ctx, state, "x: x + 1", "<test>", incrementFn);
+    assert_ctx_ok();
+    nix_init_int(ctx, argTen, 10);
+
+    // Create a lazy application: (x: x + 1) 10
+    nix_init_apply(ctx, lazyApply, incrementFn, argTen);
+    assert_ctx_ok();
+
+    BindingsBuilder * builder = nix_make_bindings_builder(ctx, state, 3);
+    nix_bindings_builder_insert(ctx, builder, "a_throwing", throwingValue);
+    nix_bindings_builder_insert(ctx, builder, "b_normal", intValue);
+    nix_bindings_builder_insert(ctx, builder, "c_lazy", lazyApply);
+    nix_make_attrs(ctx, value, builder);
+    nix_bindings_builder_free(builder);
+
+    // Proper usage: first get the size and gather all attributes into a map
+    unsigned int attrCount = nix_get_attrs_size(ctx, value);
+    assert_ctx_ok();
+    ASSERT_EQ(3u, attrCount);
+
+    // Gather all attributes into a map (proper contract usage)
+    std::map<std::string, nix_value *> attrMap;
+    const char * name;
+
+    for (unsigned int i = 0; i < attrCount; i++) {
+        nix_value * attr = nix_get_attr_byidx_lazy(ctx, value, state, i, &name);
+        assert_ctx_ok();
+        ASSERT_NE(nullptr, attr);
+        attrMap[std::string(name)] = attr;
+    }
+
+    // Now test the gathered attributes
+    ASSERT_EQ(3u, attrMap.size());
+    ASSERT_TRUE(attrMap.count("a_throwing"));
+    ASSERT_TRUE(attrMap.count("b_normal"));
+    ASSERT_TRUE(attrMap.count("c_lazy"));
+
+    // Test 1: Throwing attribute should be lazy
+    nix_value * throwingAttr = attrMap["a_throwing"];
+    nix_value_force(ctx, state, throwingAttr);
+    assert_ctx_err();
+    ASSERT_THAT(
+        nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("This should not be evaluated by the lazy accessor"));
+
+    // Test 2: Normal attribute should be already evaluated
+    nix_value * normalAttr = attrMap["b_normal"];
+    ASSERT_EQ(99, nix_get_int(ctx, normalAttr));
+
+    // Test 3: Lazy function should compute when forced
+    nix_value * lazyAttr = attrMap["c_lazy"];
+    nix_value_force(ctx, state, lazyAttr);
+    assert_ctx_ok();
+    ASSERT_EQ(11, nix_get_int(ctx, lazyAttr));
+
+    // Clean up
+    nix_gc_decref(ctx, throwingFn);
+    nix_gc_decref(ctx, throwingValue);
+    nix_gc_decref(ctx, intValue);
+    nix_gc_decref(ctx, lazyApply);
+    nix_gc_decref(ctx, incrementFn);
+    nix_gc_decref(ctx, argTen);
+    for (auto & pair : attrMap) {
+        nix_gc_decref(ctx, pair.second);
+    }
+}
+
 TEST_F(nix_api_expr_test, nix_value_init)
 {
     // Setup
@@ -320,8 +636,10 @@ TEST_F(nix_api_expr_test, nix_value_init_apply_error)
 
     // Evaluate it
     nix_value_force(ctx, state, v);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
-    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("attempt to call something which is not a function but"));
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("attempt to call something which is not a function but"));
 
     // Clean up
     nix_gc_decref(ctx, some_string);
@@ -380,7 +698,9 @@ TEST_F(nix_api_expr_test, nix_value_init_apply_lazy_arg)
     // nix_get_attr_byname isn't lazy (it could have been) so it will throw the exception
     nix_value * foo = nix_get_attr_byname(ctx, r, state, "foo");
     ASSERT_EQ(nullptr, foo);
-    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("error message for test case nix_value_init_apply_lazy_arg"));
+    ASSERT_THAT(
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("error message for test case nix_value_init_apply_lazy_arg"));
 
     // Clean up
     nix_gc_decref(ctx, f);

src/libexpr-tests/nix_api_value_internal.cc

@@ -0,0 +1,25 @@
+#include "nix_api_store.h"
+#include "nix_api_util.h"
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+#include "nix_api_expr_internal.h"
+
+#include "nix/expr/tests/nix_api_expr.hh"
+#include "nix/util/tests/string_callback.hh"
+
+#include <gmock/gmock.h>
+#include <cstddef>
+#include <cstdlib>
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+TEST_F(nix_api_expr_test, as_nix_value_ptr)
+{
+    // nix_alloc_value casts nix::Value to nix_value
+    // It should be obvious from the decl that that works, but if it doesn't,
+    // the whole implementation would be utterly broken.
+    ASSERT_EQ(sizeof(nix::Value), sizeof(nix_value));
+}
+
+} // namespace nixC

src/libexpr-tests/package.nix

@@ -62,6 +62,7 @@ mkMesonExecutable (finalAttrs: {
               mkdir -p "$HOME"
             ''
             + ''
+              export ASAN_OPTIONS=abort_on_error=1:print_summary=1:detect_leaks=0
               export _NIX_TEST_UNIT_DATA=${resolvePath ./data}
               ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage}
               touch $out

src/libexpr-tests/primops.cc

@@ -195,18 +195,18 @@ TEST_F(PrimOpTest, unsafeGetAttrPos)
     auto v = eval(expr);
     ASSERT_THAT(v, IsAttrsOfSize(3));
 
-    auto file = v.attrs()->find(createSymbol("file"));
+    auto file = v.attrs()->get(createSymbol("file"));
     ASSERT_NE(file, nullptr);
     ASSERT_THAT(*file->value, IsString());
     auto s = baseNameOf(file->value->string_view());
     ASSERT_EQ(s, "foo.nix");
 
-    auto line = v.attrs()->find(createSymbol("line"));
+    auto line = v.attrs()->get(createSymbol("line"));
     ASSERT_NE(line, nullptr);
     state.forceValue(*line->value, noPos);
     ASSERT_THAT(*line->value, IsIntEq(4));
 
-    auto column = v.attrs()->find(createSymbol("column"));
+    auto column = v.attrs()->get(createSymbol("column"));
     ASSERT_NE(column, nullptr);
     state.forceValue(*column->value, noPos);
     ASSERT_THAT(*column->value, IsIntEq(3));
@@ -246,7 +246,7 @@ TEST_F(PrimOpTest, removeAttrsRetains)
 {
     auto v = eval("builtins.removeAttrs { x = 1; y = 2; } [\"x\"]");
     ASSERT_THAT(v, IsAttrsOfSize(1));
-    ASSERT_NE(v.attrs()->find(createSymbol("y")), nullptr);
+    ASSERT_NE(v.attrs()->get(createSymbol("y")), nullptr);
 }
 
 TEST_F(PrimOpTest, listToAttrsEmptyList)
@@ -266,7 +266,7 @@ TEST_F(PrimOpTest, listToAttrs)
 {
     auto v = eval("builtins.listToAttrs [ { name = \"key\"; value = 123; } ]");
     ASSERT_THAT(v, IsAttrsOfSize(1));
-    auto key = v.attrs()->find(createSymbol("key"));
+    auto key = v.attrs()->get(createSymbol("key"));
     ASSERT_NE(key, nullptr);
     ASSERT_THAT(*key->value, IsIntEq(123));
 }
@@ -275,7 +275,7 @@ TEST_F(PrimOpTest, intersectAttrs)
 {
     auto v = eval("builtins.intersectAttrs { a = 1; b = 2; } { b = 3; c = 4; }");
     ASSERT_THAT(v, IsAttrsOfSize(1));
-    auto b = v.attrs()->find(createSymbol("b"));
+    auto b = v.attrs()->get(createSymbol("b"));
     ASSERT_NE(b, nullptr);
     ASSERT_THAT(*b->value, IsIntEq(3));
 }
@@ -293,11 +293,11 @@ TEST_F(PrimOpTest, functionArgs)
     auto v = eval("builtins.functionArgs ({ x, y ? 123}: 1)");
     ASSERT_THAT(v, IsAttrsOfSize(2));
 
-    auto x = v.attrs()->find(createSymbol("x"));
+    auto x = v.attrs()->get(createSymbol("x"));
     ASSERT_NE(x, nullptr);
     ASSERT_THAT(*x->value, IsFalse());
 
-    auto y = v.attrs()->find(createSymbol("y"));
+    auto y = v.attrs()->get(createSymbol("y"));
     ASSERT_NE(y, nullptr);
     ASSERT_THAT(*y->value, IsTrue());
 }
@@ -307,13 +307,13 @@ TEST_F(PrimOpTest, mapAttrs)
     auto v = eval("builtins.mapAttrs (name: value: value * 10) { a = 1; b = 2; }");
     ASSERT_THAT(v, IsAttrsOfSize(2));
 
-    auto a = v.attrs()->find(createSymbol("a"));
+    auto a = v.attrs()->get(createSymbol("a"));
     ASSERT_NE(a, nullptr);
     ASSERT_THAT(*a->value, IsThunk());
     state.forceValue(*a->value, noPos);
     ASSERT_THAT(*a->value, IsIntEq(10));
 
-    auto b = v.attrs()->find(createSymbol("b"));
+    auto b = v.attrs()->get(createSymbol("b"));
     ASSERT_NE(b, nullptr);
     ASSERT_THAT(*b->value, IsThunk());
     state.forceValue(*b->value, noPos);
@@ -642,7 +642,7 @@ class ToStringPrimOpTest : public PrimOpTest,
 
 TEST_P(ToStringPrimOpTest, toString)
 {
-    const auto [input, output] = GetParam();
+    const auto & [input, output] = GetParam();
     auto v = eval(input);
     ASSERT_THAT(v, IsStringEq(output));
 }
@@ -798,7 +798,7 @@ class CompareVersionsPrimOpTest : public PrimOpTest,
 
 TEST_P(CompareVersionsPrimOpTest, compareVersions)
 {
-    auto [expression, expectation] = GetParam();
+    const auto & [expression, expectation] = GetParam();
     auto v = eval(expression);
     ASSERT_THAT(v, IsIntEq(expectation));
 }
@@ -834,16 +834,16 @@ class ParseDrvNamePrimOpTest
 
 TEST_P(ParseDrvNamePrimOpTest, parseDrvName)
 {
-    auto [input, expectedName, expectedVersion] = GetParam();
+    const auto & [input, expectedName, expectedVersion] = GetParam();
     const auto expr = fmt("builtins.parseDrvName \"%1%\"", input);
     auto v = eval(expr);
     ASSERT_THAT(v, IsAttrsOfSize(2));
 
-    auto name = v.attrs()->find(createSymbol("name"));
+    auto name = v.attrs()->get(createSymbol("name"));
     ASSERT_TRUE(name);
     ASSERT_THAT(*name->value, IsStringEq(expectedName));
 
-    auto version = v.attrs()->find(createSymbol("version"));
+    auto version = v.attrs()->get(createSymbol("version"));
     ASSERT_TRUE(version);
     ASSERT_THAT(*version->value, IsStringEq(expectedVersion));
 }

src/libexpr-tests/trivial.cc

@@ -75,11 +75,11 @@ TEST_F(TrivialExpressionTest, updateAttrs)
 {
     auto v = eval("{ a = 1; } // { b = 2; a = 3; }");
     ASSERT_THAT(v, IsAttrsOfSize(2));
-    auto a = v.attrs()->find(createSymbol("a"));
+    auto a = v.attrs()->get(createSymbol("a"));
     ASSERT_NE(a, nullptr);
     ASSERT_THAT(*a->value, IsIntEq(3));
 
-    auto b = v.attrs()->find(createSymbol("b"));
+    auto b = v.attrs()->get(createSymbol("b"));
     ASSERT_NE(b, nullptr);
     ASSERT_THAT(*b->value, IsIntEq(2));
 }
@@ -176,7 +176,7 @@ TEST_P(AttrSetMergeTrvialExpressionTest, attrsetMergeLazy)
     auto v = eval(expr);
     ASSERT_THAT(v, IsAttrsOfSize(1));
 
-    auto a = v.attrs()->find(createSymbol("a"));
+    auto a = v.attrs()->get(createSymbol("a"));
     ASSERT_NE(a, nullptr);
 
     ASSERT_THAT(*a->value, IsThunk());
@@ -184,11 +184,11 @@ TEST_P(AttrSetMergeTrvialExpressionTest, attrsetMergeLazy)
 
     ASSERT_THAT(*a->value, IsAttrsOfSize(2));
 
-    auto b = a->value->attrs()->find(createSymbol("b"));
+    auto b = a->value->attrs()->get(createSymbol("b"));
     ASSERT_NE(b, nullptr);
     ASSERT_THAT(*b->value, IsIntEq(1));
 
-    auto c = a->value->attrs()->find(createSymbol("c"));
+    auto c = a->value->attrs()->get(createSymbol("c"));
     ASSERT_NE(c, nullptr);
     ASSERT_THAT(*c->value, IsIntEq(2));
 }
@@ -330,7 +330,7 @@ TEST_F(TrivialExpressionTest, bindOr)
 {
     auto v = eval("{ or = 1; }");
     ASSERT_THAT(v, IsAttrsOfSize(1));
-    auto b = v.attrs()->find(createSymbol("or"));
+    auto b = v.attrs()->get(createSymbol("or"));
     ASSERT_NE(b, nullptr);
     ASSERT_THAT(*b->value, IsIntEq(1));
 }

src/libexpr-tests/value/print.cc

@@ -35,14 +35,14 @@ TEST_F(ValuePrintingTests, tBool)
 TEST_F(ValuePrintingTests, tString)
 {
     Value vString;
-    vString.mkString("some-string");
+    vString.mkStringNoCopy("some-string");
     test(vString, "\"some-string\"");
 }
 
 TEST_F(ValuePrintingTests, tPath)
 {
     Value vPath;
-    vPath.mkString("/foo");
+    vPath.mkStringNoCopy("/foo");
     test(vPath, "\"/foo\"");
 }
 
@@ -61,7 +61,7 @@ TEST_F(ValuePrintingTests, tAttrs)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
@@ -196,11 +196,11 @@ TEST_F(ValuePrintingTests, depthAttrs)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builderEmpty(state, state.allocBindings(0));
+    BindingsBuilder builderEmpty = state.buildBindings(0);
     Value vAttrsEmpty;
     vAttrsEmpty.mkAttrs(builderEmpty.finish());
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
     builder.insert(state.symbols.create("nested"), &vAttrsEmpty);
@@ -208,7 +208,7 @@ TEST_F(ValuePrintingTests, depthAttrs)
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
 
-    BindingsBuilder builder2(state, state.allocBindings(10));
+    BindingsBuilder builder2 = state.buildBindings(10);
     builder2.insert(state.symbols.create("one"), &vOne);
     builder2.insert(state.symbols.create("two"), &vTwo);
     builder2.insert(state.symbols.create("nested"), &vAttrs);
@@ -233,14 +233,14 @@ TEST_F(ValuePrintingTests, depthList)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
 
-    BindingsBuilder builder2(state, state.allocBindings(10));
+    BindingsBuilder builder2 = state.buildBindings(10);
     builder2.insert(state.symbols.create("one"), &vOne);
     builder2.insert(state.symbols.create("two"), &vTwo);
     builder2.insert(state.symbols.create("nested"), &vAttrs);
@@ -290,12 +290,12 @@ TEST_F(StringPrintingTests, maxLengthTruncation)
 TEST_F(ValuePrintingTests, attrsTypeFirst)
 {
     Value vType;
-    vType.mkString("puppy");
+    vType.mkStringNoCopy("puppy");
 
     Value vApple;
-    vApple.mkString("apple");
+    vApple.mkStringNoCopy("apple");
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("type"), &vType);
     builder.insert(state.symbols.create("apple"), &vApple);
 
@@ -334,7 +334,7 @@ TEST_F(ValuePrintingTests, ansiColorsBool)
 TEST_F(ValuePrintingTests, ansiColorsString)
 {
     Value v;
-    v.mkString("puppy");
+    v.mkStringNoCopy("puppy");
 
     test(v, ANSI_MAGENTA "\"puppy\"" ANSI_NORMAL, PrintOptions{.ansiColors = true});
 }
@@ -342,7 +342,7 @@ TEST_F(ValuePrintingTests, ansiColorsString)
 TEST_F(ValuePrintingTests, ansiColorsStringElided)
 {
     Value v;
-    v.mkString("puppy");
+    v.mkStringNoCopy("puppy");
 
     test(
         v,
@@ -374,7 +374,7 @@ TEST_F(ValuePrintingTests, ansiColorsAttrs)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
@@ -390,10 +390,10 @@ TEST_F(ValuePrintingTests, ansiColorsAttrs)
 TEST_F(ValuePrintingTests, ansiColorsDerivation)
 {
     Value vDerivation;
-    vDerivation.mkString("derivation");
+    vDerivation.mkStringNoCopy("derivation");
 
-    BindingsBuilder builder(state, state.allocBindings(10));
-    builder.insert(state.sType, &vDerivation);
+    BindingsBuilder builder = state.buildBindings(10);
+    builder.insert(state.s.type, &vDerivation);
 
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
@@ -413,7 +413,7 @@ TEST_F(ValuePrintingTests, ansiColorsError)
 {
     Value throw_ = state.getBuiltin("throw");
     Value message;
-    message.mkString("uh oh!");
+    message.mkStringNoCopy("uh oh!");
     Value vError;
     vError.mkApp(&throw_, &message);
 
@@ -430,16 +430,16 @@ TEST_F(ValuePrintingTests, ansiColorsDerivationError)
 {
     Value throw_ = state.getBuiltin("throw");
     Value message;
-    message.mkString("uh oh!");
+    message.mkStringNoCopy("uh oh!");
     Value vError;
     vError.mkApp(&throw_, &message);
 
     Value vDerivation;
-    vDerivation.mkString("derivation");
+    vDerivation.mkStringNoCopy("derivation");
 
-    BindingsBuilder builder(state, state.allocBindings(10));
-    builder.insert(state.sType, &vDerivation);
-    builder.insert(state.sDrvPath, &vError);
+    BindingsBuilder builder = state.buildBindings(10);
+    builder.insert(state.s.type, &vDerivation);
+    builder.insert(state.s.drvPath, &vError);
 
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
@@ -553,12 +553,12 @@ TEST_F(ValuePrintingTests, ansiColorsBlackhole)
 
 TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    BindingsBuilder emptyBuilder = state.buildBindings(1);
 
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("a"), &vEmpty);
     builder.insert(state.symbols.create("b"), &vEmpty);
 
@@ -570,7 +570,7 @@ TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
 
 TEST_F(ValuePrintingTests, ansiColorsListRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    BindingsBuilder emptyBuilder = state.buildBindings(1);
 
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
@@ -586,7 +586,7 @@ TEST_F(ValuePrintingTests, ansiColorsListRepeated)
 
 TEST_F(ValuePrintingTests, listRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    BindingsBuilder emptyBuilder = state.buildBindings(1);
 
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
@@ -609,7 +609,7 @@ TEST_F(ValuePrintingTests, ansiColorsAttrsElided)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
@@ -635,8 +635,6 @@ TEST_F(ValuePrintingTests, ansiColorsAttrsElided)
 
 TEST_F(ValuePrintingTests, ansiColorsListElided)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
-
     Value vOne;
     vOne.mkInt(1);
 

src/libexpr/attr-path.cc

@@ -110,8 +110,8 @@ std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v
 {
     Value * v2;
     try {
-        auto dummyArgs = state.allocBindings(0);
-        v2 = findAlongAttrPath(state, "meta.position", *dummyArgs, v).first;
+        auto & dummyArgs = Bindings::emptyBindings;
+        v2 = findAlongAttrPath(state, "meta.position", dummyArgs, v).first;
     } catch (Error &) {
         throw NoPositionInfo("package '%s' has no source location information", what);
     }

src/libexpr/attr-set.cc

@@ -5,36 +5,37 @@
 
 namespace nix {
 
+Bindings Bindings::emptyBindings;
+
 /* Allocate a new array of attributes for an attribute set with a specific
    capacity. The space is implicitly reserved after the Bindings
    structure. */
-Bindings * EvalState::allocBindings(size_t capacity)
+Bindings * EvalMemory::allocBindings(size_t capacity)
 {
     if (capacity == 0)
-        return &emptyBindings;
-    if (capacity > std::numeric_limits<Bindings::size_t>::max())
+        return &Bindings::emptyBindings;
+    if (capacity > std::numeric_limits<Bindings::size_type>::max())
         throw Error("attribute set of size %d is too big", capacity);
-    nrAttrsets++;
-    nrAttrsInAttrsets += capacity;
-    return new (allocBytes(sizeof(Bindings) + sizeof(Attr) * capacity)) Bindings((Bindings::size_t) capacity);
+    stats.nrAttrsets++;
+    stats.nrAttrsInAttrsets += capacity;
+    return new (allocBytes(sizeof(Bindings) + sizeof(Attr) * capacity)) Bindings();
 }
 
 Value & BindingsBuilder::alloc(Symbol name, PosIdx pos)
 {
-    auto value = state.allocValue();
+    auto value = mem.get().allocValue();
     bindings->push_back(Attr(name, value, pos));
     return *value;
 }
 
 Value & BindingsBuilder::alloc(std::string_view name, PosIdx pos)
 {
-    return alloc(state.symbols.create(name), pos);
+    return alloc(symbols.get().create(name), pos);
 }
 
 void Bindings::sort()
 {
-    if (size_)
-        std::sort(begin(), end());
+    std::sort(attrs, attrs + numAttrs);
 }
 
 Value & Value::mkAttrs(BindingsBuilder & bindings)

src/libexpr/eval-cache.cc

@@ -330,7 +330,7 @@ AttrCursor::AttrCursor(
 AttrKey AttrCursor::getKey()
 {
     if (!parent)
-        return {0, root->state.sEpsilon};
+        return {0, root->state.s.epsilon};
     if (!parent->first->cachedValue) {
         parent->first->cachedValue = root->db->getAttr(parent->first->getKey());
         assert(parent->first->cachedValue);
@@ -702,7 +702,7 @@ bool AttrCursor::isDerivation()
 
 StorePath AttrCursor::forceDerivation()
 {
-    auto aDrvPath = getAttr(root->state.sDrvPath);
+    auto aDrvPath = getAttr(root->state.s.drvPath);
     auto drvPath = root->state.store->parseStorePath(aDrvPath->getString());
     drvPath.requireDerivation();
     if (!root->state.store->isValidPath(drvPath) && !settings.readOnlyMode) {

src/libexpr/eval-gc.cc

@@ -16,6 +16,7 @@
 #  endif
 
 #  include <gc/gc_allocator.h>
+#  include <gc/gc_tiny_fl.h> // For GC_GRANULE_BYTES
 
 #  include <boost/coroutine2/coroutine.hpp>
 #  include <boost/coroutine2/protected_fixedsize_stack.hpp>
@@ -26,6 +27,18 @@
 namespace nix {
 
 #if NIX_USE_BOEHMGC
+
+/*
+ * Ensure that Boehm satisfies our alignment requirements. This is the default configuration [^]
+ * and this assertion should never break for any platform. Let's assert it just in case.
+ *
+ * This alignment is particularly useful to be able to use aligned
+ * load/store instructions for loading/writing Values.
+ *
+ * [^]: https://github.com/bdwgc/bdwgc/blob/54ac18ccbc5a833dd7edaff94a10ab9b65044d61/include/gc/gc_tiny_fl.h#L31-L33
+ */
+static_assert(sizeof(void *) * 2 == GC_GRANULE_BYTES, "Boehm GC must use GC_GRANULE_WORDS = 2");
+
 /* Called when the Boehm GC runs out of memory. */
 static void * oomHandler(size_t requested)
 {

src/libexpr/eval-profiler.cc

@@ -185,7 +185,7 @@ FrameInfo SampleStack::getPrimOpFrameInfo(const PrimOp & primOp, std::span<Value
                 /* Error context strings don't actually matter, since we ignore all eval errors. */
                 state.forceAttrs(*args[0], pos, "");
                 auto attrs = args[0]->attrs();
-                auto nameAttr = state.getAttr(state.sName, attrs, "");
+                auto nameAttr = state.getAttr(state.s.name, attrs, "");
                 auto drvName = std::string(state.forceStringNoCtx(*nameAttr->value, pos, ""));
                 return DerivationStrictFrameInfo{.callPos = pos, .drvName = std::move(drvName)};
             } catch (...) {
@@ -211,7 +211,7 @@ FrameInfo SampleStack::getFrameInfoFromValueAndPos(const Value & v, std::span<Va
         /* Resolve primOp eagerly. Must not hold on to a reference to a Value. */
         return PrimOpFrameInfo{.expr = v.primOpAppPrimOp(), .callPos = pos};
     else if (state.isFunctor(v)) {
-        const auto functor = v.attrs()->get(state.sFunctor);
+        const auto functor = v.attrs()->get(state.s.functor);
         if (auto pos_ = posCache.lookup(pos); std::holds_alternative<std::monostate>(pos_.origin))
             /* HACK: In case callsite position is unresolved. */
             return FunctorFrameInfo{.pos = functor->pos};
@@ -324,7 +324,7 @@ void SampleStack::saveProfile()
             std::visit([&](auto && info) { info.symbolize(state, os, posCache); }, pos);
         }
         os << " " << count;
-        writeLine(profileFd.get(), std::move(os).str());
+        writeLine(profileFd.get(), os.str());
         /* Clear ostringstream. */
         os.str("");
         os.clear();

src/libexpr/eval.cc

@@ -17,11 +17,13 @@
 #include "nix/expr/print.hh"
 #include "nix/fetchers/filtering-source-accessor.hh"
 #include "nix/util/memory-source-accessor.hh"
+#include "nix/util/mounted-source-accessor.hh"
 #include "nix/expr/gc-small-vector.hh"
 #include "nix/util/url.hh"
 #include "nix/fetchers/fetch-to-store.hh"
 #include "nix/fetchers/tarball.hh"
 #include "nix/fetchers/input-cache.hh"
+#include "nix/util/current-process.hh"
 
 #include "parser-tab.hh"
 
@@ -37,10 +39,7 @@
 
 #include <nlohmann/json.hpp>
 #include <boost/container/small_vector.hpp>
-
-#ifndef _WIN32 // TODO use portable implementation
-#  include <sys/resource.h>
-#endif
+#include <boost/unordered/concurrent_flat_map.hpp>
 
 #include "nix/util/strings-inline.hh"
 
@@ -195,6 +194,15 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
 
 static constexpr size_t BASE_ENV_SIZE = 128;
 
+EvalMemory::EvalMemory()
+#if NIX_USE_BOEHMGC
+    : valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
+    , env1AllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
+#endif
+{
+    assertGCInitialized();
+}
+
 EvalState::EvalState(
     const LookupPath & lookupPathFromArguments,
     ref<Store> store,
@@ -203,138 +211,75 @@ EvalState::EvalState(
     std::shared_ptr<Store> buildStore)
     : fetchSettings{fetchSettings}
     , settings{settings}
-    , sWith(symbols.create("<with>"))
-    , sOutPath(symbols.create("outPath"))
-    , sDrvPath(symbols.create("drvPath"))
-    , sType(symbols.create("type"))
-    , sMeta(symbols.create("meta"))
-    , sName(symbols.create("name"))
-    , sValue(symbols.create("value"))
-    , sSystem(symbols.create("system"))
-    , sOverrides(symbols.create("__overrides"))
-    , sOutputs(symbols.create("outputs"))
-    , sOutputName(symbols.create("outputName"))
-    , sIgnoreNulls(symbols.create("__ignoreNulls"))
-    , sFile(symbols.create("file"))
-    , sLine(symbols.create("line"))
-    , sColumn(symbols.create("column"))
-    , sFunctor(symbols.create("__functor"))
-    , sToString(symbols.create("__toString"))
-    , sRight(symbols.create("right"))
-    , sWrong(symbols.create("wrong"))
-    , sStructuredAttrs(symbols.create("__structuredAttrs"))
-    , sJson(symbols.create("__json"))
-    , sAllowedReferences(symbols.create("allowedReferences"))
-    , sAllowedRequisites(symbols.create("allowedRequisites"))
-    , sDisallowedReferences(symbols.create("disallowedReferences"))
-    , sDisallowedRequisites(symbols.create("disallowedRequisites"))
-    , sMaxSize(symbols.create("maxSize"))
-    , sMaxClosureSize(symbols.create("maxClosureSize"))
-    , sBuilder(symbols.create("builder"))
-    , sArgs(symbols.create("args"))
-    , sContentAddressed(symbols.create("__contentAddressed"))
-    , sImpure(symbols.create("__impure"))
-    , sOutputHash(symbols.create("outputHash"))
-    , sOutputHashAlgo(symbols.create("outputHashAlgo"))
-    , sOutputHashMode(symbols.create("outputHashMode"))
-    , sRecurseForDerivations(symbols.create("recurseForDerivations"))
-    , sDescription(symbols.create("description"))
-    , sSelf(symbols.create("self"))
-    , sEpsilon(symbols.create(""))
-    , sStartSet(symbols.create("startSet"))
-    , sOperator(symbols.create("operator"))
-    , sKey(symbols.create("key"))
-    , sPath(symbols.create("path"))
-    , sPrefix(symbols.create("prefix"))
-    , sOutputSpecified(symbols.create("outputSpecified"))
-    , exprSymbols{
-        .sub = symbols.create("__sub"),
-        .lessThan = symbols.create("__lessThan"),
-        .mul = symbols.create("__mul"),
-        .div = symbols.create("__div"),
-        .or_ = symbols.create("or"),
-        .findFile = symbols.create("__findFile"),
-        .nixPath = symbols.create("__nixPath"),
-        .body = symbols.create("body"),
-    }
+    , symbols(StaticEvalSymbols::staticSymbolTable())
     , repair(NoRepair)
-    , emptyBindings(0)
-    , storeFS(
-        makeMountedSourceAccessor(
-            {
-                {CanonPath::root, makeEmptySourceAccessor()},
-                /* In the pure eval case, we can simply require
-                   valid paths. However, in the *impure* eval
-                   case this gets in the way of the union
-                   mechanism, because an invalid access in the
-                   upper layer will *not* be caught by the union
-                   source accessor, but instead abort the entire
-                   lookup.
+    , storeFS(makeMountedSourceAccessor({
+          {CanonPath::root, makeEmptySourceAccessor()},
+          /* In the pure eval case, we can simply require
+             valid paths. However, in the *impure* eval
+             case this gets in the way of the union
+             mechanism, because an invalid access in the
+             upper layer will *not* be caught by the union
+             source accessor, but instead abort the entire
+             lookup.
 
-                   This happens when the store dir in the
-                   ambient file system has a path (e.g. because
-                   another Nix store there), but the relocated
-                   store does not.
+             This happens when the store dir in the
+             ambient file system has a path (e.g. because
+             another Nix store there), but the relocated
+             store does not.
 
-                   TODO make the various source accessors doing
-                   access control all throw the same type of
-                   exception, and make union source accessor
-                   catch it, so we don't need to do this hack.
-                 */
-                {CanonPath(store->storeDir), store->getFSAccessor(settings.pureEval)},
-            }))
-    , rootFS(
-        ({
-            /* In pure eval mode, we provide a filesystem that only
-               contains the Nix store.
+             TODO make the various source accessors doing
+             access control all throw the same type of
+             exception, and make union source accessor
+             catch it, so we don't need to do this hack.
+           */
+          {CanonPath(store->storeDir), store->getFSAccessor(settings.pureEval)},
+      }))
+    , rootFS([&] {
+        /* In pure eval mode, we provide a filesystem that only
+           contains the Nix store.
 
-               If we have a chroot store and pure eval is not enabled,
-               use a union accessor to make the chroot store available
-               at its logical location while still having the
-               underlying directory available. This is necessary for
-               instance if we're evaluating a file from the physical
-               /nix/store while using a chroot store. */
-            auto accessor = getFSSourceAccessor();
+           Otherwise, use a union accessor to make the augmented store
+           available at its logical location while still having the
+           underlying directory available. This is necessary for
+           instance if we're evaluating a file from the physical
+           /nix/store while using a chroot store, and also for lazy
+           mounted fetchTree. */
+        auto accessor = settings.pureEval ? storeFS.cast<SourceAccessor>()
+                                          : makeUnionSourceAccessor({getFSSourceAccessor(), storeFS});
 
-            auto realStoreDir = dirOf(store->toRealPath(StorePath::dummy));
-            if (settings.pureEval || store->storeDir != realStoreDir) {
-                accessor = settings.pureEval
-                    ? storeFS
-                    : makeUnionSourceAccessor({accessor, storeFS});
-            }
+        /* Apply access control if needed. */
+        if (settings.restrictEval || settings.pureEval)
+            accessor = AllowListSourceAccessor::create(
+                accessor, {}, {}, [&settings](const CanonPath & path) -> RestrictedPathError {
+                    auto modeInformation = settings.pureEval ? "in pure evaluation mode (use '--impure' to override)"
+                                                             : "in restricted mode";
+                    throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", path, modeInformation);
+                });
 
-            /* Apply access control if needed. */
-            if (settings.restrictEval || settings.pureEval)
-                accessor = AllowListSourceAccessor::create(accessor, {}, {},
-                    [&settings](const CanonPath & path) -> RestrictedPathError {
-                        auto modeInformation = settings.pureEval
-                            ? "in pure evaluation mode (use '--impure' to override)"
-                            : "in restricted mode";
-                        throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", path, modeInformation);
-                    });
-
-            accessor;
-        }))
+        return accessor;
+    }())
     , corepkgsFS(make_ref<MemorySourceAccessor>())
     , internalFS(make_ref<MemorySourceAccessor>())
-    , derivationInternal{corepkgsFS->addFile(
-        CanonPath("derivation-internal.nix"),
+    , derivationInternal{internalFS->addFile(
+          CanonPath("derivation-internal.nix"),
 #include "primops/derivation.nix.gen.hh"
-    )}
+          )}
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , inputCache(fetchers::InputCache::create())
     , debugRepl(nullptr)
     , debugStop(false)
     , trylevel(0)
+    , srcToStore(make_ref<decltype(srcToStore)::element_type>())
+    , importResolutionCache(make_ref<decltype(importResolutionCache)::element_type>())
+    , fileEvalCache(make_ref<decltype(fileEvalCache)::element_type>())
     , regexCache(makeRegexCache())
 #if NIX_USE_BOEHMGC
-    , valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
-    , env1AllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
-    , baseEnvP(std::allocate_shared<Env *>(traceable_allocator<Env *>(), &allocEnv(BASE_ENV_SIZE)))
+    , baseEnvP(std::allocate_shared<Env *>(traceable_allocator<Env *>(), &mem.allocEnv(BASE_ENV_SIZE)))
     , baseEnv(**baseEnvP)
 #else
-    , baseEnv(allocEnv(BASE_ENV_SIZE))
+    , baseEnv(mem.allocEnv(BASE_ENV_SIZE))
 #endif
     , staticBaseEnv{std::make_shared<StaticEnv>(nullptr, nullptr)}
 {
@@ -343,18 +288,8 @@ EvalState::EvalState(
 
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
-    assertGCInitialized();
-
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
-
-    vEmptyList.mkList(buildList(0));
-    vNull.mkNull();
-    vTrue.mkBool(true);
-    vFalse.mkBool(false);
-    vStringRegular.mkString("regular");
-    vStringDirectory.mkString("directory");
-    vStringSymlink.mkString("symlink");
-    vStringUnknown.mkString("unknown");
+    static_assert(sizeof(Counter) == 64, "counters must be 64 bytes");
 
     /* Construct the Nix expression search path. */
     assert(lookupPath.elements.empty());
@@ -401,7 +336,7 @@ EvalState::EvalState(
 
 EvalState::~EvalState() {}
 
-void EvalState::allowPath(const Path & path)
+void EvalState::allowPathLegacy(const Path & path)
 {
     if (auto rootFS2 = rootFS.dynamic_pointer_cast<AllowListSourceAccessor>())
         rootFS2->allowPrefix(CanonPath(path));
@@ -649,12 +584,12 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
             .name = name,
             .arity = 0, // FIXME: figure out how deep by syntax only? It's not semantically useful though...
             .args = {},
-            .doc = makeImmutableString(toView(s)), // NOTE: memory leak when compiled without GC
+            .doc = makeImmutableString(s.view()), // NOTE: memory leak when compiled without GC
         };
     }
     if (isFunctor(v)) {
         try {
-            Value & functor = *v.attrs()->find(sFunctor)->value;
+            Value & functor = *v.attrs()->get(s.functor)->value;
             Value * vp[] = {&v};
             Value partiallyApplied;
             // The first parameter is not user-provided, and may be
@@ -883,7 +818,7 @@ DebugTraceStacker::DebugTraceStacker(EvalState & evalState, DebugTrace t)
 
 void Value::mkString(std::string_view s)
 {
-    mkString(makeImmutableString(s));
+    mkStringNoCopy(makeImmutableString(s));
 }
 
 static const char ** encodeContext(const NixStringContext & context)
@@ -902,12 +837,12 @@ static const char ** encodeContext(const NixStringContext & context)
 
 void Value::mkString(std::string_view s, const NixStringContext & context)
 {
-    mkString(makeImmutableString(s), encodeContext(context));
+    mkStringNoCopy(makeImmutableString(s), encodeContext(context));
 }
 
 void Value::mkStringMove(const char * s, const NixStringContext & context)
 {
-    mkString(s, encodeContext(context));
+    mkStringNoCopy(s, encodeContext(context));
 }
 
 void Value::mkPath(const SourcePath & path)
@@ -948,19 +883,18 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
     }
 }
 
-ListBuilder::ListBuilder(EvalState & state, size_t size)
+ListBuilder::ListBuilder(size_t size)
     : size(size)
     , elems(size <= 2 ? inlineElems : (Value **) allocBytes(size * sizeof(Value *)))
 {
-    state.nrListElems += size;
 }
 
 Value * EvalState::getBool(bool b)
 {
-    return b ? &vTrue : &vFalse;
+    return b ? &Value::vTrue : &Value::vFalse;
 }
 
-unsigned long nrThunks = 0;
+static Counter nrThunks;
 
 static inline void mkThunk(Value & v, Env & env, Expr * expr)
 {
@@ -978,8 +912,8 @@ void EvalState::mkPos(Value & v, PosIdx p)
     auto origin = positions.originOf(p);
     if (auto path = std::get_if<SourcePath>(&origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(sFile).mkString(path->path.abs());
-        makePositionThunks(*this, p, attrs.alloc(sLine), attrs.alloc(sColumn));
+        attrs.alloc(s.file).mkString(path->path.abs());
+        makePositionThunks(*this, p, attrs.alloc(s.line), attrs.alloc(s.column));
         v.mkAttrs(attrs);
     } else
         v.mkNull();
@@ -1051,10 +985,6 @@ void EvalState::mkSingleDerivedPathString(const SingleDerivedPath & p, Value & v
         });
 }
 
-/* Create a thunk for the delayed computation of the given expression
-   in the given environment.  But if the expression is a variable,
-   then look it up right away.  This significantly reduces the number
-   of thunks allocated. */
 Value * Expr::maybeThunk(EvalState & state, Env & env)
 {
     Value * v = state.allocValue();
@@ -1098,61 +1028,90 @@ Value * ExprPath::maybeThunk(EvalState & state, Env & env)
     return &v;
 }
 
+/**
+ * A helper `Expr` class to lets us parse and evaluate Nix expressions
+ * from a thunk, ensuring that every file is parsed/evaluated only
+ * once (via the thunk stored in `EvalState::fileEvalCache`).
+ */
+struct ExprParseFile : Expr, gc
+{
+    // FIXME: make this a reference (see below).
+    SourcePath path;
+    bool mustBeTrivial;
+
+    ExprParseFile(SourcePath & path, bool mustBeTrivial)
+        : path(path)
+        , mustBeTrivial(mustBeTrivial)
+    {
+    }
+
+    void eval(EvalState & state, Env & env, Value & v) override
+    {
+        printTalkative("evaluating file '%s'", path);
+
+        auto e = state.parseExprFromFile(path);
+
+        try {
+            auto dts =
+                state.debugRepl
+                    ? makeDebugTraceStacker(
+                          state, *e, state.baseEnv, e->getPos(), "while evaluating the file '%s':", path.to_string())
+                    : nullptr;
+
+            // Enforce that 'flake.nix' is a direct attrset, not a
+            // computation.
+            if (mustBeTrivial && !(dynamic_cast<ExprAttrs *>(e)))
+                state.error<EvalError>("file '%s' must be an attribute set", path).debugThrow();
+
+            state.eval(e, v);
+        } catch (Error & e) {
+            state.addErrorTrace(e, "while evaluating the file '%s':", path.to_string());
+            throw;
+        }
+    }
+};
+
 void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
 {
-    FileEvalCache::iterator i;
-    if ((i = fileEvalCache.find(path)) != fileEvalCache.end()) {
-        v = i->second;
+    auto resolvedPath = getConcurrent(*importResolutionCache, path);
+
+    if (!resolvedPath) {
+        resolvedPath = resolveExprPath(path);
+        importResolutionCache->emplace(path, *resolvedPath);
+    }
+
+    if (auto v2 = getConcurrent(*fileEvalCache, *resolvedPath)) {
+        forceValue(**v2, noPos);
+        v = **v2;
         return;
     }
 
-    auto resolvedPath = resolveExprPath(path);
-    if ((i = fileEvalCache.find(resolvedPath)) != fileEvalCache.end()) {
-        v = i->second;
-        return;
-    }
+    Value * vExpr;
+    // FIXME: put ExprParseFile on the stack instead of the heap once
+    // https://github.com/NixOS/nix/pull/13930 is merged. That will ensure
+    // the post-condition that `expr` is unreachable after
+    // `forceValue()` returns.
+    auto expr = new ExprParseFile{*resolvedPath, mustBeTrivial};
 
-    printTalkative("evaluating file '%1%'", resolvedPath);
-    Expr * e = nullptr;
+    fileEvalCache->try_emplace_and_cvisit(
+        *resolvedPath,
+        nullptr,
+        [&](auto & i) {
+            vExpr = allocValue();
+            vExpr->mkThunk(&baseEnv, expr);
+            i.second = vExpr;
+        },
+        [&](auto & i) { vExpr = i.second; });
 
-    auto j = fileParseCache.find(resolvedPath);
-    if (j != fileParseCache.end())
-        e = j->second;
+    forceValue(*vExpr, noPos);
 
-    if (!e)
-        e = parseExprFromFile(resolvedPath);
-
-    fileParseCache.emplace(resolvedPath, e);
-
-    try {
-        auto dts = debugRepl ? makeDebugTraceStacker(
-                                   *this,
-                                   *e,
-                                   this->baseEnv,
-                                   e->getPos(),
-                                   "while evaluating the file '%1%':",
-                                   resolvedPath.to_string())
-                             : nullptr;
-
-        // Enforce that 'flake.nix' is a direct attrset, not a
-        // computation.
-        if (mustBeTrivial && !(dynamic_cast<ExprAttrs *>(e)))
-            error<EvalError>("file '%s' must be an attribute set", path).debugThrow();
-        eval(e, v);
-    } catch (Error & e) {
-        addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath.to_string());
-        throw;
-    }
-
-    fileEvalCache.emplace(resolvedPath, v);
-    if (path != resolvedPath)
-        fileEvalCache.emplace(path, v);
+    v = *vExpr;
 }
 
 void EvalState::resetFileCache()
 {
-    fileEvalCache.clear();
-    fileParseCache.clear();
+    importResolutionCache->clear();
+    fileEvalCache->clear();
     inputCache->clear();
 }
 
@@ -1221,7 +1180,7 @@ void ExprPath::eval(EvalState & state, Env & env, Value & v)
 
 Env * ExprAttrs::buildInheritFromEnv(EvalState & state, Env & up)
 {
-    Env & inheritEnv = state.allocEnv(inheritFromExprs->size());
+    Env & inheritEnv = state.mem.allocEnv(inheritFromExprs->size());
     inheritEnv.up = &up;
 
     Displacement displ = 0;
@@ -1240,12 +1199,12 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
     if (recursive) {
         /* Create a new environment that contains the attributes in
            this `rec'. */
-        Env & env2(state.allocEnv(attrs.size()));
+        Env & env2(state.mem.allocEnv(attrs.size()));
         env2.up = &env;
         dynamicEnv = &env2;
         Env * inheritEnv = inheritFromExprs ? buildInheritFromEnv(state, env2) : nullptr;
 
-        AttrDefs::iterator overrides = attrs.find(state.sOverrides);
+        AttrDefs::iterator overrides = attrs.find(state.s.overrides);
         bool hasOverrides = overrides != attrs.end();
 
         /* The recursive attributes are evaluated in the new
@@ -1277,7 +1236,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
                 *vOverrides,
                 [&]() { return vOverrides->determinePos(noPos); },
                 "while evaluating the `__overrides` attribute");
-            bindings.grow(state.allocBindings(bindings.capacity() + vOverrides->attrs()->size()));
+            bindings.grow(state.buildBindings(bindings.capacity() + vOverrides->attrs()->size()));
             for (auto & i : *vOverrides->attrs()) {
                 AttrDefs::iterator j = attrs.find(i.name);
                 if (j != attrs.end()) {
@@ -1332,7 +1291,7 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
 {
     /* Create a new environment that contains the attributes in this
        `let'. */
-    Env & env2(state.allocEnv(attrs->attrs.size()));
+    Env & env2(state.mem.allocEnv(attrs->attrs.size()));
     env2.up = &env;
 
     Env * inheritEnv = attrs->inheritFromExprs ? attrs->buildInheritFromEnv(state, env2) : nullptr;
@@ -1363,7 +1322,7 @@ void ExprList::eval(EvalState & state, Env & env, Value & v)
 Value * ExprList::maybeThunk(EvalState & state, Env & env)
 {
     if (elems.empty()) {
-        return &state.vEmptyList;
+        return &Value::vEmptyList;
     }
     return Expr::maybeThunk(state, env);
 }
@@ -1375,7 +1334,7 @@ void ExprVar::eval(EvalState & state, Env & env, Value & v)
     v = *v2;
 }
 
-static std::string showAttrPath(EvalState & state, Env & env, const AttrPath & attrPath)
+static std::string showAttrPath(EvalState & state, Env & env, std::span<const AttrName> attrPath)
 {
     std::ostringstream out;
     bool first = true;
@@ -1411,10 +1370,10 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                                          env,
                                          getPos(),
                                          "while evaluating the attribute '%1%'",
-                                         showAttrPath(state, env, attrPath))
+                                         showAttrPath(state, env, getAttrPath()))
                                    : nullptr;
 
-        for (auto & i : attrPath) {
+        for (auto & i : getAttrPath()) {
             state.nrLookups++;
             const Attr * j;
             auto name = getName(i, state, env);
@@ -1452,7 +1411,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
             auto origin = std::get_if<SourcePath>(&pos2r.origin);
             if (!(origin && *origin == state.derivationInternal))
                 state.addErrorTrace(
-                    e, pos2, "while evaluating the attribute '%1%'", showAttrPath(state, env, attrPath));
+                    e, pos2, "while evaluating the attribute '%1%'", showAttrPath(state, env, getAttrPath()));
         }
         throw;
     }
@@ -1463,13 +1422,13 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
 Symbol ExprSelect::evalExceptFinalSelect(EvalState & state, Env & env, Value & attrs)
 {
     Value vTmp;
-    Symbol name = getName(attrPath[attrPath.size() - 1], state, env);
+    Symbol name = getName(attrPathStart[nAttrPath - 1], state, env);
 
-    if (attrPath.size() == 1) {
+    if (nAttrPath == 1) {
         e->eval(state, env, vTmp);
     } else {
         ExprSelect init(*this);
-        init.attrPath.pop_back();
+        init.nAttrPath--;
         init.eval(state, env, vTmp);
     }
     attrs = vTmp;
@@ -1538,7 +1497,7 @@ void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes,
             ExprLambda & lambda(*vCur.lambda().fun);
 
             auto size = (!lambda.arg ? 0 : 1) + (lambda.hasFormals() ? lambda.formals->formals.size() : 0);
-            Env & env2(allocEnv(size));
+            Env & env2(mem.allocEnv(size));
             env2.up = vCur.lambda().env;
 
             Displacement displ = 0;
@@ -1717,7 +1676,7 @@ void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes,
             }
         }
 
-        else if (vCur.type() == nAttrs && (functor = vCur.attrs()->get(sFunctor))) {
+        else if (vCur.type() == nAttrs && (functor = vCur.attrs()->get(s.functor))) {
             /* 'vCur' may be allocated on the stack of the calling
                function, but for functors we may keep a reference, so
                heap-allocate a copy and use that instead. */
@@ -1779,8 +1738,8 @@ void EvalState::autoCallFunction(const Bindings & args, Value & fun, Value & res
     forceValue(fun, pos);
 
     if (fun.type() == nAttrs) {
-        auto found = fun.attrs()->find(sFunctor);
-        if (found != fun.attrs()->end()) {
+        auto found = fun.attrs()->get(s.functor);
+        if (found) {
             Value * v = allocValue();
             callFunction(*found->value, fun, *v, pos);
             forceValue(*v, pos);
@@ -1827,7 +1786,7 @@ https://nix.dev/manual/nix/stable/language/syntax.html#functions.)",
 
 void ExprWith::eval(EvalState & state, Env & env, Value & v)
 {
-    Env & env2(state.allocEnv(1));
+    Env & env2(state.mem.allocEnv(1));
     env2.up = &env;
     env2.values[0] = attrs->maybeThunk(state, env);
 
@@ -1845,7 +1804,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
     if (!state.evalBool(env, cond, pos, "in the condition of the assert statement")) {
         std::ostringstream out;
         cond->show(state.symbols, out);
-        auto exprStr = toView(out);
+        auto exprStr = out.view();
 
         if (auto eq = dynamic_cast<ExprOpEq *>(cond)) {
             try {
@@ -1909,51 +1868,113 @@ void ExprOpImpl::eval(EvalState & state, Env & env, Value & v)
         || state.evalBool(env, e2, pos, "in the right operand of the IMPL (->) operator"));
 }
 
-void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
+void ExprOpUpdate::eval(EvalState & state, Value & v, Value & v1, Value & v2)
 {
-    Value v1, v2;
-    state.evalAttrs(env, e1, v1, pos, "in the left operand of the update (//) operator");
-    state.evalAttrs(env, e2, v2, pos, "in the right operand of the update (//) operator");
-
     state.nrOpUpdates++;
 
-    if (v1.attrs()->size() == 0) {
+    const Bindings & bindings1 = *v1.attrs();
+    if (bindings1.empty()) {
         v = v2;
         return;
     }
-    if (v2.attrs()->size() == 0) {
+
+    const Bindings & bindings2 = *v2.attrs();
+    if (bindings2.empty()) {
         v = v1;
         return;
     }
 
-    auto attrs = state.buildBindings(v1.attrs()->size() + v2.attrs()->size());
+    /* Simple heuristic for determining whether attrs2 should be "layered" on top of
+       attrs1 instead of copying to a new Bindings. */
+    bool shouldLayer = [&]() -> bool {
+        if (bindings1.isLayerListFull())
+            return false;
+
+        if (bindings2.size() > state.settings.bindingsUpdateLayerRhsSizeThreshold)
+            return false;
+
+        return true;
+    }();
+
+    if (shouldLayer) {
+        auto attrs = state.buildBindings(bindings2.size());
+        attrs.layerOnTopOf(bindings1);
+
+        std::ranges::copy(bindings2, std::back_inserter(attrs));
+        v.mkAttrs(attrs.alreadySorted());
+
+        state.nrOpUpdateValuesCopied += bindings2.size();
+        return;
+    }
+
+    auto attrs = state.buildBindings(bindings1.size() + bindings2.size());
 
     /* Merge the sets, preferring values from the second set.  Make
        sure to keep the resulting vector in sorted order. */
-    auto i = v1.attrs()->begin();
-    auto j = v2.attrs()->begin();
+    auto i = bindings1.begin();
+    auto j = bindings2.begin();
 
-    while (i != v1.attrs()->end() && j != v2.attrs()->end()) {
+    while (i != bindings1.end() && j != bindings2.end()) {
         if (i->name == j->name) {
             attrs.insert(*j);
             ++i;
             ++j;
-        } else if (i->name < j->name)
-            attrs.insert(*i++);
-        else
-            attrs.insert(*j++);
+        } else if (i->name < j->name) {
+            attrs.insert(*i);
+            ++i;
+        } else {
+            attrs.insert(*j);
+            ++j;
+        }
     }
 
-    while (i != v1.attrs()->end())
-        attrs.insert(*i++);
-    while (j != v2.attrs()->end())
-        attrs.insert(*j++);
+    while (i != bindings1.end()) {
+        attrs.insert(*i);
+        ++i;
+    }
+
+    while (j != bindings2.end()) {
+        attrs.insert(*j);
+        ++j;
+    }
 
     v.mkAttrs(attrs.alreadySorted());
 
     state.nrOpUpdateValuesCopied += v.attrs()->size();
 }
 
+void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
+{
+    UpdateQueue q;
+    evalForUpdate(state, env, q);
+
+    v.mkAttrs(&Bindings::emptyBindings);
+    for (auto & rhs : std::views::reverse(q)) {
+        /* Remember that queue is sorted rightmost attrset first. */
+        eval(state, /*v=*/v, /*v1=*/v, /*v2=*/rhs);
+    }
+}
+
+void Expr::evalForUpdate(EvalState & state, Env & env, UpdateQueue & q, std::string_view errorCtx)
+{
+    Value v;
+    state.evalAttrs(env, this, v, getPos(), errorCtx);
+    q.push_back(v);
+}
+
+void ExprOpUpdate::evalForUpdate(EvalState & state, Env & env, UpdateQueue & q)
+{
+    /* Output rightmost attrset first to the merge queue as the one
+       with the most priority. */
+    e2->evalForUpdate(state, env, q, "in the right operand of the update (//) operator");
+    e1->evalForUpdate(state, env, q, "in the left operand of the update (//) operator");
+}
+
+void ExprOpUpdate::evalForUpdate(EvalState & state, Env & env, UpdateQueue & q, std::string_view errorCtx)
+{
+    evalForUpdate(state, env, q);
+}
+
 void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1;
@@ -2230,10 +2251,10 @@ bool EvalState::forceBool(Value & v, const PosIdx pos, std::string_view errorCtx
     return v.boolean();
 }
 
-Bindings::const_iterator EvalState::getAttr(Symbol attrSym, const Bindings * attrSet, std::string_view errorCtx)
+const Attr * EvalState::getAttr(Symbol attrSym, const Bindings * attrSet, std::string_view errorCtx)
 {
-    auto value = attrSet->find(attrSym);
-    if (value == attrSet->end()) {
+    auto value = attrSet->get(attrSym);
+    if (!value) {
         error<TypeError>("attribute '%s' missing", symbols[attrSym]).withTrace(noPos, errorCtx).debugThrow();
     }
     return value;
@@ -2241,7 +2262,7 @@ Bindings::const_iterator EvalState::getAttr(Symbol attrSym, const Bindings * att
 
 bool EvalState::isFunctor(const Value & fun) const
 {
-    return fun.type() == nAttrs && fun.attrs()->find(sFunctor) != fun.attrs()->end();
+    return fun.type() == nAttrs && fun.attrs()->get(s.functor);
 }
 
 void EvalState::forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx)
@@ -2310,7 +2331,7 @@ bool EvalState::isDerivation(Value & v)
 {
     if (v.type() != nAttrs)
         return false;
-    auto i = v.attrs()->get(sType);
+    auto i = v.attrs()->get(s.type);
     if (!i)
         return false;
     forceValue(*i->value, i->pos);
@@ -2322,8 +2343,8 @@ bool EvalState::isDerivation(Value & v)
 std::optional<std::string>
 EvalState::tryAttrsToString(const PosIdx pos, Value & v, NixStringContext & context, bool coerceMore, bool copyToStore)
 {
-    auto i = v.attrs()->find(sToString);
-    if (i != v.attrs()->end()) {
+    auto i = v.attrs()->get(s.toString);
+    if (i) {
         Value v1;
         callFunction(*i->value, v, v1, pos);
         return coerceToString(
@@ -2368,8 +2389,8 @@ BackedStringView EvalState::coerceToString(
         auto maybeString = tryAttrsToString(pos, v, context, coerceMore, copyToStore);
         if (maybeString)
             return std::move(*maybeString);
-        auto i = v.attrs()->find(sOutPath);
-        if (i == v.attrs()->end()) {
+        auto i = v.attrs()->get(s.outPath);
+        if (!i) {
             error<TypeError>(
                 "cannot coerce %1% to a string: %2%", showType(v), ValuePrinter(*this, v, errorPrintOptions))
                 .withTrace(pos, errorCtx)
@@ -2437,7 +2458,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
     if (nix::isDerivation(path.path.abs()))
         error<EvalError>("file names are not allowed to end in '%1%'", drvExtension).debugThrow();
 
-    auto dstPathCached = get(*srcToStore.lock(), path);
+    auto dstPathCached = getConcurrent(*srcToStore, path);
 
     auto dstPath = dstPathCached ? *dstPathCached : [&]() {
         auto dstPath = fetchToStore(
@@ -2450,7 +2471,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
             nullptr,
             repair);
         allowPath(dstPath);
-        srcToStore.lock()->try_emplace(path, dstPath);
+        srcToStore->try_emplace(path, dstPath);
         printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
         return dstPath;
     }();
@@ -2475,8 +2496,8 @@ SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext
     /* Similarly, handle __toString where the result may be a path
        value. */
     if (v.type() == nAttrs) {
-        auto i = v.attrs()->find(sToString);
-        if (i != v.attrs()->end()) {
+        auto i = v.attrs()->get(s.toString);
+        if (i) {
             Value v1;
             callFunction(*i->value, v, v1, pos);
             return coerceToPath(pos, v1, context, errorCtx);
@@ -2665,8 +2686,8 @@ void EvalState::assertEqValues(Value & v1, Value & v2, const PosIdx pos, std::st
 
     case nAttrs: {
         if (isDerivation(v1) && isDerivation(v2)) {
-            auto i = v1.attrs()->get(sOutPath);
-            auto j = v2.attrs()->get(sOutPath);
+            auto i = v1.attrs()->get(s.outPath);
+            auto j = v2.attrs()->get(s.outPath);
             if (i && j) {
                 try {
                     assertEqValues(*i->value, *j->value, pos, errorCtx);
@@ -2819,8 +2840,8 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
         /* If both sets denote a derivation (type = "derivation"),
            then compare their outPaths. */
         if (isDerivation(v1) && isDerivation(v2)) {
-            auto i = v1.attrs()->get(sOutPath);
-            auto j = v2.attrs()->get(sOutPath);
+            auto i = v1.attrs()->get(s.outPath);
+            auto j = v2.attrs()->get(s.outPath);
             if (i && j)
                 return eqValues(*i->value, *j->value, pos, errorCtx);
         }
@@ -2872,11 +2893,11 @@ bool EvalState::fullGC()
 #endif
 }
 
+bool Counter::enabled = getEnv("NIX_SHOW_STATS").value_or("0") != "0";
+
 void EvalState::maybePrintStats()
 {
-    bool showStats = getEnv("NIX_SHOW_STATS").value_or("0") != "0";
-
-    if (showStats) {
+    if (Counter::enabled) {
         // Make the final heap size more deterministic.
 #if NIX_USE_BOEHMGC
         if (!fullGC()) {
@@ -2889,16 +2910,15 @@ void EvalState::maybePrintStats()
 
 void EvalState::printStatistics()
 {
-#ifndef _WIN32 // TODO use portable implementation
-    struct rusage buf;
-    getrusage(RUSAGE_SELF, &buf);
-    float cpuTime = buf.ru_utime.tv_sec + ((float) buf.ru_utime.tv_usec / 1000000);
-#endif
+    std::chrono::microseconds cpuTimeDuration = getCpuUserTime();
+    float cpuTime = std::chrono::duration_cast<std::chrono::duration<float>>(cpuTimeDuration).count();
 
-    uint64_t bEnvs = nrEnvs * sizeof(Env) + nrValuesInEnvs * sizeof(Value *);
-    uint64_t bLists = nrListElems * sizeof(Value *);
-    uint64_t bValues = nrValues * sizeof(Value);
-    uint64_t bAttrsets = nrAttrsets * sizeof(Bindings) + nrAttrsInAttrsets * sizeof(Attr);
+    auto & memstats = mem.getStats();
+
+    uint64_t bEnvs = memstats.nrEnvs * sizeof(Env) + memstats.nrValuesInEnvs * sizeof(Value *);
+    uint64_t bLists = memstats.nrListElems * sizeof(Value *);
+    uint64_t bValues = memstats.nrValues * sizeof(Value);
+    uint64_t bAttrsets = memstats.nrAttrsets * sizeof(Bindings) + memstats.nrAttrsInAttrsets * sizeof(Attr);
 
 #if NIX_USE_BOEHMGC
     GC_word heapSize, totalBytes;
@@ -2915,33 +2935,27 @@ void EvalState::printStatistics()
     if (outPath != "-")
         fs.open(outPath, std::fstream::out);
     json topObj = json::object();
-#ifndef _WIN32 // TODO implement
     topObj["cpuTime"] = cpuTime;
-#endif
     topObj["time"] = {
-#ifndef _WIN32 // TODO implement
         {"cpu", cpuTime},
-#endif
 #if NIX_USE_BOEHMGC
         {GC_is_incremental_mode() ? "gcNonIncremental" : "gc", gcFullOnlyTime},
-#  ifndef _WIN32 // TODO implement
         {GC_is_incremental_mode() ? "gcNonIncrementalFraction" : "gcFraction", gcFullOnlyTime / cpuTime},
-#  endif
 #endif
     };
     topObj["envs"] = {
-        {"number", nrEnvs},
-        {"elements", nrValuesInEnvs},
+        {"number", memstats.nrEnvs.load()},
+        {"elements", memstats.nrValuesInEnvs.load()},
         {"bytes", bEnvs},
     };
-    topObj["nrExprs"] = Expr::nrExprs;
+    topObj["nrExprs"] = Expr::nrExprs.load();
     topObj["list"] = {
-        {"elements", nrListElems},
+        {"elements", memstats.nrListElems.load()},
         {"bytes", bLists},
-        {"concats", nrListConcats},
+        {"concats", nrListConcats.load()},
     };
     topObj["values"] = {
-        {"number", nrValues},
+        {"number", memstats.nrValues.load()},
         {"bytes", bValues},
     };
     topObj["symbols"] = {
@@ -2949,9 +2963,9 @@ void EvalState::printStatistics()
         {"bytes", symbols.totalSize()},
     };
     topObj["sets"] = {
-        {"number", nrAttrsets},
+        {"number", memstats.nrAttrsets.load()},
         {"bytes", bAttrsets},
-        {"elements", nrAttrsInAttrsets},
+        {"elements", memstats.nrAttrsInAttrsets.load()},
     };
     topObj["sizes"] = {
         {"Env", sizeof(Env)},
@@ -2959,13 +2973,13 @@ void EvalState::printStatistics()
         {"Bindings", sizeof(Bindings)},
         {"Attr", sizeof(Attr)},
     };
-    topObj["nrOpUpdates"] = nrOpUpdates;
-    topObj["nrOpUpdateValuesCopied"] = nrOpUpdateValuesCopied;
-    topObj["nrThunks"] = nrThunks;
-    topObj["nrAvoided"] = nrAvoided;
-    topObj["nrLookups"] = nrLookups;
-    topObj["nrPrimOpCalls"] = nrPrimOpCalls;
-    topObj["nrFunctionCalls"] = nrFunctionCalls;
+    topObj["nrOpUpdates"] = nrOpUpdates.load();
+    topObj["nrOpUpdateValuesCopied"] = nrOpUpdateValuesCopied.load();
+    topObj["nrThunks"] = nrThunks.load();
+    topObj["nrAvoided"] = nrAvoided.load();
+    topObj["nrLookups"] = nrLookups.load();
+    topObj["nrPrimOpCalls"] = nrPrimOpCalls.load();
+    topObj["nrFunctionCalls"] = nrFunctionCalls.load();
 #if NIX_USE_BOEHMGC
     topObj["gc"] = {
         {"heapSize", heapSize},
@@ -3112,6 +3126,11 @@ SourcePath EvalState::findFile(const LookupPath & lookupPath, const std::string_
         auto res = (r / CanonPath(suffix)).resolveSymlinks();
         if (res.pathExists())
             return res;
+
+        // Backward compatibility hack: throw an exception if access
+        // to this path is not allowed.
+        if (auto accessor = res.accessor.dynamic_pointer_cast<FilteringSourceAccessor>())
+            accessor->checkAccess(res.path);
     }
 
     if (hasPrefix(path, "nix/"))
@@ -3166,7 +3185,7 @@ std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Pat
 
         /* Allow access to paths in the search path. */
         if (initAccessControl) {
-            allowPath(path.path.abs());
+            allowPathLegacy(path.path.abs());
             if (store->isInStore(path.path.abs())) {
                 try {
                     allowClosure(store->toStorePath(path.path.abs()).first);
@@ -3178,6 +3197,11 @@ std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Pat
         if (path.resolveSymlinks().pathExists())
             return finish(std::move(path));
         else {
+            // Backward compatibility hack: throw an exception if access
+            // to this path is not allowed.
+            if (auto accessor = path.accessor.dynamic_pointer_cast<FilteringSourceAccessor>())
+                accessor->checkAccess(path.path);
+
             logWarning({.msg = HintFmt("Nix search path entry '%1%' does not exist, ignoring", value)});
         }
     }
@@ -3197,7 +3221,7 @@ Expr * EvalState::parse(
     }
 
     auto result = parseExprFromBuf(
-        text, length, origin, basePath, symbols, settings, positions, *docComments, rootFS, exprSymbols);
+        text, length, origin, basePath, mem.exprs.alloc, symbols, settings, positions, *docComments, rootFS);
 
     result->bindVars(*this, staticEnv);
 

src/libexpr/get-drvs.cc

@@ -45,8 +45,8 @@ PackageInfo::PackageInfo(EvalState & state, ref<Store> store, const std::string
 std::string PackageInfo::queryName() const
 {
     if (name == "" && attrs) {
-        auto i = attrs->find(state->sName);
-        if (i == attrs->end())
+        auto i = attrs->get(state->s.name);
+        if (!i)
             state->error<TypeError>("derivation name missing").debugThrow();
         name = state->forceStringNoCtx(*i->value, noPos, "while evaluating the 'name' attribute of a derivation");
     }
@@ -56,11 +56,10 @@ std::string PackageInfo::queryName() const
 std::string PackageInfo::querySystem() const
 {
     if (system == "" && attrs) {
-        auto i = attrs->find(state->sSystem);
+        auto i = attrs->get(state->s.system);
         system =
-            i == attrs->end()
-                ? "unknown"
-                : state->forceStringNoCtx(*i->value, i->pos, "while evaluating the 'system' attribute of a derivation");
+            !i ? "unknown"
+               : state->forceStringNoCtx(*i->value, i->pos, "while evaluating the 'system' attribute of a derivation");
     }
     return system;
 }
@@ -68,7 +67,7 @@ std::string PackageInfo::querySystem() const
 std::optional<StorePath> PackageInfo::queryDrvPath() const
 {
     if (!drvPath && attrs) {
-        if (auto i = attrs->get(state->sDrvPath)) {
+        if (auto i = attrs->get(state->s.drvPath)) {
             NixStringContext context;
             auto found = state->coerceToStorePath(
                 i->pos, *i->value, context, "while evaluating the 'drvPath' attribute of a derivation");
@@ -95,9 +94,9 @@ StorePath PackageInfo::requireDrvPath() const
 StorePath PackageInfo::queryOutPath() const
 {
     if (!outPath && attrs) {
-        auto i = attrs->find(state->sOutPath);
+        auto i = attrs->get(state->s.outPath);
         NixStringContext context;
-        if (i != attrs->end())
+        if (i)
             outPath = state->coerceToStorePath(
                 i->pos, *i->value, context, "while evaluating the output path of a derivation");
     }
@@ -111,7 +110,7 @@ PackageInfo::Outputs PackageInfo::queryOutputs(bool withPaths, bool onlyOutputsT
     if (outputs.empty()) {
         /* Get the ‘outputs’ list. */
         const Attr * i;
-        if (attrs && (i = attrs->get(state->sOutputs))) {
+        if (attrs && (i = attrs->get(state->s.outputs))) {
             state->forceList(*i->value, i->pos, "while evaluating the 'outputs' attribute of a derivation");
 
             /* For each output... */
@@ -127,7 +126,7 @@ PackageInfo::Outputs PackageInfo::queryOutputs(bool withPaths, bool onlyOutputsT
                     state->forceAttrs(*out->value, i->pos, "while evaluating an output of a derivation");
 
                     /* And evaluate its ‘outPath’ attribute. */
-                    auto outPath = out->value->attrs()->get(state->sOutPath);
+                    auto outPath = out->value->attrs()->get(state->s.outPath);
                     if (!outPath)
                         continue; // FIXME: throw error?
                     NixStringContext context;
@@ -146,7 +145,7 @@ PackageInfo::Outputs PackageInfo::queryOutputs(bool withPaths, bool onlyOutputsT
         return outputs;
 
     const Attr * i;
-    if (attrs && (i = attrs->get(state->sOutputSpecified))
+    if (attrs && (i = attrs->get(state->s.outputSpecified))
         && state->forceBool(*i->value, i->pos, "while evaluating the 'outputSpecified' attribute of a derivation")) {
         Outputs result;
         auto out = outputs.find(queryOutputName());
@@ -181,7 +180,7 @@ PackageInfo::Outputs PackageInfo::queryOutputs(bool withPaths, bool onlyOutputsT
 std::string PackageInfo::queryOutputName() const
 {
     if (outputName == "" && attrs) {
-        auto i = attrs->get(state->sOutputName);
+        auto i = attrs->get(state->s.outputName);
         outputName =
             i ? state->forceStringNoCtx(*i->value, noPos, "while evaluating the output name of a derivation") : "";
     }
@@ -194,7 +193,7 @@ const Bindings * PackageInfo::getMeta()
         return meta;
     if (!attrs)
         return 0;
-    auto a = attrs->get(state->sMeta);
+    auto a = attrs->get(state->s.meta);
     if (!a)
         return 0;
     state->forceAttrs(*a->value, a->pos, "while evaluating the 'meta' attribute of a derivation");
@@ -221,7 +220,7 @@ bool PackageInfo::checkMeta(Value & v)
                 return false;
         return true;
     } else if (v.type() == nAttrs) {
-        if (v.attrs()->get(state->sOutPath))
+        if (v.attrs()->get(state->s.outPath))
             return false;
         for (auto & i : *v.attrs())
             if (!checkMeta(*i.value))
@@ -411,7 +410,7 @@ static void getDerivations(
                     should we recurse into it?  => Only if it has a
                     `recurseForDerivations = true' attribute. */
                     if (i->value->type() == nAttrs) {
-                        auto j = i->value->attrs()->get(state.sRecurseForDerivations);
+                        auto j = i->value->attrs()->get(state.s.recurseForDerivations);
                         if (j
                             && state.forceBool(
                                 *j->value, j->pos, "while evaluating the attribute `recurseForDerivations`"))

src/libexpr/include/nix/expr/attr-set.hh

@@ -4,11 +4,17 @@
 #include "nix/expr/nixexpr.hh"
 #include "nix/expr/symbol-table.hh"
 
+#include <boost/container/static_vector.hpp>
+#include <boost/iterator/function_output_iterator.hpp>
+
 #include <algorithm>
+#include <functional>
+#include <ranges>
+#include <optional>
 
 namespace nix {
 
-class EvalState;
+class EvalMemory;
 struct Value;
 
 /**
@@ -46,141 +52,466 @@ static_assert(
  * by its size and its capacity, the capacity being the number of Attr
  * elements allocated after this structure, while the size corresponds to
  * the number of elements already inserted in this structure.
+ *
+ * Bindings can be efficiently `//`-composed into an intrusive linked list of "layers"
+ * that saves on copies and allocations. Each lookup (@see Bindings::get) traverses
+ * this linked list until a matching attribute is found (thus overlays earlier in
+ * the list take precedence). For iteration over the whole Bindings, an on-the-fly
+ * k-way merge is performed by Bindings::iterator class.
  */
 class Bindings
 {
 public:
-    typedef uint32_t size_t;
+    using size_type = uint32_t;
+
     PosIdx pos;
 
+    /**
+     * An instance of bindings objects with 0 attributes.
+     * This object must never be modified.
+     */
+    static Bindings emptyBindings;
+
 private:
-    size_t size_, capacity_;
+    /**
+     * Number of attributes in the attrs FAM (Flexible Array Member).
+     */
+    size_type numAttrs = 0;
+
+    /**
+     * Number of attributes with unique names in the layer chain.
+     *
+     * This is the *real* user-facing size of bindings, whereas @ref numAttrs is
+     * an implementation detail of the data structure.
+     */
+    size_type numAttrsInChain = 0;
+
+    /**
+     * Length of the layers list.
+     */
+    uint32_t numLayers = 1;
+
+    /**
+     * Bindings that this attrset is "layered" on top of.
+     */
+    const Bindings * baseLayer = nullptr;
+
+    /**
+     * Flexible array member of attributes.
+     */
     Attr attrs[0];
 
-    Bindings(size_t capacity)
-        : size_(0)
-        , capacity_(capacity)
-    {
-    }
+    Bindings() = default;
+    Bindings(const Bindings &) = delete;
+    Bindings(Bindings &&) = delete;
+    Bindings & operator=(const Bindings &) = delete;
+    Bindings & operator=(Bindings &&) = delete;
 
-    Bindings(const Bindings & bindings) = delete;
+    friend class BindingsBuilder;
+
+    /**
+     * Maximum length of the Bindings layer chains.
+     */
+    static constexpr unsigned maxLayers = 8;
 
 public:
-    size_t size() const
+    size_type size() const
     {
-        return size_;
+        return numAttrsInChain;
     }
 
     bool empty() const
     {
-        return !size_;
+        return size() == 0;
     }
 
-    typedef Attr * iterator;
+    class iterator
+    {
+    public:
+        using value_type = Attr;
+        using pointer = const value_type *;
+        using reference = const value_type &;
+        using difference_type = std::ptrdiff_t;
+        using iterator_category = std::forward_iterator_tag;
 
-    typedef const Attr * const_iterator;
+        friend class Bindings;
+
+    private:
+        struct BindingsCursor
+        {
+            /**
+             * Attr that the cursor currently points to.
+             */
+            pointer current;
+
+            /**
+             * One past the end pointer to the contiguous buffer of Attrs.
+             */
+            pointer end;
+
+            /**
+             * Priority of the value. Lesser values have more priority (i.e. they override
+             * attributes that appear later in the linked list of Bindings).
+             */
+            uint32_t priority;
+
+            pointer operator->() const noexcept
+            {
+                return current;
+            }
+
+            reference get() const noexcept
+            {
+                return *current;
+            }
+
+            bool empty() const noexcept
+            {
+                return current == end;
+            }
+
+            void increment() noexcept
+            {
+                ++current;
+            }
+
+            void consume(Symbol name) noexcept
+            {
+                while (!empty() && current->name <= name)
+                    ++current;
+            }
+
+            GENERATE_CMP(BindingsCursor, me->current->name, me->priority)
+        };
+
+        using QueueStorageType = boost::container::static_vector<BindingsCursor, maxLayers>;
+
+        /**
+         * Comparator implementing the override priority / name ordering
+         * for BindingsCursor.
+         */
+        static constexpr auto comp = std::greater<BindingsCursor>();
+
+        /**
+         * A priority queue used to implement an on-the-fly k-way merge.
+         */
+        QueueStorageType cursorHeap;
+
+        /**
+         * The attribute the iterator currently points to.
+         */
+        pointer current = nullptr;
+
+        /**
+         * Whether iterating over a single attribute and not a merge chain.
+         */
+        bool doMerge = true;
+
+        void push(BindingsCursor cursor) noexcept
+        {
+            cursorHeap.push_back(cursor);
+            std::ranges::make_heap(cursorHeap, comp);
+        }
+
+        [[nodiscard]] BindingsCursor pop() noexcept
+        {
+            std::ranges::pop_heap(cursorHeap, comp);
+            auto cursor = cursorHeap.back();
+            cursorHeap.pop_back();
+            return cursor;
+        }
+
+        iterator & finished() noexcept
+        {
+            current = nullptr;
+            return *this;
+        }
+
+        void next(BindingsCursor cursor) noexcept
+        {
+            current = &cursor.get();
+            cursor.increment();
+
+            if (!cursor.empty())
+                push(cursor);
+        }
+
+        std::optional<BindingsCursor> consumeAllUntilCurrentName() noexcept
+        {
+            auto cursor = pop();
+            Symbol lastHandledName = current->name;
+
+            while (cursor->name <= lastHandledName) {
+                cursor.consume(lastHandledName);
+                if (!cursor.empty())
+                    push(cursor);
+
+                if (cursorHeap.empty())
+                    return std::nullopt;
+
+                cursor = pop();
+            }
+
+            return cursor;
+        }
+
+        explicit iterator(const Bindings & attrs) noexcept
+            : doMerge(attrs.baseLayer)
+        {
+            auto pushBindings = [this, priority = unsigned{0}](const Bindings & layer) mutable {
+                auto first = layer.attrs;
+                push(
+                    BindingsCursor{
+                        .current = first,
+                        .end = first + layer.numAttrs,
+                        .priority = priority++,
+                    });
+            };
+
+            if (!doMerge) {
+                if (attrs.empty())
+                    return;
+
+                current = attrs.attrs;
+                pushBindings(attrs);
+
+                return;
+            }
+
+            const Bindings * layer = &attrs;
+            while (layer) {
+                if (layer->numAttrs != 0)
+                    pushBindings(*layer);
+                layer = layer->baseLayer;
+            }
+
+            if (cursorHeap.empty())
+                return;
+
+            next(pop());
+        }
+
+    public:
+        iterator() = default;
+
+        reference operator*() const noexcept
+        {
+            return *current;
+        }
+
+        pointer operator->() const noexcept
+        {
+            return current;
+        }
+
+        iterator & operator++() noexcept
+        {
+            if (!doMerge) {
+                ++current;
+                if (current == cursorHeap.front().end)
+                    return finished();
+                return *this;
+            }
+
+            if (cursorHeap.empty())
+                return finished();
+
+            auto cursor = consumeAllUntilCurrentName();
+            if (!cursor)
+                return finished();
+
+            next(*cursor);
+            return *this;
+        }
+
+        iterator operator++(int) noexcept
+        {
+            iterator tmp = *this;
+            ++*this;
+            return tmp;
+        }
+
+        bool operator==(const iterator & rhs) const noexcept
+        {
+            return current == rhs.current;
+        }
+    };
+
+    using const_iterator = iterator;
 
     void push_back(const Attr & attr)
     {
-        assert(size_ < capacity_);
-        attrs[size_++] = attr;
+        attrs[numAttrs++] = attr;
+        numAttrsInChain = numAttrs;
     }
 
-    const_iterator find(Symbol name) const
+    /**
+     * Get attribute by name or nullptr if no such attribute exists.
+     */
+    const Attr * get(Symbol name) const noexcept
     {
-        Attr key(name, 0);
-        const_iterator i = std::lower_bound(begin(), end(), key);
-        if (i != end() && i->name == name)
-            return i;
-        return end();
-    }
+        auto getInChunk = [key = Attr{name, nullptr}](const Bindings & chunk) -> const Attr * {
+            auto first = chunk.attrs;
+            auto last = first + chunk.numAttrs;
+            const Attr * i = std::lower_bound(first, last, key);
+            if (i != last && i->name == key.name)
+                return i;
+            return nullptr;
+        };
+
+        const Bindings * currentChunk = this;
+        while (currentChunk) {
+            const Attr * maybeAttr = getInChunk(*currentChunk);
+            if (maybeAttr)
+                return maybeAttr;
+            currentChunk = currentChunk->baseLayer;
+        }
 
-    const Attr * get(Symbol name) const
-    {
-        Attr key(name, 0);
-        const_iterator i = std::lower_bound(begin(), end(), key);
-        if (i != end() && i->name == name)
-            return &*i;
         return nullptr;
     }
 
-    iterator begin()
+    /**
+     * Check if the layer chain is full.
+     */
+    bool isLayerListFull() const noexcept
     {
-        return &attrs[0];
+        return numLayers == Bindings::maxLayers;
     }
 
-    iterator end()
+    /**
+     * Test if the length of the linked list of layers is greater than 1.
+     */
+    bool isLayered() const noexcept
     {
-        return &attrs[size_];
+        return numLayers > 1;
     }
 
     const_iterator begin() const
     {
-        return &attrs[0];
+        return const_iterator(*this);
     }
 
     const_iterator end() const
     {
-        return &attrs[size_];
+        return const_iterator();
     }
 
-    Attr & operator[](size_t pos)
+    Attr & operator[](size_type pos)
     {
+        if (isLayered()) [[unlikely]]
+            unreachable();
         return attrs[pos];
     }
 
-    const Attr & operator[](size_t pos) const
+    const Attr & operator[](size_type pos) const
     {
+        if (isLayered()) [[unlikely]]
+            unreachable();
         return attrs[pos];
     }
 
     void sort();
 
-    size_t capacity() const
-    {
-        return capacity_;
-    }
-
     /**
      * Returns the attributes in lexicographically sorted order.
      */
     std::vector<const Attr *> lexicographicOrder(const SymbolTable & symbols) const
     {
         std::vector<const Attr *> res;
-        res.reserve(size_);
-        for (size_t n = 0; n < size_; n++)
-            res.emplace_back(&attrs[n]);
-        std::sort(res.begin(), res.end(), [&](const Attr * a, const Attr * b) {
+        res.reserve(size());
+        std::ranges::transform(*this, std::back_inserter(res), [](const Attr & a) { return &a; });
+        std::ranges::sort(res, [&](const Attr * a, const Attr * b) {
             std::string_view sa = symbols[a->name], sb = symbols[b->name];
             return sa < sb;
         });
         return res;
     }
 
-    friend class EvalState;
+    friend class EvalMemory;
 };
 
+static_assert(std::forward_iterator<Bindings::iterator>);
+static_assert(std::ranges::forward_range<Bindings>);
+
 /**
  * A wrapper around Bindings that ensures that its always in sorted
  * order at the end. The only way to consume a BindingsBuilder is to
  * call finish(), which sorts the bindings.
  */
-class BindingsBuilder
+class BindingsBuilder final
 {
-    Bindings * bindings;
-
 public:
     // needed by std::back_inserter
     using value_type = Attr;
+    using size_type = Bindings::size_type;
 
-    EvalState & state;
+private:
+    Bindings * bindings;
+    Bindings::size_type capacity_;
 
-    BindingsBuilder(EvalState & state, Bindings * bindings)
+    friend class EvalMemory;
+
+    BindingsBuilder(EvalMemory & mem, SymbolTable & symbols, Bindings * bindings, size_type capacity)
         : bindings(bindings)
-        , state(state)
+        , capacity_(capacity)
+        , mem(mem)
+        , symbols(symbols)
     {
     }
 
+    bool hasBaseLayer() const noexcept
+    {
+        return bindings->baseLayer;
+    }
+
+    /**
+     * If the bindings gets "layered" on top of another we need to recalculate
+     * the number of unique attributes in the chain.
+     *
+     * This is done by either iterating over the base "layer" and the newly added
+     * attributes and counting duplicates. If the base "layer" is big this approach
+     * is inefficient and we fall back to doing per-element binary search in the base
+     * "layer".
+     */
+    void finishSizeIfNecessary()
+    {
+        if (!hasBaseLayer())
+            return;
+
+        auto & base = *bindings->baseLayer;
+        auto attrs = std::span(bindings->attrs, bindings->numAttrs);
+
+        Bindings::size_type duplicates = 0;
+
+        /* If the base bindings is smaller than the newly added attributes
+           iterate using std::set_intersection to run in O(|base| + |attrs|) =
+           O(|attrs|). Otherwise use an O(|attrs| * log(|base|)) per-attr binary
+           search to check for duplicates. Note that if we are in this code path then
+           |attrs| <= bindingsUpdateLayerRhsSizeThreshold, which 16 by default. We are
+           optimizing for the case when a small attribute set gets "layered" on top of
+           a much larger one. When attrsets are already small it's fine to do a linear
+           scan, but we should avoid expensive iterations over large "base" attrsets. */
+        if (attrs.size() > base.size()) {
+            std::set_intersection(
+                base.begin(),
+                base.end(),
+                attrs.begin(),
+                attrs.end(),
+                boost::make_function_output_iterator([&]([[maybe_unused]] auto && _) { ++duplicates; }));
+        } else {
+            for (const auto & attr : attrs) {
+                if (base.get(attr.name))
+                    ++duplicates;
+            }
+        }
+
+        bindings->numAttrsInChain = base.numAttrsInChain + attrs.size() - duplicates;
+    }
+
+public:
+    std::reference_wrapper<EvalMemory> mem;
+    std::reference_wrapper<SymbolTable> symbols;
+
     void insert(Symbol name, Value * value, PosIdx pos = noPos)
     {
         insert(Attr(name, value, pos));
@@ -193,9 +524,26 @@ public:
 
     void push_back(const Attr & attr)
     {
+        assert(bindings->numAttrs < capacity_);
         bindings->push_back(attr);
     }
 
+    /**
+     * "Layer" the newly constructured Bindings on top of another attribute set.
+     *
+     * This effectively performs an attribute set merge, while giving preference
+     * to attributes from the newly constructed Bindings in case of duplicate attribute
+     * names.
+     *
+     * This operation amortizes the need to copy over all attributes and allows
+     * for efficient implementation of attribute set merges (ExprOpUpdate::eval).
+     */
+    void layerOnTopOf(const Bindings & base) noexcept
+    {
+        bindings->baseLayer = &base;
+        bindings->numLayers = base.numLayers + 1;
+    }
+
     Value & alloc(Symbol name, PosIdx pos = noPos);
 
     Value & alloc(std::string_view name, PosIdx pos = noPos);
@@ -203,24 +551,26 @@ public:
     Bindings * finish()
     {
         bindings->sort();
+        finishSizeIfNecessary();
         return bindings;
     }
 
     Bindings * alreadySorted()
     {
+        finishSizeIfNecessary();
         return bindings;
     }
 
-    size_t capacity()
+    size_t capacity() const noexcept
     {
-        return bindings->capacity();
+        return capacity_;
     }
 
-    void grow(Bindings * newBindings)
+    void grow(BindingsBuilder newBindings)
     {
         for (auto & i : *bindings)
-            newBindings->push_back(i);
-        bindings = newBindings;
+            newBindings.push_back(i);
+        std::swap(*this, newBindings);
     }
 
     friend struct ExprAttrs;

src/libexpr/include/nix/expr/counter.hh

@@ -0,0 +1,70 @@
+#pragma once
+
+#include <atomic>
+#include <cstdint>
+
+namespace nix {
+
+/**
+ * An atomic counter aligned on a cache line to prevent false sharing.
+ * The counter is only enabled when the `NIX_SHOW_STATS` environment
+ * variable is set. This is to prevent contention on these counters
+ * when multi-threaded evaluation is enabled.
+ */
+struct alignas(64) Counter
+{
+    using value_type = uint64_t;
+
+    std::atomic<value_type> inner{0};
+
+    static bool enabled;
+
+    Counter() {}
+
+    operator value_type() const noexcept
+    {
+        return inner;
+    }
+
+    void operator=(value_type n) noexcept
+    {
+        inner = n;
+    }
+
+    value_type load() const noexcept
+    {
+        return inner;
+    }
+
+    value_type operator++() noexcept
+    {
+        return enabled ? ++inner : 0;
+    }
+
+    value_type operator++(int) noexcept
+    {
+        return enabled ? inner++ : 0;
+    }
+
+    value_type operator--() noexcept
+    {
+        return enabled ? --inner : 0;
+    }
+
+    value_type operator--(int) noexcept
+    {
+        return enabled ? inner-- : 0;
+    }
+
+    value_type operator+=(value_type n) noexcept
+    {
+        return enabled ? inner += n : 0;
+    }
+
+    value_type operator-=(value_type n) noexcept
+    {
+        return enabled ? inner -= n : 0;
+    }
+};
+
+} // namespace nix

src/libexpr/include/nix/expr/eval-inline.hh

@@ -26,7 +26,7 @@ inline void * allocBytes(size_t n)
 }
 
 [[gnu::always_inline]]
-Value * EvalState::allocValue()
+Value * EvalMemory::allocValue()
 {
 #if NIX_USE_BOEHMGC
     /* We use the boehm batch allocator to speed up allocations of Values (of which there are many).
@@ -48,15 +48,15 @@ Value * EvalState::allocValue()
     void * p = allocBytes(sizeof(Value));
 #endif
 
-    nrValues++;
+    stats.nrValues++;
     return (Value *) p;
 }
 
 [[gnu::always_inline]]
-Env & EvalState::allocEnv(size_t size)
+Env & EvalMemory::allocEnv(size_t size)
 {
-    nrEnvs++;
-    nrValuesInEnvs += size;
+    stats.nrEnvs++;
+    stats.nrValuesInEnvs += size;
 
     Env * env;
 

src/libexpr/include/nix/expr/eval-settings.hh

@@ -342,6 +342,25 @@ struct EvalSettings : Config
           This is useful for improving code readability and making path literals
           more explicit.
     )"};
+
+    Setting<unsigned> bindingsUpdateLayerRhsSizeThreshold{
+        this,
+        sizeof(void *) == 4 ? 8192 : 16,
+        "eval-attrset-update-layer-rhs-threshold",
+        R"(
+          Tunes the maximum size of an attribute set that, when used
+          as a right operand in an [attribute set update expression](@docroot@/language/operators.md#update),
+          uses a more space-efficient linked-list representation of attribute sets.
+
+          Setting this to larger values generally leads to less memory allocations,
+          but may lead to worse evaluation performance.
+
+          A value of `0` disables this optimization completely.
+
+          This is an advanced performance tuning option and typically should not be changed.
+          The default value is chosen to balance performance and memory usage. On 32 bit systems
+          where memory is scarce, the default is a large value to reduce the amount of allocations.
+    )"};
 };
 
 /**

src/libexpr/include/nix/expr/eval.hh

@@ -16,10 +16,14 @@
 #include "nix/expr/search-path.hh"
 #include "nix/expr/repl-exit-status.hh"
 #include "nix/util/ref.hh"
+#include "nix/expr/counter.hh"
 
 // For `NIX_USE_BOEHMGC`, and if that's set, `GC_THREADS`
 #include "nix/expr/config.hh"
 
+#include <boost/unordered/unordered_flat_map.hpp>
+#include <boost/unordered/concurrent_flat_map_fwd.hpp>
+
 #include <map>
 #include <optional>
 #include <functional>
@@ -38,6 +42,7 @@ class Store;
 namespace fetchers {
 struct Settings;
 struct InputCache;
+struct Input;
 } // namespace fetchers
 struct EvalSettings;
 class EvalState;
@@ -45,6 +50,7 @@ class StorePath;
 struct SingleDerivedPath;
 enum RepairFlag : bool;
 struct MemorySourceAccessor;
+struct MountedSourceAccessor;
 
 namespace eval_cache {
 class EvalCache;
@@ -162,7 +168,7 @@ typedef std::
     map<std::string, Value *, std::less<std::string>, traceable_allocator<std::pair<const std::string, Value *>>>
         ValMap;
 
-typedef std::unordered_map<PosIdx, DocComment> DocCommentMap;
+typedef boost::unordered_flat_map<PosIdx, DocComment, std::hash<PosIdx>> DocCommentMap;
 
 struct Env
 {
@@ -213,22 +219,163 @@ struct DebugTrace
     }
 };
 
+struct StaticEvalSymbols
+{
+    Symbol with, outPath, drvPath, type, meta, name, value, system, overrides, outputs, outputName, ignoreNulls, file,
+        line, column, functor, toString, right, wrong, structuredAttrs, json, allowedReferences, allowedRequisites,
+        disallowedReferences, disallowedRequisites, maxSize, maxClosureSize, builder, args, contentAddressed, impure,
+        outputHash, outputHashAlgo, outputHashMode, recurseForDerivations, description, self, epsilon, startSet,
+        operator_, key, path, prefix, outputSpecified;
+
+    Expr::AstSymbols exprSymbols;
+
+    static constexpr auto preallocate()
+    {
+        StaticSymbolTable alloc;
+
+        StaticEvalSymbols staticSymbols = {
+            .with = alloc.create("<with>"),
+            .outPath = alloc.create("outPath"),
+            .drvPath = alloc.create("drvPath"),
+            .type = alloc.create("type"),
+            .meta = alloc.create("meta"),
+            .name = alloc.create("name"),
+            .value = alloc.create("value"),
+            .system = alloc.create("system"),
+            .overrides = alloc.create("__overrides"),
+            .outputs = alloc.create("outputs"),
+            .outputName = alloc.create("outputName"),
+            .ignoreNulls = alloc.create("__ignoreNulls"),
+            .file = alloc.create("file"),
+            .line = alloc.create("line"),
+            .column = alloc.create("column"),
+            .functor = alloc.create("__functor"),
+            .toString = alloc.create("__toString"),
+            .right = alloc.create("right"),
+            .wrong = alloc.create("wrong"),
+            .structuredAttrs = alloc.create("__structuredAttrs"),
+            .json = alloc.create("__json"),
+            .allowedReferences = alloc.create("allowedReferences"),
+            .allowedRequisites = alloc.create("allowedRequisites"),
+            .disallowedReferences = alloc.create("disallowedReferences"),
+            .disallowedRequisites = alloc.create("disallowedRequisites"),
+            .maxSize = alloc.create("maxSize"),
+            .maxClosureSize = alloc.create("maxClosureSize"),
+            .builder = alloc.create("builder"),
+            .args = alloc.create("args"),
+            .contentAddressed = alloc.create("__contentAddressed"),
+            .impure = alloc.create("__impure"),
+            .outputHash = alloc.create("outputHash"),
+            .outputHashAlgo = alloc.create("outputHashAlgo"),
+            .outputHashMode = alloc.create("outputHashMode"),
+            .recurseForDerivations = alloc.create("recurseForDerivations"),
+            .description = alloc.create("description"),
+            .self = alloc.create("self"),
+            .epsilon = alloc.create(""),
+            .startSet = alloc.create("startSet"),
+            .operator_ = alloc.create("operator"),
+            .key = alloc.create("key"),
+            .path = alloc.create("path"),
+            .prefix = alloc.create("prefix"),
+            .outputSpecified = alloc.create("outputSpecified"),
+            .exprSymbols = {
+                .sub = alloc.create("__sub"),
+                .lessThan = alloc.create("__lessThan"),
+                .mul = alloc.create("__mul"),
+                .div = alloc.create("__div"),
+                .or_ = alloc.create("or"),
+                .findFile = alloc.create("__findFile"),
+                .nixPath = alloc.create("__nixPath"),
+                .body = alloc.create("body"),
+            }};
+
+        return std::pair{staticSymbols, alloc};
+    }
+
+    static consteval StaticEvalSymbols create()
+    {
+        return preallocate().first;
+    }
+
+    static constexpr StaticSymbolTable staticSymbolTable()
+    {
+        return preallocate().second;
+    }
+};
+
+class EvalMemory
+{
+#if NIX_USE_BOEHMGC
+    /**
+     * Allocation cache for GC'd Value objects.
+     */
+    std::shared_ptr<void *> valueAllocCache;
+
+    /**
+     * Allocation cache for size-1 Env objects.
+     */
+    std::shared_ptr<void *> env1AllocCache;
+#endif
+
+public:
+    struct Statistics
+    {
+        Counter nrEnvs;
+        Counter nrValuesInEnvs;
+        Counter nrValues;
+        Counter nrAttrsets;
+        Counter nrAttrsInAttrsets;
+        Counter nrListElems;
+    };
+
+    EvalMemory();
+
+    EvalMemory(const EvalMemory &) = delete;
+    EvalMemory(EvalMemory &&) = delete;
+    EvalMemory & operator=(const EvalMemory &) = delete;
+    EvalMemory & operator=(EvalMemory &&) = delete;
+
+    inline Value * allocValue();
+    inline Env & allocEnv(size_t size);
+
+    Bindings * allocBindings(size_t capacity);
+
+    BindingsBuilder buildBindings(SymbolTable & symbols, size_t capacity)
+    {
+        return BindingsBuilder(*this, symbols, allocBindings(capacity), capacity);
+    }
+
+    ListBuilder buildList(size_t size)
+    {
+        stats.nrListElems += size;
+        return ListBuilder(size);
+    }
+
+    const Statistics & getStats() const &
+    {
+        return stats;
+    }
+
+    /**
+     * Storage for the AST nodes
+     */
+    Exprs exprs;
+
+private:
+    Statistics stats;
+};
+
 class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
+    static constexpr StaticEvalSymbols s = StaticEvalSymbols::create();
+
     const fetchers::Settings & fetchSettings;
     const EvalSettings & settings;
     SymbolTable symbols;
     PosTable positions;
 
-    const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue, sSystem, sOverrides, sOutputs, sOutputName,
-        sIgnoreNulls, sFile, sLine, sColumn, sFunctor, sToString, sRight, sWrong, sStructuredAttrs, sJson,
-        sAllowedReferences, sAllowedRequisites, sDisallowedReferences, sDisallowedRequisites, sMaxSize, sMaxClosureSize,
-        sBuilder, sArgs, sContentAddressed, sImpure, sOutputHash, sOutputHashAlgo, sOutputHashMode,
-        sRecurseForDerivations, sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath, sPrefix,
-        sOutputSpecified;
-
-    const Expr::AstSymbols exprSymbols;
+    EvalMemory mem;
 
     /**
      * If set, force copying files to the Nix store even if they
@@ -236,47 +383,10 @@ public:
      */
     RepairFlag repair;
 
-    Bindings emptyBindings;
-
-    /**
-     * Empty list constant.
-     */
-    Value vEmptyList;
-
-    /**
-     * `null` constant.
-     *
-     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
-     */
-    Value vNull;
-
-    /**
-     * `true` constant.
-     *
-     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
-     */
-    Value vTrue;
-
-    /**
-     * `true` constant.
-     *
-     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
-     */
-    Value vFalse;
-
-    /** `"regular"` */
-    Value vStringRegular;
-    /** `"directory"` */
-    Value vStringDirectory;
-    /** `"symlink"` */
-    Value vStringSymlink;
-    /** `"unknown"` */
-    Value vStringUnknown;
-
     /**
      * The accessor corresponding to `store`.
      */
-    const ref<SourceAccessor> storeFS;
+    const ref<MountedSourceAccessor> storeFS;
 
     /**
      * The accessor for the root filesystem.
@@ -318,7 +428,7 @@ public:
     bool inDebugger = false;
     int trylevel;
     std::list<DebugTrace> debugTraces;
-    std::map<const Expr *, const std::shared_ptr<const StaticEnv>> exprEnvs;
+    boost::unordered_flat_map<const Expr *, const std::shared_ptr<const StaticEnv>> exprEnvs;
 
     const std::shared_ptr<const StaticEnv> getStaticEnv(const Expr & expr) const
     {
@@ -361,59 +471,41 @@ private:
 
     /* Cache for calls to addToStore(); maps source paths to the store
        paths. */
-    Sync<std::unordered_map<SourcePath, StorePath>> srcToStore;
+    ref<boost::concurrent_flat_map<SourcePath, StorePath>> srcToStore;
 
     /**
-     * A cache from path names to parse trees.
+     * A cache that maps paths to "resolved" paths for importing Nix
+     * expressions, i.e. `/foo` to `/foo/default.nix`.
      */
-    typedef std::unordered_map<
-        SourcePath,
-        Expr *,
-        std::hash<SourcePath>,
-        std::equal_to<SourcePath>,
-        traceable_allocator<std::pair<const SourcePath, Expr *>>>
-        FileParseCache;
-    FileParseCache fileParseCache;
+    ref<boost::concurrent_flat_map<SourcePath, SourcePath>> importResolutionCache;
 
     /**
-     * A cache from path names to values.
+     * A cache from resolved paths to values.
      */
-    typedef std::unordered_map<
+    ref<boost::concurrent_flat_map<
         SourcePath,
-        Value,
+        Value *,
         std::hash<SourcePath>,
         std::equal_to<SourcePath>,
-        traceable_allocator<std::pair<const SourcePath, Value>>>
-        FileEvalCache;
-    FileEvalCache fileEvalCache;
+        traceable_allocator<std::pair<const SourcePath, Value *>>>>
+        fileEvalCache;
 
     /**
      * Associate source positions of certain AST nodes with their preceding doc comment, if they have one.
      * Grouped by file.
      */
-    std::unordered_map<SourcePath, DocCommentMap> positionToDocComment;
+    boost::unordered_flat_map<SourcePath, DocCommentMap> positionToDocComment;
 
     LookupPath lookupPath;
 
-    std::map<std::string, std::optional<SourcePath>> lookupPathResolved;
+    boost::unordered_flat_map<std::string, std::optional<SourcePath>, StringViewHash, std::equal_to<>>
+        lookupPathResolved;
 
     /**
      * Cache used by prim_match().
      */
     std::shared_ptr<RegexCache> regexCache;
 
-#if NIX_USE_BOEHMGC
-    /**
-     * Allocation cache for GC'd Value objects.
-     */
-    std::shared_ptr<void *> valueAllocCache;
-
-    /**
-     * Allocation cache for size-1 Env objects.
-     */
-    std::shared_ptr<void *> env1AllocCache;
-#endif
-
 public:
 
     EvalState(
@@ -424,6 +516,15 @@ public:
         std::shared_ptr<Store> buildStore = nullptr);
     ~EvalState();
 
+    /**
+     * A wrapper around EvalMemory::allocValue() to avoid code churn when it
+     * was introduced.
+     */
+    inline Value * allocValue()
+    {
+        return mem.allocValue();
+    }
+
     LookupPath getLookupPath()
     {
         return lookupPath;
@@ -451,8 +552,11 @@ public:
 
     /**
      * Allow access to a path.
+     *
+     * Only for restrict eval: pure eval just whitelist store paths,
+     * never arbitrary paths.
      */
-    void allowPath(const Path & path);
+    void allowPathLegacy(const Path & path);
 
     /**
      * Allow access to a store path. Note that this gets remapped to
@@ -472,6 +576,11 @@ public:
 
     void checkURI(const std::string & uri);
 
+    /**
+     * Mount an input on the Nix store.
+     */
+    StorePath mountInput(fetchers::Input & input, const fetchers::Input & originalInput, ref<SourceAccessor> accessor);
+
     /**
      * Parse a Nix expression from the specified file.
      */
@@ -570,7 +679,7 @@ public:
     /**
      * Get attribute from an attribute set and throw an error if it doesn't exist.
      */
-    Bindings::const_iterator getAttr(Symbol attrSym, const Bindings * attrSet, std::string_view errorCtx);
+    const Attr * getAttr(Symbol attrSym, const Bindings * attrSet, std::string_view errorCtx);
 
     template<typename... Args>
     [[gnu::noinline]]
@@ -669,11 +778,11 @@ public:
     /**
      * Internal primops not exposed to the user.
      */
-    std::unordered_map<
+    boost::unordered_flat_map<
         std::string,
         Value *,
-        std::hash<std::string>,
-        std::equal_to<std::string>,
+        StringViewHash,
+        std::equal_to<>,
         traceable_allocator<std::pair<const std::string, Value *>>>
         internalPrimOps;
 
@@ -792,22 +901,14 @@ public:
      */
     void autoCallFunction(const Bindings & args, Value & fun, Value & res);
 
-    /**
-     * Allocation primitives.
-     */
-    inline Value * allocValue();
-    inline Env & allocEnv(size_t size);
-
-    Bindings * allocBindings(size_t capacity);
-
     BindingsBuilder buildBindings(size_t capacity)
     {
-        return BindingsBuilder(*this, allocBindings(capacity));
+        return mem.buildBindings(symbols, capacity);
     }
 
     ListBuilder buildList(size_t size)
     {
-        return ListBuilder(*this, size);
+        return mem.buildList(size);
     }
 
     /**
@@ -924,26 +1025,20 @@ private:
      */
     std::string mkSingleDerivedPathStringRaw(const SingleDerivedPath & p);
 
-    unsigned long nrEnvs = 0;
-    unsigned long nrValuesInEnvs = 0;
-    unsigned long nrValues = 0;
-    unsigned long nrListElems = 0;
-    unsigned long nrLookups = 0;
-    unsigned long nrAttrsets = 0;
-    unsigned long nrAttrsInAttrsets = 0;
-    unsigned long nrAvoided = 0;
-    unsigned long nrOpUpdates = 0;
-    unsigned long nrOpUpdateValuesCopied = 0;
-    unsigned long nrListConcats = 0;
-    unsigned long nrPrimOpCalls = 0;
-    unsigned long nrFunctionCalls = 0;
+    Counter nrLookups;
+    Counter nrAvoided;
+    Counter nrOpUpdates;
+    Counter nrOpUpdateValuesCopied;
+    Counter nrListConcats;
+    Counter nrPrimOpCalls;
+    Counter nrFunctionCalls;
 
     bool countCalls;
 
-    typedef std::map<std::string, size_t> PrimOpCalls;
+    typedef boost::unordered_flat_map<std::string, size_t, StringViewHash, std::equal_to<>> PrimOpCalls;
     PrimOpCalls primOpCalls;
 
-    typedef std::map<ExprLambda *, size_t> FunctionCalls;
+    typedef boost::unordered_flat_map<ExprLambda *, size_t> FunctionCalls;
     FunctionCalls functionCalls;
 
     /** Evaluation/call profiler. */
@@ -951,7 +1046,7 @@ private:
 
     void incrFunctionCall(ExprLambda * fun);
 
-    typedef std::map<PosIdx, size_t> AttrSelects;
+    typedef boost::unordered_flat_map<PosIdx, size_t, std::hash<PosIdx>> AttrSelects;
     AttrSelects attrSelects;
 
     friend struct ExprOpUpdate;

src/libexpr/include/nix/expr/gc-small-vector.hh

@@ -26,4 +26,20 @@ using SmallValueVector = SmallVector<Value *, nItems>;
 template<size_t nItems>
 using SmallTemporaryValueVector = SmallVector<Value, nItems>;
 
+/**
+ * For functions where we do not expect deep recursion, we can use a sizable
+ * part of the stack a free allocation space.
+ *
+ * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
+ */
+constexpr size_t nonRecursiveStackReservation = 128;
+
+/**
+ * Functions that maybe applied to self-similar inputs, such as concatMap on a
+ * tree, should reserve a smaller part of the stack for allocation.
+ *
+ * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
+ */
+constexpr size_t conservativeStackReservation = 16;
+
 } // namespace nix

src/libexpr/include/nix/expr/meson.build

@@ -10,6 +10,7 @@ config_pub_h = configure_file(
 headers = [ config_pub_h ] + files(
   'attr-path.hh',
   'attr-set.hh',
+  'counter.hh',
   'eval-cache.hh',
   'eval-error.hh',
   'eval-gc.hh',

src/libexpr/include/nix/expr/nixexpr.hh

@@ -2,12 +2,17 @@
 ///@file
 
 #include <map>
+#include <span>
 #include <vector>
+#include <memory_resource>
+#include <algorithm>
 
+#include "nix/expr/gc-small-vector.hh"
 #include "nix/expr/value.hh"
 #include "nix/expr/symbol-table.hh"
 #include "nix/expr/eval-error.hh"
 #include "nix/util/pos-idx.hh"
+#include "nix/expr/counter.hh"
 
 namespace nix {
 
@@ -76,9 +81,20 @@ struct AttrName
         : expr(e) {};
 };
 
+static_assert(std::is_trivially_copy_constructible_v<AttrName>);
+
 typedef std::vector<AttrName> AttrPath;
 
-std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath);
+std::string showAttrPath(const SymbolTable & symbols, std::span<const AttrName> attrPath);
+
+using UpdateQueue = SmallTemporaryValueVector<conservativeStackReservation>;
+
+class Exprs
+{
+    std::pmr::monotonic_buffer_resource buffer;
+public:
+    std::pmr::polymorphic_allocator<char> alloc{&buffer};
+};
 
 /* Abstract syntax of Nix expressions. */
 
@@ -89,7 +105,7 @@ struct Expr
         Symbol sub, lessThan, mul, div, or_, findFile, nixPath, body;
     };
 
-    static unsigned long nrExprs;
+    static Counter nrExprs;
 
     Expr()
     {
@@ -99,8 +115,25 @@ struct Expr
     virtual ~Expr() {};
     virtual void show(const SymbolTable & symbols, std::ostream & str) const;
     virtual void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
+
+    /** Normal evaluation, implemented directly by all subclasses. */
     virtual void eval(EvalState & state, Env & env, Value & v);
+
+    /**
+     * Create a thunk for the delayed computation of the given expression
+     * in the given environment. But if the expression is a variable,
+     * then look it up right away. This significantly reduces the number
+     * of thunks allocated.
+     */
     virtual Value * maybeThunk(EvalState & state, Env & env);
+
+    /**
+     * Only called when performing an attrset update: `//` or similar.
+     * Instead of writing to a Value &, this function writes to an UpdateQueue.
+     * This allows the expression to perform multiple updates in a delayed manner, gathering up all the updates before
+     * applying them.
+     */
+    virtual void evalForUpdate(EvalState & state, Env & env, UpdateQueue & q, std::string_view errorCtx);
     virtual void setName(Symbol name);
     virtual void setDocComment(DocComment docComment) {};
 
@@ -158,7 +191,7 @@ struct ExprString : Expr
     ExprString(std::string && s)
         : s(std::move(s))
     {
-        v.mkString(this->s.data());
+        v.mkStringNoCopy(this->s.data());
     };
 
     Value * maybeThunk(EvalState & state, Env & env) override;
@@ -168,14 +201,16 @@ struct ExprString : Expr
 struct ExprPath : Expr
 {
     ref<SourceAccessor> accessor;
-    std::string s;
     Value v;
 
-    ExprPath(ref<SourceAccessor> accessor, std::string s)
+    ExprPath(std::pmr::polymorphic_allocator<char> & alloc, ref<SourceAccessor> accessor, std::string_view sv)
         : accessor(accessor)
-        , s(std::move(s))
     {
-        v.mkPath(&*accessor, this->s.c_str());
+        auto len = sv.length();
+        char * s = alloc.allocate(len + 1);
+        sv.copy(s, len);
+        s[len] = '\0';
+        v.mkPath(&*accessor, s);
     }
 
     Value * maybeThunk(EvalState & state, Env & env) override;
@@ -242,20 +277,33 @@ struct ExprInheritFrom : ExprVar
 struct ExprSelect : Expr
 {
     PosIdx pos;
+    uint32_t nAttrPath;
     Expr *e, *def;
-    AttrPath attrPath;
-    ExprSelect(const PosIdx & pos, Expr * e, AttrPath attrPath, Expr * def)
+    AttrName * attrPathStart;
+
+    ExprSelect(
+        std::pmr::polymorphic_allocator<char> & alloc,
+        const PosIdx & pos,
+        Expr * e,
+        std::span<const AttrName> attrPath,
+        Expr * def)
         : pos(pos)
+        , nAttrPath(attrPath.size())
         , e(e)
         , def(def)
-        , attrPath(std::move(attrPath)) {};
+        , attrPathStart(alloc.allocate_object<AttrName>(nAttrPath))
+    {
+        std::ranges::copy(attrPath, attrPathStart);
+    };
 
-    ExprSelect(const PosIdx & pos, Expr * e, Symbol name)
+    ExprSelect(std::pmr::polymorphic_allocator<char> & alloc, const PosIdx & pos, Expr * e, Symbol name)
         : pos(pos)
+        , nAttrPath(1)
         , e(e)
         , def(0)
+        , attrPathStart((alloc.allocate_object<AttrName>()))
     {
-        attrPath.push_back(AttrName(name));
+        *attrPathStart = AttrName(name);
     };
 
     PosIdx getPos() const override
@@ -263,6 +311,11 @@ struct ExprSelect : Expr
         return pos;
     }
 
+    std::span<const AttrName> getAttrPath() const
+    {
+        return {attrPathStart, nAttrPath};
+    }
+
     /**
      * Evaluate the `a.b.c` part of `a.b.c.d`. This exists mostly for the purpose of :doc in the repl.
      *
@@ -280,10 +333,14 @@ struct ExprSelect : Expr
 struct ExprOpHasAttr : Expr
 {
     Expr * e;
-    AttrPath attrPath;
-    ExprOpHasAttr(Expr * e, AttrPath attrPath)
+    std::span<AttrName> attrPath;
+
+    ExprOpHasAttr(std::pmr::polymorphic_allocator<char> & alloc, Expr * e, std::vector<AttrName> attrPath)
         : e(e)
-        , attrPath(std::move(attrPath)) {};
+        , attrPath({alloc.allocate_object<AttrName>(attrPath.size()), attrPath.size()})
+    {
+        std::ranges::copy(attrPath, this->attrPath.begin());
+    };
 
     PosIdx getPos() const override
     {
@@ -565,42 +622,61 @@ struct ExprOpNot : Expr
     COMMON_METHODS
 };
 
-#define MakeBinOp(name, s)                                                                   \
-    struct name : Expr                                                                       \
-    {                                                                                        \
-        PosIdx pos;                                                                          \
-        Expr *e1, *e2;                                                                       \
-        name(Expr * e1, Expr * e2)                                                           \
-            : e1(e1)                                                                         \
-            , e2(e2) {};                                                                     \
-        name(const PosIdx & pos, Expr * e1, Expr * e2)                                       \
-            : pos(pos)                                                                       \
-            , e1(e1)                                                                         \
-            , e2(e2) {};                                                                     \
-        void show(const SymbolTable & symbols, std::ostream & str) const override            \
-        {                                                                                    \
-            str << "(";                                                                      \
-            e1->show(symbols, str);                                                          \
-            str << " " s " ";                                                                \
-            e2->show(symbols, str);                                                          \
-            str << ")";                                                                      \
-        }                                                                                    \
-        void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env) override \
-        {                                                                                    \
-            e1->bindVars(es, env);                                                           \
-            e2->bindVars(es, env);                                                           \
-        }                                                                                    \
-        void eval(EvalState & state, Env & env, Value & v) override;                         \
-        PosIdx getPos() const override                                                       \
-        {                                                                                    \
-            return pos;                                                                      \
-        }                                                                                    \
-    };
+#define MakeBinOpMembers(name, s)                                                        \
+    PosIdx pos;                                                                          \
+    Expr *e1, *e2;                                                                       \
+    name(Expr * e1, Expr * e2)                                                           \
+        : e1(e1)                                                                         \
+        , e2(e2){};                                                                      \
+    name(const PosIdx & pos, Expr * e1, Expr * e2)                                       \
+        : pos(pos)                                                                       \
+        , e1(e1)                                                                         \
+        , e2(e2){};                                                                      \
+    void show(const SymbolTable & symbols, std::ostream & str) const override            \
+    {                                                                                    \
+        str << "(";                                                                      \
+        e1->show(symbols, str);                                                          \
+        str << " " s " ";                                                                \
+        e2->show(symbols, str);                                                          \
+        str << ")";                                                                      \
+    }                                                                                    \
+    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env) override \
+    {                                                                                    \
+        e1->bindVars(es, env);                                                           \
+        e2->bindVars(es, env);                                                           \
+    }                                                                                    \
+    void eval(EvalState & state, Env & env, Value & v) override;                         \
+    PosIdx getPos() const override                                                       \
+    {                                                                                    \
+        return pos;                                                                      \
+    }
 
-MakeBinOp(ExprOpEq, "==") MakeBinOp(ExprOpNEq, "!=") MakeBinOp(ExprOpAnd, "&&") MakeBinOp(ExprOpOr, "||")
-    MakeBinOp(ExprOpImpl, "->") MakeBinOp(ExprOpUpdate, "//") MakeBinOp(ExprOpConcatLists, "++")
+#define MakeBinOp(name, s)        \
+    struct name : Expr            \
+    {                             \
+        MakeBinOpMembers(name, s) \
+    }
 
-        struct ExprConcatStrings : Expr
+MakeBinOp(ExprOpEq, "==");
+MakeBinOp(ExprOpNEq, "!=");
+MakeBinOp(ExprOpAnd, "&&");
+MakeBinOp(ExprOpOr, "||");
+MakeBinOp(ExprOpImpl, "->");
+MakeBinOp(ExprOpConcatLists, "++");
+
+struct ExprOpUpdate : Expr
+{
+private:
+    /** Special case for merging of two attrsets. */
+    void eval(EvalState & state, Value & v, Value & v1, Value & v2);
+    void evalForUpdate(EvalState & state, Env & env, UpdateQueue & q);
+
+public:
+    MakeBinOpMembers(ExprOpUpdate, "//");
+    virtual void evalForUpdate(EvalState & state, Env & env, UpdateQueue & q, std::string_view errorCtx) override;
+};
+
+struct ExprConcatStrings : Expr
 {
     PosIdx pos;
     bool forceString;

src/libexpr/include/nix/expr/parser-state.hh

@@ -24,7 +24,6 @@ struct StringToken
     }
 };
 
-// This type must be trivially copyable; see YYLTYPE_IS_TRIVIAL in parser.y.
 struct ParserLocation
 {
     int beginOffset;
@@ -44,9 +43,6 @@ struct ParserLocation
         beginOffset = stashedBeginOffset;
         endOffset = stashedEndOffset;
     }
-
-    /** Latest doc comment position, or 0. */
-    int doc_comment_first_column, doc_comment_last_column;
 };
 
 struct LexerState
@@ -71,7 +67,7 @@ struct LexerState
     /**
      * @brief Maps some positions to a DocComment, where the comment is relevant to the location.
      */
-    std::unordered_map<PosIdx, DocComment> & positionToDocComment;
+    DocCommentMap & positionToDocComment;
 
     PosTable & positions;
     PosTable::Origin origin;
@@ -82,13 +78,14 @@ struct LexerState
 struct ParserState
 {
     const LexerState & lexerState;
+    std::pmr::polymorphic_allocator<char> & alloc;
     SymbolTable & symbols;
     PosTable & positions;
     Expr * result;
     SourcePath basePath;
     PosTable::Origin origin;
     const ref<SourceAccessor> rootFS;
-    const Expr::AstSymbols & s;
+    static constexpr Expr::AstSymbols s = StaticEvalSymbols::create().exprSymbols;
     const EvalSettings & settings;
 
     void dupAttr(const AttrPath & attrPath, const PosIdx pos, const PosIdx prevPos);

src/libexpr/include/nix/expr/primops.hh

@@ -8,31 +8,11 @@
 
 namespace nix {
 
-/**
- * For functions where we do not expect deep recursion, we can use a sizable
- * part of the stack a free allocation space.
- *
- * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
- */
-constexpr size_t nonRecursiveStackReservation = 128;
-
-/**
- * Functions that maybe applied to self-similar inputs, such as concatMap on a
- * tree, should reserve a smaller part of the stack for allocation.
- *
- * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
- */
-constexpr size_t conservativeStackReservation = 16;
-
 struct RegisterPrimOp
 {
     typedef std::vector<PrimOp> PrimOps;
 
-    static PrimOps & primOps()
-    {
-        static PrimOps primOps;
-        return primOps;
-    }
+    static PrimOps & primOps();
 
     /**
      * You can register a constant by passing an arity of 0. fun

src/libexpr/include/nix/expr/print-options.hh

@@ -110,7 +110,7 @@ struct PrintOptions
  * `PrintOptions` for unknown and therefore potentially large values in error messages,
  * to avoid printing "too much" output.
  */
-static PrintOptions errorPrintOptions = PrintOptions{
+static constexpr PrintOptions errorPrintOptions = PrintOptions{
     .ansiColors = true,
     .maxDepth = 10,
     .maxAttrs = 10,