develop: Allow shells to declare the minimal bash version they require

Allow derivations to export a `NIX_ENV_MIN_BASH_VERSION` that sets the minimal (major) bash version that is required to parse their setup script, and use that to gracefully fail if the current bash is too old. Fix #10263 (from the Nix side at least, needs https://github.com/NixOS/nixpkgs/pull/299490 from the Nixpkgs side to be useful in practice)
Merge pull request #10299 from detroyejr/remove-repl-flake
2024-03-29 18:13:42 +01:00 · 2024-03-29 15:06:23 +01:00 · 2024-03-29 08:57:41 -04:00 · 2024-03-29 13:38:37 +01:00 · 2024-03-29 09:10:09 +01:00 · 2024-03-28 19:54:03 +01:00
241 changed files with 4102 additions and 5230 deletions
@@ -14,4 +14,4 @@
 src/libexpr/primops.cc @roberth

 # Libstore layer
-/src/libstore @thufschmitt
+/src/libstore @thufschmitt @ericson2314
@@ -20,7 +20,7 @@ jobs:
    - uses: actions/checkout@v4
      with:
        fetch-depth: 0
-    - uses: cachix/install-nix-action@v25
+    - uses: cachix/install-nix-action@v26
      with:
        # The sandbox would otherwise be disabled by default on Darwin
        extra_nix_config: "sandbox = true"
@@ -62,9 +62,9 @@ jobs:
      with:
        fetch-depth: 0
    - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
-    - uses: cachix/install-nix-action@v25
+    - uses: cachix/install-nix-action@v26
      with:
-        install_url: https://releases.nixos.org/nix/nix-2.13.3/install
+        install_url: https://releases.nixos.org/nix/nix-2.20.3/install
    - uses: cachix/cachix-action@v14
      with:
        name: '${{ env.CACHIX_NAME }}'
@@ -84,7 +84,7 @@ jobs:
    steps:
    - uses: actions/checkout@v4
    - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
-    - uses: cachix/install-nix-action@v25
+    - uses: cachix/install-nix-action@v26
      with:
        install_url: '${{needs.installer.outputs.installerURL}}'
        install_options: "--tarball-url-prefix https://${{ env.CACHIX_NAME }}.cachix.org/serve"
@@ -114,9 +114,9 @@ jobs:
    - uses: actions/checkout@v4
      with:
        fetch-depth: 0
-    - uses: cachix/install-nix-action@v25
+    - uses: cachix/install-nix-action@v26
      with:
-        install_url: https://releases.nixos.org/nix/nix-2.13.3/install
+        install_url: https://releases.nixos.org/nix/nix-2.20.3/install
    - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
    - run: echo NIX_VERSION="$(nix --experimental-features 'nix-command flakes' eval .\#default.version | tr -d \")" >> $GITHUB_ENV
    - uses: cachix/cachix-action@v14
@@ -153,6 +153,17 @@ jobs:
        IMAGE_ID=$(echo $IMAGE_ID | tr '[A-Z]' '[a-z]')

        docker tag nix:$NIX_VERSION $IMAGE_ID:$NIX_VERSION
-        docker tag nix:$NIX_VERSION $IMAGE_ID:master
+        docker tag nix:$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
+
+  vm_tests:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+      - uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - run: nix build -L .#hydraJobs.tests.githubFlakes .#hydraJobs.tests.tarballFlakes
@@ -10,6 +10,7 @@ perl/Makefile.config
 /stamp-h1
 /svn-revision
 /libtool
+/config/config.*

 # /doc/manual/
 /doc/manual/*.1
@@ -45,13 +46,16 @@ perl/Makefile.config
 /src/libexpr/parser-tab.hh
 /src/libexpr/parser-tab.output
 /src/libexpr/nix.tbl
+/src/libexpr/tests
 /tests/unit/libexpr/libnixexpr-tests

 # /src/libstore/
 *.gen.*
+/src/libstore/tests
 /tests/unit/libstore/libnixstore-tests

 # /src/libutil/
+/src/libutil/tests
 /tests/unit/libutil/libnixutil-tests

 /src/nix/nix
@@ -138,6 +142,7 @@ GTAGS

 # auto-generated compilation database
 compile_commands.json
+*.compile_commands.json

 nix-rust/target

@@ -1 +1 @@
-2.21.0
+2.22.0
@@ -67,7 +67,7 @@ Check out the [security policy](https://github.com/NixOS/nix/security/policy).
   - [ ] API documentation in header files
   - [ ] Code and comments are self-explanatory
   - [ ] Commit message explains **why** the change was made
-   - [ ] New feature or incompatible change: updated [release notes](./doc/manual/src/release-notes/rl-next.md)
+   - [ ] New feature or incompatible change: [add a release note](https://nixos.org/manual/nix/stable/contributing/hacking#add-a-release-note)

 7. If you need additional feedback or help to getting pull request into shape, ask other contributors using [@mentions](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#mentioning-people-and-teams).

@@ -42,6 +42,7 @@ ifeq ($(ENABLE_FUNCTIONAL_TESTS), yes)
 makefiles += \
  tests/functional/local.mk \
  tests/functional/ca/local.mk \
+  tests/functional/git-hashing/local.mk \
  tests/functional/dyn-drv/local.mk \
  tests/functional/test-libstoreconsumer/local.mk \
  tests/functional/plugins/local.mk
@@ -67,6 +68,7 @@ ifeq ($(OPTIMIZE), 1)
  GLOBAL_LDFLAGS += $(CXXLTO)
 else
  GLOBAL_CXXFLAGS += -O0 -U_FORTIFY_SOURCE
+  unexport NIX_HARDENING_ENABLE
 endif

 include mk/platform.mk
@@ -81,7 +83,7 @@ ifdef HOST_WINDOWS
  GLOBAL_LDFLAGS += -Wl,--export-all-symbols
 endif

-GLOBAL_CXXFLAGS += -g -Wall -include $(buildprefix)config.h -std=c++2a -I src
+GLOBAL_CXXFLAGS += -g -Wall -Wimplicit-fallthrough -include $(buildprefix)config.h -std=c++2a -I src

 # Include the main lib, causing rules to be defined

@@ -175,6 +175,16 @@ $(d)/src/SUMMARY-rl-next.md: $(d)/src/release-notes/rl-next.md
 # Generate the HTML manual.
 .PHONY: manual-html
 manual-html: $(docdir)/manual/index.html
+
+# Open the built HTML manual in the default browser.
+manual-html-open: $(docdir)/manual/index.html
+	@echo "  OPEN  " $<; \
+	  xdg-open $< \
+		  || open $< \
+			|| { \
+		echo "Could not open the manual in a browser. Please open '$<'" >&2; \
+		false; \
+		}
 install: $(docdir)/manual/index.html

 # Generate 'nix' manpages.
@@ -14,7 +14,7 @@

 const redirects = {
 "index.html": {
-    "part-advanced-topics": "advanced-topics/advanced-topics.html",
+    "part-advanced-topics": "advanced-topics/index.html",
    "chap-tuning-cores-and-jobs": "advanced-topics/cores-vs-jobs.html",
    "chap-diff-hook": "advanced-topics/diff-hook.html",
    "check-dirs-are-unregistered": "advanced-topics/diff-hook.html#check-dirs-are-unregistered",
@@ -22,7 +22,7 @@ const redirects = {
    "chap-post-build-hook": "advanced-topics/post-build-hook.html",
    "chap-post-build-hook-caveats": "advanced-topics/post-build-hook.html#implementation-caveats",
    "chap-writing-nix-expressions": "language/index.html",
-    "part-command-ref": "command-ref/command-ref.html",
+    "part-command-ref": "command-ref/index.html",
    "conf-allow-import-from-derivation": "command-ref/conf-file.html#conf-allow-import-from-derivation",
    "conf-allow-new-privileges": "command-ref/conf-file.html#conf-allow-new-privileges",
    "conf-allowed-uris": "command-ref/conf-file.html#conf-allowed-uris",
@@ -261,7 +261,7 @@ const redirects = {
    "sec-installer-proxy-settings": "installation/env-variables.html#proxy-environment-variables",
    "sec-nix-ssl-cert-file": "installation/env-variables.html#nix_ssl_cert_file",
    "sec-nix-ssl-cert-file-with-nix-daemon-and-macos": "installation/env-variables.html#nix_ssl_cert_file-with-macos-and-the-nix-daemon",
-    "chap-installation": "installation/installation.html",
+    "chap-installation": "installation/index.html",
    "ch-installing-binary": "installation/installing-binary.html",
    "sect-macos-installation": "installation/installing-binary.html#macos-installation",
    "sect-macos-installation-change-store-prefix": "installation/installing-binary.html#macos-installation",
@@ -288,7 +288,7 @@ const redirects = {
    "ssec-copy-closure": "package-management/copy-closure.html",
    "sec-garbage-collection": "package-management/garbage-collection.html",
    "ssec-gc-roots": "package-management/garbage-collector-roots.html",
-    "chap-package-management": "package-management/package-management.html",
+    "chap-package-management": "package-management/index.html",
    "sec-profiles": "package-management/profiles.html",
    "ssec-s3-substituter": "package-management/s3-substituter.html",
    "ssec-s3-substituter-anonymous-reads": "package-management/s3-substituter.html#anonymous-reads-to-your-s3-compatible-binary-cache",
@@ -297,7 +297,7 @@ const redirects = {
    "sec-sharing-packages": "package-management/sharing-packages.html",
    "ssec-ssh-substituter": "package-management/ssh-substituter.html",
    "chap-quick-start": "quick-start.html",
-    "sec-relnotes": "release-notes/release-notes.html",
+    "sec-relnotes": "release-notes/index.html",
    "ch-relnotes-0.10.1": "release-notes/rl-0.10.1.html",
    "ch-relnotes-0.10": "release-notes/rl-0.10.html",
    "ssec-relnotes-0.11": "release-notes/rl-0.11.html",
@@ -1,40 +0,0 @@
---
-synopsis: Concise error printing in `nix repl`
-prs: 9928
---
-
-Previously, if an element of a list or attribute set threw an error while
-evaluating, `nix repl` would print the entire error (including source location
-information) inline. This output was clumsy and difficult to parse:
-
-```
-nix-repl> { err = builtins.throw "uh oh!"; }
-{ err = «error:
-       … while calling the 'throw' builtin
-         at «string»:1:9:
-            1| { err = builtins.throw "uh oh!"; }
-             |         ^
-
-       error: uh oh!»; }
-```
-
-Now, only the error message is displayed, making the output much more readable.
-```
-nix-repl> { err = builtins.throw "uh oh!"; }
-{ err = «error: uh oh!»; }
-```
-
-However, if the whole expression being evaluated throws an error, source
-locations and (if applicable) a stack trace are printed, just like you'd expect:
-
-```
-nix-repl> builtins.throw "uh oh!"
-error:
-       … while calling the 'throw' builtin
-         at «string»:1:1:
-            1| builtins.throw "uh oh!"
-             | ^
-
-       error: uh oh!
-```
-
@@ -1,9 +0,0 @@
---
-synopsis: "`--debugger` can now access bindings from `let` expressions"
-prs: 9918
-issues: 8827.
---
-
-Breakpoints and errors in the bindings of a `let` expression can now access
-those bindings in the debugger. Previously, only the body of `let` expressions
-could access those bindings.
@@ -1,9 +0,0 @@
---
-synopsis: Enter the `--debugger` when `builtins.trace` is called if `debugger-on-trace` is set
-prs: 9914
---
-
-If the `debugger-on-trace` option is set and `--debugger` is given,
-`builtins.trace` calls will behave similarly to `builtins.break` and will enter
-the debug REPL. This is useful for determining where warnings are being emitted
-from.
@@ -1,25 +0,0 @@
---
-synopsis: Debugger prints source position information
-prs: 9913
---
-
-The `--debugger` now prints source location information, instead of the
-pointers of source location information. Before:
-
-```
-nix-repl> :bt
-0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
-0x600001522598
-```
-
-After:
-
-```
-0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
-/nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27
-
-   131|
-   132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
-      |                           ^
-   133|     in
-```
@@ -1,25 +0,0 @@
---
-synopsis: The `--debugger` will start more reliably in `let` expressions and function calls
-prs: 9917
-issues: 6649
---
-
-Previously, if you attempted to evaluate this file with the debugger:
-
-```nix
-let
-  a = builtins.trace "before inner break" (
-    builtins.break "hello"
-  );
-  b = builtins.trace "before outer break" (
-    builtins.break a
-  );
-in
-  b
-```
-
-Nix would correctly enter the debugger at `builtins.break a`, but if you asked
-it to `:continue`, it would skip over the `builtins.break "hello"` expression
-entirely.
-
-Now, Nix will correctly enter the debugger at both breakpoints.
@@ -1,50 +0,0 @@
---
-synopsis: Functions are printed with more detail
-prs: 9606
-issues: 7145
---
-
-Functions and `builtins` are printed with more detail in `nix repl`, `nix
-eval`, `builtins.trace`, and most other places values are printed.
-
-Before:
-
-```
-$ nix repl nixpkgs
-nix-repl> builtins.map
-«primop»
-
-nix-repl> builtins.map lib.id
-«primop-app»
-
-nix-repl> builtins.trace lib.id "my-value"
-trace: <LAMBDA>
-"my-value"
-
-$ nix eval --file functions.nix
-{ id = <LAMBDA>; primop = <PRIMOP>; primop-app = <PRIMOP-APP>; }
-```
-
-After:
-
-```
-$ nix repl nixpkgs
-nix-repl> builtins.map
-«primop map»
-
-nix-repl> builtins.map lib.id
-«partially applied primop map»
-
-nix-repl> builtins.trace lib.id "my-value"
-trace: «lambda id @ /nix/store/8rrzq23h2zq7sv5l2vhw44kls5w0f654-source/lib/trivial.nix:26:5»
-"my-value"
-
-$ nix eval --file functions.nix
-{ id = «lambda id @ /Users/wiggles/nix/functions.nix:2:8»; primop = «primop map»; primop-app = «partially applied primop map»; }
-```
-
-This was actually released in Nix 2.20, but wasn't added to the release notes
-so we're announcing it here. The historical release notes have been updated as well.
-
-[type-error]: https://github.com/NixOS/nix/pull/9753
-[coercion-error]: https://github.com/NixOS/nix/pull/9754
@@ -1,10 +0,0 @@
---
-synopsis: Store paths are allowed to start with `.`
-issues: 912
-prs: 9867 9091 9095 9120 9121 9122 9130 9219 9224
---
-
-Leading periods were allowed by accident in Nix 2.4. The Nix team has considered this to be a bug, but this behavior has since been relied on by users, leading to unnecessary difficulties.
-From now on, leading periods are officially, definitively supported. The names `.` and `..` are disallowed, as well as those starting with `.-` or `..-`.
-
-Nix versions that denied leading periods are documented [in the issue](https://github.com/NixOS/nix/issues/912#issuecomment-1919583286).
@@ -1,13 +0,0 @@
---
-synopsis: Nix commands respect Ctrl-C
-prs: 9687 6995
-issues: 7245
---
-
-Previously, many Nix commands would hang indefinitely if Ctrl-C was pressed
-while performing various operations (including `nix develop`, `nix flake
-update`, and so on). With several fixes to Nix's signal handlers, Nix commands
-will now exit quickly after Ctrl-C is pressed.
-
-This was actually released in Nix 2.20, but wasn't added to the release notes
-so we're announcing it here. The historical release notes have been updated as well.
@@ -0,0 +1,13 @@
+---
+synopsis: "`nix eval` prints derivations as `.drv` paths"
+prs: 10200
+---
+
+`nix eval` will now print derivations as their `.drv` paths, rather than as
+attribute sets. This makes commands like `nix eval nixpkgs#bash` terminate
+instead of infinitely looping into recursive self-referential attributes:
+
+```ShellSession
+$ nix eval nixpkgs#bash
+«derivation /nix/store/m32cbgbd598f4w299g0hwyv7gbw6rqcg-bash-5.2p26.drv»
+```
@@ -1,24 +0,0 @@
---
-synopsis: "`nix repl` pretty-prints values"
-prs: 9931
---
-
-`nix repl` will now pretty-print values:
-
-```
-{
-  attrs = {
-    a = {
-      b = {
-        c = { };
-      };
-    };
-  };
-  list = [ 1 ];
-  list' = [
-    1
-    2
-    3
-  ];
-}
-```
@@ -1,37 +0,0 @@
---
-synopsis: "Visual clutter in `--debugger` is reduced"
-prs: 9919
---
-
-Before:
-```
-info: breakpoint reached
-
-
-Starting REPL to allow you to inspect the current state of the evaluator.
-
-Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.
-
-nix-repl> :continue
-error: uh oh
-
-
-Starting REPL to allow you to inspect the current state of the evaluator.
-
-Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.
-
-nix-repl>
-```
-
-After:
-
-```
-info: breakpoint reached
-
-Nix 2.20.0pre20231222_dirty debugger
-Type :? for help.
-nix-repl> :continue
-error: uh oh
-
-nix-repl>
-```
@@ -0,0 +1,8 @@
+---
+synopsis: Remove experimental repl-flake
+significance: significant
+issues: 10103
+prs: 10299
+---
+
+The `repl-flake` experimental feature has been removed. The `nix repl` command now works like the rest of the new CLI in that `nix repl {path}` now tries to load a flake at `{path}` (or fails if the `flakes` experimental feature isn't enabled).*
@@ -1,8 +0,0 @@
---
-synopsis: "`nix repl` now respects Ctrl-C while printing values"
-prs: 9927
---
-
-`nix repl` will now halt immediately when Ctrl-C is pressed while it's printing
-a value. This is useful if you got curious about what would happen if you
-printed all of Nixpkgs.
@@ -1,22 +0,0 @@
---
-synopsis: Cycle detection in `nix repl` is simpler and more reliable
-prs: 9926
-issues: 8672
---
-
-The cycle detection in `nix repl`, `nix eval`, `builtins.trace`, and everywhere
-else values are printed is now simpler and matches the cycle detection in
-`nix-instantiate --eval` output.
-
-Before:
-
-```
-nix eval --expr 'let self = { inherit self; }; in self'
-{ self = { self = «repeated»; }; }
-```
-
-After:
-
-```
-{ self = «repeated»; }
-```
@@ -1,9 +0,0 @@
---
-synopsis: Stack size is increased on macOS
-prs: 9860
---
-
-Previously, Nix would set the stack size to 64MiB on Linux, but would leave the
-stack size set to the default (approximately 8KiB) on macOS. Now, the stack
-size is correctly set to 64MiB on macOS as well, which should reduce stack
-overflow segfaults in deeply-recursive Nix expressions.
@@ -121,6 +121,7 @@
  - [C++ style guide](contributing/cxx.md)
 - [Release Notes](release-notes/index.md)
 {{#include ./SUMMARY-rl-next.md}}
+  - [Release 2.21 (2024-03-11)](release-notes/rl-2.21.md)
  - [Release 2.20 (2024-01-29)](release-notes/rl-2.20.md)
  - [Release 2.19 (2023-11-17)](release-notes/rl-2.19.md)
  - [Release 2.18 (2023-09-20)](release-notes/rl-2.18.md)
@@ -27,11 +27,9 @@ and open `./result-doc/share/doc/nix/manual/index.html`.
 To build the manual incrementally, [enter the development shell](./hacking.md) and run:

 ```console
-make manual-html -j $NIX_BUILD_CORES
+make manual-html-open -j $NIX_BUILD_CORES
 ```

-and open `./outputs/out/share/doc/nix/manual/language/index.html`.
-
 In order to reflect changes to the [Makefile for the manual], clear all generated files before re-building:

 [Makefile for the manual]: https://github.com/NixOS/nix/blob/master/doc/manual/local.mk
@@ -258,10 +258,10 @@ See [supported compilation environments](#compilation-environments) and instruct
 To use the LSP with your editor, you first need to [set up `clangd`](https://clangd.llvm.org/installation#project-setup) by running:

 ```console
-make clean && bear -- make -j$NIX_BUILD_CORES default check install
+make compile_commands.json
 ```

-Configure your editor to use the `clangd` from the shell, either by running it inside the development shell, or by using [nix-direnv](https://github.com/nix-community/nix-direnv) and [the appropriate editor plugin](https://github.com/direnv/direnv/wiki#editor-integration).
+Configure your editor to use the `clangd` from the `.#native-clangStdenvPackages` shell. You can do that either by running it inside the development shell, or by using [nix-direnv](https://github.com/nix-community/nix-direnv) and [the appropriate editor plugin](https://github.com/direnv/direnv/wiki#editor-integration).

 > **Note**
 >
@@ -86,7 +86,7 @@

  [store path]: #gloss-store-path

- [file system object]{#gloss-store-object}
+- [file system object]{#gloss-file-system-object}

  The Nix data model for representing simplified file system data.

@@ -28,7 +28,7 @@ $ sudo su
 ## macOS multi-user

 ```console
-$ sudo nix-env --install --file '<nixpkgs>' --attr nix -I nixpkgs=channel:nixpkgs-unstable
+$ sudo nix-env --install --file '<nixpkgs>' --attr nix cacert -I nixpkgs=channel:nixpkgs-unstable
 $ sudo launchctl remove org.nixos.nix-daemon
 $ sudo launchctl load /Library/LaunchDaemons/org.nixos.nix-daemon.plist
 ```
@@ -188,9 +188,13 @@ Derivations can declare some infrequently used optional attributes.
    }
    ```

-    The `outputHashAlgo` attribute specifies the hash algorithm used to
-    compute the hash. It can currently be `"sha1"`, `"sha256"` or
-    `"sha512"`.
+    The `outputHash` attribute must be a string containing the hash in either hexadecimal or "nix32" encoding, or following the format for integrity metadata as defined by [SRI](https://www.w3.org/TR/SRI/).
+    The "nix32" encoding is an adaptation of base-32 encoding.
+    The [`convertHash`](@docroot@/language/builtins.md#builtins-convertHash) function shows how to convert between different encodings, and the [`nix-hash` command](../command-ref/nix-hash.md) has information about obtaining the hash for some contents, as well as converting to and from encodings.
+
+    The `outputHashAlgo` attribute specifies the hash algorithm used to compute the hash.
+    It can currently be `"sha1"`, `"sha256"`, `"sha512"`, or `null`.
+    `outputHashAlgo` can only be `null` when `outputHash` follows the SRI format.

    The `outputHashMode` attribute determines how the hash is computed.
    It must be one of the following two values:
@@ -209,11 +213,6 @@ Derivations can declare some infrequently used optional attributes.
        this case, the output can be anything, including a directory
        tree.

-    The `outputHash` attribute, finally, must be a string containing
-    the hash in either hexadecimal or base-32 notation. (See the
-    [`nix-hash` command](../command-ref/nix-hash.md) for information
-    about converting to and from base-32 notation.)
-
  - [`__contentAddressed`]{#adv-attr-__contentAddressed}
    > **Warning**
    > This attribute is part of an [experimental feature](@docroot@/contributing/experimental-features.md).
@@ -1,7 +1,13 @@
 # Nix Language

 The Nix language is designed for conveniently creating and composing *derivations* – precise descriptions of how contents of existing files are used to derive new files.
-It is:
+
+> **Tip**
+>
+> These pages are written as a reference.
+> If you are learning Nix, nix.dev has a good [introduction to the Nix language](https://nix.dev/tutorials/nix-language).
+
+The language is:

 - *domain-specific*

@@ -432,6 +438,32 @@ This is an incomplete overview of language features, by example.

  </td>
 </tr>
+ <tr>
+  <td>
+
+   `inherit pkgs src;`
+
+  </td>
+  <td>
+
+   Adds the variables to the current scope (attribute set or `let` binding).
+   Desugars to `pkgs = pkgs; src = src;`
+
+  </td>
+ </tr>
+ <tr>
+  <td>
+
+   `inherit (pkgs) lib stdenv;`
+
+  </td>
+  <td>
+
+   Adds the attributes, from the attribute set in parentheses, to the current scope (attribute set or `let` binding).
+   Desugars to `lib = pkgs.lib; stdenv = pkgs.stdenv;`
+
+  </td>
+ </tr>
 <tr>
  <td>

@@ -89,15 +89,20 @@ where

      - `rec` = one of:

+        - ```ebnf
+          | ""
+          ```
+          (empty string) for hashes of the flat (single file) serialization
+
        - ```ebnf
          | "r:"
          ```
          hashes of the for [Nix Archive (NAR)] (arbitrary file system object) serialization

        - ```ebnf
-          | ""
+          | "git:"
          ```
-          (empty string) for hashes of the flat (single file) serialization
+          hashes of the [Git blob/tree](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects) [Merkel tree](https://en.wikipedia.org/wiki/Merkle_tree) format

      - ```ebnf
        algo = "md5" | "sha1" | "sha256"
@@ -34,7 +34,7 @@ For more in-depth information you are kindly referred to subsequent chapters.
   lolcat: command not found
   ```

-1. Search for more packages on <search.nixos.org> to try them out.
+1. Search for more packages on [search.nixos.org](https://search.nixos.org/) to try them out.

 1. Free up storage space:

@@ -0,0 +1,302 @@
+# Release 2.21.0 (2024-03-11)
+
+- Fix a fixed-output derivation sandbox escape (CVE-2024-27297)
+
+  Cooperating Nix derivations could send file descriptors to files in the Nix
+  store to each other via Unix domain sockets in the abstract namespace. This
+  allowed one derivation to modify the output of the other derivation, after Nix
+  has registered the path as "valid" and immutable in the Nix database.
+  In particular, this allowed the output of fixed-output derivations to be
+  modified from their expected content.
+
+  This isn't the case any more.
+
+- CLI options `--arg-from-file` and `--arg-from-stdin` [#10122](https://github.com/NixOS/nix/pull/10122)
+
+  The new CLI option `--arg-from-file` *name* *path* passes the contents
+  of file *path* as a string value via the function argument *name* to a
+  Nix expression. Similarly, the new option `--arg-from-stdin` *name*
+  reads the contents of the string from standard input.
+
+- Concise error printing in `nix repl` [#9928](https://github.com/NixOS/nix/pull/9928)
+
+  Previously, if an element of a list or attribute set threw an error while
+  evaluating, `nix repl` would print the entire error (including source location
+  information) inline. This output was clumsy and difficult to parse:
+
+  ```
+  nix-repl> { err = builtins.throw "uh oh!"; }
+  { err = «error:
+         … while calling the 'throw' builtin
+           at «string»:1:9:
+              1| { err = builtins.throw "uh oh!"; }
+               |         ^
+
+         error: uh oh!»; }
+  ```
+
+  Now, only the error message is displayed, making the output much more readable.
+  ```
+  nix-repl> { err = builtins.throw "uh oh!"; }
+  { err = «error: uh oh!»; }
+  ```
+
+  However, if the whole expression being evaluated throws an error, source
+  locations and (if applicable) a stack trace are printed, just like you'd expect:
+
+  ```
+  nix-repl> builtins.throw "uh oh!"
+  error:
+         … while calling the 'throw' builtin
+           at «string»:1:1:
+              1| builtins.throw "uh oh!"
+               | ^
+
+         error: uh oh!
+  ```
+
+- `--debugger` can now access bindings from `let` expressions [#8827](https://github.com/NixOS/nix/issues/8827) [#9918](https://github.com/NixOS/nix/pull/9918)
+
+  Breakpoints and errors in the bindings of a `let` expression can now access
+  those bindings in the debugger. Previously, only the body of `let` expressions
+  could access those bindings.
+
+- Enter the `--debugger` when `builtins.trace` is called if `debugger-on-trace` is set [#9914](https://github.com/NixOS/nix/pull/9914)
+
+  If the `debugger-on-trace` option is set and `--debugger` is given,
+  `builtins.trace` calls will behave similarly to `builtins.break` and will enter
+  the debug REPL. This is useful for determining where warnings are being emitted
+  from.
+
+- Debugger prints source position information [#9913](https://github.com/NixOS/nix/pull/9913)
+
+  The `--debugger` now prints source location information, instead of the
+  pointers of source location information. Before:
+
+  ```
+  nix-repl> :bt
+  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
+  0x600001522598
+  ```
+
+  After:
+
+  ```
+  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
+  /nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27
+
+     131|
+     132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
+        |                           ^
+     133|     in
+  ```
+
+- The `--debugger` will start more reliably in `let` expressions and function calls [#6649](https://github.com/NixOS/nix/issues/6649) [#9917](https://github.com/NixOS/nix/pull/9917)
+
+  Previously, if you attempted to evaluate this file with the debugger:
+
+  ```nix
+  let
+    a = builtins.trace "before inner break" (
+      builtins.break "hello"
+    );
+    b = builtins.trace "before outer break" (
+      builtins.break a
+    );
+  in
+    b
+  ```
+
+  Nix would correctly enter the debugger at `builtins.break a`, but if you asked
+  it to `:continue`, it would skip over the `builtins.break "hello"` expression
+  entirely.
+
+  Now, Nix will correctly enter the debugger at both breakpoints.
+
+- Nested debuggers are no longer supported [#9920](https://github.com/NixOS/nix/pull/9920)
+
+  Previously, evaluating an expression that throws an error in the debugger would
+  enter a second, nested debugger:
+
+  ```
+  nix-repl> builtins.throw "what"
+  error: what
+
+
+  Starting REPL to allow you to inspect the current state of the evaluator.
+
+  Welcome to Nix 2.18.1. Type :? for help.
+
+  nix-repl>
+  ```
+
+  Now, it just prints the error message like `nix repl`:
+
+  ```
+  nix-repl> builtins.throw "what"
+  error:
+         … while calling the 'throw' builtin
+           at «string»:1:1:
+              1| builtins.throw "what"
+               | ^
+
+         error: what
+  ```
+
+- Consistent order of function arguments in printed expressions [#9874](https://github.com/NixOS/nix/pull/9874)
+
+  Function arguments are now printed in lexicographic order rather than the internal, creation-time based symbol order.
+
+- Fix duplicate attribute error positions for `inherit` [#9874](https://github.com/NixOS/nix/pull/9874)
+
+  When an `inherit` caused a duplicate attribute error the position of the error was not reported correctly, placing the error with the inherit itself or at the start of the bindings block instead of the offending attribute name.
+
+- `inherit (x) ...` evaluates `x` only once [#9847](https://github.com/NixOS/nix/pull/9847)
+
+  `inherit (x) a b ...` now evaluates the expression `x` only once for all inherited attributes rather than once for each inherited attribute.
+  This does not usually have a measurable impact, but side-effects (such as `builtins.trace`) would be duplicated and expensive expressions (such as derivations) could cause a measurable slowdown.
+
+- Store paths are allowed to start with `.` [#912](https://github.com/NixOS/nix/issues/912) [#9091](https://github.com/NixOS/nix/pull/9091) [#9095](https://github.com/NixOS/nix/pull/9095) [#9120](https://github.com/NixOS/nix/pull/9120) [#9121](https://github.com/NixOS/nix/pull/9121) [#9122](https://github.com/NixOS/nix/pull/9122) [#9130](https://github.com/NixOS/nix/pull/9130) [#9219](https://github.com/NixOS/nix/pull/9219) [#9224](https://github.com/NixOS/nix/pull/9224) [#9867](https://github.com/NixOS/nix/pull/9867)
+
+  Leading periods were allowed by accident in Nix 2.4. The Nix team has considered this to be a bug, but this behavior has since been relied on by users, leading to unnecessary difficulties.
+  From now on, leading periods are supported. The names `.` and `..` are disallowed, as well as those starting with `.-` or `..-`.
+
+  Nix versions that denied leading periods are documented [in the issue](https://github.com/NixOS/nix/issues/912#issuecomment-1919583286).
+
+- `nix repl` pretty-prints values [#9931](https://github.com/NixOS/nix/pull/9931)
+
+  `nix repl` will now pretty-print values:
+
+  ```
+  {
+    attrs = {
+      a = {
+        b = {
+          c = { };
+        };
+      };
+    };
+    list = [ 1 ];
+    list' = [
+      1
+      2
+      3
+    ];
+  }
+  ```
+
+- Introduction of `--regex` and `--all` in `nix profile remove` and `nix profile upgrade` [#10166](https://github.com/NixOS/nix/pull/10166)
+
+  Previously the command-line arguments for `nix profile remove` and `nix profile upgrade` matched the package entries using regular expression.
+  For instance:
+
+  ```
+  nix profile remove '.*vim.*'
+  ```
+
+  This would remove all packages that contain `vim` in their name.
+
+  In most cases, only singular package names were used to remove and upgrade packages. Mixing this with regular expressions sometimes lead to unintended behavior. For instance, `python3.1` could match `python311`.
+
+  To avoid unintended behavior, the arguments are now only matching exact names.
+
+  Matching using regular expressions is still possible by using the new `--regex` flag:
+
+  ```
+  nix profile remove --regex '.*vim.*'
+  ```
+
+  One of the most useful cases for using regular expressions was to upgrade all packages. This was previously accomplished by:
+
+  ```
+  nix profile upgrade '.*'
+  ```
+
+  With the introduction of the `--all` flag, this now becomes more straightforward:
+
+  ```
+  nix profile upgrade --all
+  ```
+
+- Visual clutter in `--debugger` is reduced [#9919](https://github.com/NixOS/nix/pull/9919)
+
+  Before:
+  ```
+  info: breakpoint reached
+
+
+  Starting REPL to allow you to inspect the current state of the evaluator.
+
+  Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.
+
+  nix-repl> :continue
+  error: uh oh
+
+
+  Starting REPL to allow you to inspect the current state of the evaluator.
+
+  Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.
+
+  nix-repl>
+  ```
+
+  After:
+
+  ```
+  info: breakpoint reached
+
+  Nix 2.20.0pre20231222_dirty debugger
+  Type :? for help.
+  nix-repl> :continue
+  error: uh oh
+
+  nix-repl>
+  ```
+
+- Cycle detection in `nix repl` is simpler and more reliable [#8672](https://github.com/NixOS/nix/issues/8672) [#9926](https://github.com/NixOS/nix/pull/9926)
+
+  The cycle detection in `nix repl`, `nix eval`, `builtins.trace`, and everywhere
+  else values are printed is now simpler and matches the cycle detection in
+  `nix-instantiate --eval` output.
+
+  Before:
+
+  ```
+  nix eval --expr 'let self = { inherit self; }; in self'
+  { self = { self = «repeated»; }; }
+  ```
+
+  After:
+
+  ```
+  { self = «repeated»; }
+  ```
+
+- In the debugger, `while evaluating the attribute` errors now include position information [#9915](https://github.com/NixOS/nix/pull/9915)
+
+  Before:
+
+  ```
+  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
+  0x600001522598
+  ```
+
+  After:
+
+  ```
+  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
+  /nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27
+
+     131|
+     132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
+        |                           ^
+     133|     in
+  ```
+
+- Stack size is increased on macOS [#9860](https://github.com/NixOS/nix/pull/9860)
+
+  Previously, Nix would set the stack size to 64MiB on Linux, but would leave the
+  stack size set to the default (approximately 8KiB) on macOS. Now, the stack
+  size is correctly set to 64MiB on macOS as well, which should reduce stack
+  overflow segfaults in deeply-recursive Nix expressions.
+
@@ -34,16 +34,16 @@
    },
    "nixpkgs": {
      "locked": {
-        "lastModified": 1705033721,
-        "narHash": "sha256-K5eJHmL1/kev6WuqyqqbS1cdNnSidIZ3jeqJ7GbrYnQ=",
+        "lastModified": 1709083642,
+        "narHash": "sha256-7kkJQd4rZ+vFrzWu8sTRtta5D1kBG0LSRYAfhtmMlSo=",
        "owner": "NixOS",
        "repo": "nixpkgs",
-        "rev": "a1982c92d8980a0114372973cbdfe0a307f1bdea",
+        "rev": "b550fe4b4776908ac2a861124307045f8e717c8e",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
-        "ref": "nixos-23.05-small",
+        "ref": "release-23.11",
        "repo": "nixpkgs",
        "type": "github"
      }
@@ -1,7 +1,9 @@
 {
  description = "The purely functional package manager";

-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05-small";
+  # TODO switch to nixos-23.11-small
+  #      https://nixpk.gs/pr-tracker.html?pr=291954
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/release-23.11";
  inputs.nixpkgs-regression.url = "github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
  inputs.flake-compat = { url = "github:edolstra/flake-compat"; flake = false; };
  inputs.libgit2 = { url = "github:libgit2/libgit2"; flake = false; };
@@ -10,20 +12,10 @@

    let
      inherit (nixpkgs) lib;
-
-      # Experimental fileset library: https://github.com/NixOS/nixpkgs/pull/222981
-      # Not an "idiomatic" flake input because:
-      #  - Propagation to dependent locks: https://github.com/NixOS/nix/issues/7730
-      #  - Subflake would download redundant and huge parent flake
-      #  - No git tree hash support: https://github.com/NixOS/nix/issues/6044
-      inherit (import (builtins.fetchTarball { url = "https://github.com/NixOS/nix/archive/1bdcd7fc8a6a40b2e805bad759b36e64e911036b.tar.gz"; sha256 = "sha256:14ljlpdsp4x7h1fkhbmc4bd3vsqnx8zdql4h3037wh09ad6a0893"; }))
-        fileset;
+      inherit (lib) fileset;

      officialRelease = false;

-      # Set to true to build the release notes for the next release.
-      buildUnreleasedNotes = false;
-
      version = lib.fileContents ./.version + versionSuffix;
      versionSuffix =
        if officialRelease
@@ -39,7 +31,6 @@
      crossSystems = [
        "armv6l-unknown-linux-gnueabihf"
        "armv7l-unknown-linux-gnueabihf"
-        "x86_64-unknown-freebsd13"
        "x86_64-unknown-netbsd"
      ];

@@ -307,8 +298,11 @@
              ''
                type -p nix-env
                # Note: we're filtering out nixos-install-tools because https://github.com/NixOS/nixpkgs/pull/153594#issuecomment-1020530593.
-                time nix-env --store dummy:// -f ${nixpkgs-regression} -qaP --drv-path | sort | grep -v nixos-install-tools > packages
-                [[ $(sha1sum < packages | cut -c1-40) = ff451c521e61e4fe72bdbe2d0ca5d1809affa733 ]]
+                (
+                  set -x
+                  time nix-env --store dummy:// -f ${nixpkgs-regression} -qaP --drv-path | sort | grep -v nixos-install-tools > packages
+                  [[ $(sha1sum < packages | cut -c1-40) = e01b031fc9785a572a38be6bc473957e3b6faad7 ]]
+                )
                mkdir $out
              '';

@@ -349,7 +343,6 @@

      checks = forAllSystems (system: {
        binaryTarball = self.hydraJobs.binaryTarball.${system};
-        perlBindings = self.hydraJobs.perlBindings.${system};
        installTests = self.hydraJobs.installTests.${system};
        nixpkgsLibTests = self.hydraJobs.tests.nixpkgsLibTests.${system};
        rl-next =
@@ -359,6 +352,11 @@
        '';
      } // (lib.optionalAttrs (builtins.elem system linux64BitSystems)) {
        dockerImage = self.hydraJobs.dockerImage.${system};
+      } // (lib.optionalAttrs (!(builtins.elem system linux32BitSystems))) {
+        # Some perl dependencies are broken on i686-linux.
+        # Since the support is only best-effort there, disable the perl
+        # bindings
+        perlBindings = self.hydraJobs.perlBindings.${system};
      });

      packages = forAllSystems (system: rec {
@@ -404,8 +402,11 @@
            # Make bash completion work.
            XDG_DATA_DIRS+=:$out/share
          '';
+
          nativeBuildInputs = attrs.nativeBuildInputs or []
-            ++ lib.optional stdenv.cc.isClang pkgs.buildPackages.bear
+            # TODO: Remove the darwin check once
+            # https://github.com/NixOS/nixpkgs/pull/291814 is available
+            ++ lib.optional (stdenv.cc.isClang && !stdenv.buildPlatform.isDarwin) pkgs.buildPackages.bear
            ++ lib.optional (stdenv.cc.isClang && stdenv.hostPlatform == stdenv.buildPlatform) pkgs.buildPackages.clang-tools;
        });
        in
@@ -417,8 +418,9 @@
              (forAllStdenvs (stdenvName: makeShell pkgs pkgs.${stdenvName}));
          in
            (makeShells "native" nixpkgsFor.${system}.native) //
-            (makeShells "static" nixpkgsFor.${system}.static) //
-            (lib.genAttrs shellCrossSystems (crossSystem: let pkgs = nixpkgsFor.${system}.cross.${crossSystem}; in makeShell pkgs pkgs.stdenv)) //
+            (lib.optionalAttrs (!nixpkgsFor.${system}.native.stdenv.isDarwin)
+              (makeShells "static" nixpkgsFor.${system}.static)) //
+              (lib.genAttrs shellCrossSystems (crossSystem: let pkgs = nixpkgsFor.${system}.cross.${crossSystem}; in makeShell pkgs pkgs.stdenv)) //
            {
              default = self.devShells.${system}.native-stdenvPackages;
            }
@@ -7,4 +7,4 @@ $(foreach i, config.h $(wildcard src/lib*/*.hh), \

 $(GCH): src/libutil/util.hh config.h

-GCH_CXXFLAGS = -I src/libutil
+GCH_CXXFLAGS = $(INCLUDE_libutil)
@@ -11,6 +11,8 @@ use JSON::PP;
 use LWP::UserAgent;
 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 $releasesBucketName = "nix-releases";
@@ -36,9 +38,9 @@ sub fetch {
 my $evalUrl = "https://hydra.nixos.org/eval/$evalId";
 my $evalInfo = decode_json(fetch($evalUrl, 'application/json'));
 #print Dumper($evalInfo);
-my $flakeUrl = $evalInfo->{flake} or die;
-my $flakeInfo = decode_json(`nix flake metadata --json "$flakeUrl"` or die);
-my $nixRev = $flakeInfo->{revision} or die;
+my $flakeUrl = $evalInfo->{flake};
+my $flakeInfo = decode_json(`nix flake metadata --json "$flakeUrl"` or die) if $flakeUrl;
+my $nixRev = ($flakeInfo ? $flakeInfo->{revision} : $evalInfo->{jobsetevalinputs}->{nix}->{revision}) or die;

 my $buildInfo = decode_json(fetch("$evalUrl/job/build.x86_64-linux", 'application/json'));
 #print Dumper($buildInfo);
@@ -83,12 +85,19 @@ my $channelsBucket = $s3_us->bucket($channelsBucketName) or die;
 sub getStorePath {
    my ($jobName, $output) = @_;
    my $buildInfo = decode_json(fetch("$evalUrl/job/$jobName", 'application/json'));
-    return $buildInfo->{buildoutputs}->{$output or "out"}->{path} or die "cannot get store path for '$jobName'";
+    return $buildInfo->{buildoutputs}->{$output or "out"}->{path} // die "cannot get store path for '$jobName'";
 }

 sub copyManual {
-    my $manual = getStorePath("build.x86_64-linux", "doc");
-    print "$manual\n";
+    my $manual;
+    eval {
+        $manual = getStorePath("build.x86_64-linux", "doc");
+    };
+    if ($@) {
+        warn "$@";
+        return;
+    }
+    print "Manual: $manual\n";

    my $manualNar = "$tmpDir/$releaseName-manual.nar.xz";
    print "$manualNar\n";
@@ -154,19 +163,33 @@ downloadFile("binaryTarball.x86_64-linux", "1");
 downloadFile("binaryTarball.aarch64-linux", "1");
 downloadFile("binaryTarball.x86_64-darwin", "1");
 downloadFile("binaryTarball.aarch64-darwin", "1");
-downloadFile("binaryTarballCross.x86_64-linux.armv6l-unknown-linux-gnueabihf", "1");
-downloadFile("binaryTarballCross.x86_64-linux.armv7l-unknown-linux-gnueabihf", "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 $@;
 downloadFile("installerScript", "1");

 # Upload docker images to dockerhub.
 my $dockerManifest = "";
 my $dockerManifestLatest = "";
+my $haveDocker = 0;

 for my $platforms (["x86_64-linux", "amd64"], ["aarch64-linux", "arm64"]) {
    my $system = $platforms->[0];
    my $dockerPlatform = $platforms->[1];
    my $fn = "nix-$version-docker-image-$dockerPlatform.tar.gz";
-    downloadFile("dockerImage.$system", "1", $fn);
+    eval {
+        downloadFile("dockerImage.$system", "1", $fn);
+    };
+    if ($@) {
+        warn "$@" if $@;
+        next;
+    }
+    $haveDocker = 1;

    print STDERR "loading docker image for $dockerPlatform...\n";
    system("docker load -i $tmpDir/$fn") == 0 or die;
@@ -194,21 +217,23 @@ for my $platforms (["x86_64-linux", "amd64"], ["aarch64-linux", "arm64"]) {
    $dockerManifestLatest .= " --amend $latestTag"
 }

-print STDERR "creating multi-platform docker manifest...\n";
-system("docker manifest rm nixos/nix:$version");
-system("docker manifest create nixos/nix:$version $dockerManifest") == 0 or die;
-if ($isLatest) {
-    print STDERR "creating latest multi-platform docker manifest...\n";
-    system("docker manifest rm nixos/nix:latest");
-    system("docker manifest create nixos/nix:latest $dockerManifestLatest") == 0 or die;
-}
+if ($haveDocker) {
+    print STDERR "creating multi-platform docker manifest...\n";
+    system("docker manifest rm nixos/nix:$version");
+    system("docker manifest create nixos/nix:$version $dockerManifest") == 0 or die;
+    if ($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;
+    }

-print STDERR "pushing multi-platform docker manifest...\n";
-system("docker manifest push nixos/nix:$version") == 0 or die;
+    print STDERR "pushing multi-platform docker manifest...\n";
+    system("docker manifest push nixos/nix:$version") == 0 or die;

-if ($isLatest) {
-    print STDERR "pushing latest multi-platform docker manifest...\n";
-    system("docker manifest push nixos/nix:latest") == 0 or die;
+    if ($isLatest) {
+        print STDERR "pushing latest multi-platform docker manifest...\n";
+        system("docker manifest push nixos/nix:latest") == 0 or die;
+    }
 }

 # Upload nix-fallback-paths.nix.
@@ -0,0 +1,11 @@
+compile-commands-json-files :=
+
+define write-compile-commands
+  _srcs := $$(sort $$(foreach src, $$($(1)_SOURCES), $$(src)))
+
+  $(1)_COMPILE_COMMANDS_JSON := $$(addprefix $(buildprefix), $$(addsuffix .compile_commands.json, $$(basename $$(_srcs))))
+
+  compile-commands-json-files += $$($(1)_COMPILE_COMMANDS_JSON)
+
+  clean-files += $$($(1)_COMPILE_COMMANDS_JSON)
+endef
@@ -1,5 +1,5 @@
 %.gen.hh: %
-	@echo 'R"foo(' >> $@.tmp
+	@echo 'R"__NIX_STR(' >> $@.tmp
 	$(trace-gen) cat $< >> $@.tmp
-	@echo ')foo"' >> $@.tmp
+	@echo ')__NIX_STR"' >> $@.tmp
 	@mv $@.tmp $@
@@ -68,6 +68,7 @@ include mk/patterns.mk
 include mk/templates.mk
 include mk/cxx-big-literal.mk
 include mk/tests.mk
+include mk/compilation-database.mk


 # Include all sub-Makefiles.
@@ -97,6 +98,13 @@ $(foreach test-group, $(install-tests-groups), \
    $(eval $(call run-test,$(test),$(install_test_init))) \
    $(eval $(test-group).test-group: $(test).test)))

+# Compilation database.
+$(foreach lib, $(libraries), $(eval $(call write-compile-commands,$(lib))))
+$(foreach prog, $(programs), $(eval $(call write-compile-commands,$(prog))))
+
+compile_commands.json: $(compile-commands-json-files)
+	@jq --slurp '.' $^ >$@
+
 # Include makefiles requiring built programs.
 $(foreach mf, $(makefiles-late), $(eval $(call include-sub-makefile,$(mf))))

@@ -1,11 +1,41 @@
+
+# These are the complete command lines we use to compile C and C++ files.
+# - $< is the source file.
+# - $1 is the object file to create.
+CC_CMD=$(CC) -o $1 -c $< $(CPPFLAGS) $(GLOBAL_CFLAGS) $(CFLAGS) $($1_CFLAGS) -MMD -MF $(call filename-to-dep,$1) -MP
+CXX_CMD=$(CXX) -o $1 -c $< $(CPPFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($1_CXXFLAGS) $(ERROR_SWITCH_ENUM) -MMD -MF $(call filename-to-dep,$1) -MP
+
+# We use COMPILE_COMMANDS_JSON_CMD to turn a compilation command (like CC_CMD
+# or CXX_CMD above) into a comple_commands.json file. We rely on bash native
+# word splitting to define the positional arguments.
+# - $< is the source file being compiled.
+COMPILE_COMMANDS_JSON_CMD=jq --null-input '{ directory: $$ENV.PWD, file: "$<", arguments: $$ARGS.positional }' --args --
+
+
 $(buildprefix)%.o: %.cc
 	@mkdir -p "$(dir $@)"
-	$(trace-cxx) $(CXX) -o $@ -c $< $(CPPFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) $(ERROR_SWITCH_ENUM) -MMD -MF $(call filename-to-dep, $@) -MP
+	$(trace-cxx) $(call CXX_CMD,$@)

 $(buildprefix)%.o: %.cpp
 	@mkdir -p "$(dir $@)"
-	$(trace-cxx) $(CXX) -o $@ -c $< $(CPPFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) $(ERROR_SWITCH_ENUM) -MMD -MF $(call filename-to-dep, $@) -MP
+	$(trace-cxx) $(call CXX_CMD,$@)

 $(buildprefix)%.o: %.c
 	@mkdir -p "$(dir $@)"
-	$(trace-cc) $(CC) -o $@ -c $< $(CPPFLAGS) $(GLOBAL_CFLAGS) $(CFLAGS) $($@_CFLAGS) -MMD -MF $(call filename-to-dep, $@) -MP
+	$(trace-cc) $(call CC_CMD,$@)
+
+# In the following we need to replace the .compile_commands.json extension in $@ with .o
+# to make the object file. This is needed because CC_CMD and CXX_CMD do further expansions
+# based on the object file name (i.e. *_CXXFLAGS and filename-to-dep).
+
+$(buildprefix)%.compile_commands.json: %.cc
+	@mkdir -p "$(dir $@)"
+	$(trace-jq) $(COMPILE_COMMANDS_JSON_CMD) $(call CXX_CMD,$(@:.compile_commands.json=.o)) > $@
+
+$(buildprefix)%.compile_commands.json: %.cpp
+	@mkdir -p "$(dir $@)"
+	$(trace-jq) $(COMPILE_COMMANDS_JSON_CMD) $(call CXX_CMD,$(@:.compile_commands.json=.o)) > $@
+
+$(buildprefix)%.compile_commands.json: %.c
+	@mkdir -p "$(dir $@)"
+	$(trace-jq) $(COMPILE_COMMANDS_JSON_CMD) $(call CC_CMD,$(@:.compile_commands.json=.o)) > $@
@@ -10,6 +10,8 @@ ifeq ($(V), 0)
  trace-install = @echo "  INST  " $@;
  trace-mkdir   = @echo "  MKDIR " $@;
  trace-test    = @echo "  TEST  " $@;
+  trace-sh      = @echo "  SH    " $@;
+  trace-jq      = @echo "  JQ    " $@;

  suppress  = @

@@ -24,6 +24,7 @@
 , libgit2
 , libseccomp
 , libsodium
+, man
 , lowdown
 , mdbook
 , mdbook-linkcheck
@@ -74,7 +75,10 @@
 # sounds so long as evaluation just takes places within short-lived
 # processes. (When the process exits, the memory is reclaimed; it is
 # only leaked *within* the process.)
-, enableGC ? true
+#
+# Temporarily disabled on Windows because the `GC_throw_bad_alloc`
+# symbol is missing during linking.
+, enableGC ? !stdenv.hostPlatform.isWindows

 # Whether to enable Markdown rendering in the Nix binary.
 , enableMarkdown ? !stdenv.hostPlatform.isWindows
@@ -154,7 +158,7 @@ in {
    in
      fileset.toSource {
        root = ./.;
-        fileset = fileset.intersect baseFiles (fileset.unions ([
+        fileset = fileset.intersection baseFiles (fileset.unions ([
          # For configure
          ./.version
          ./configure.ac
@@ -209,6 +213,11 @@ in {
    (lib.getBin lowdown)
    mdbook
    mdbook-linkcheck
+  ] ++ lib.optionals doInstallCheck [
+    git
+    mercurial
+    openssh
+    man # for testing `nix-* --help`
  ] ++ lib.optionals (doInstallCheck || enableManual) [
    jq # Also for custom mdBook preprocessor.
  ] ++ lib.optional stdenv.hostPlatform.isLinux util-linux
@@ -249,12 +258,6 @@ in {
  dontBuild = !attrs.doBuild;
  doCheck = attrs.doCheck;

-  nativeCheckInputs = [
-    git
-    mercurial
-    openssh
-  ];
-
  disallowedReferences = [ boost ];

  preConfigure = lib.optionalString (doBuild && ! stdenv.hostPlatform.isStatic) (
@@ -335,6 +338,12 @@ in {
    echo "doc internal-api-docs $out/share/doc/nix/internal-api/html" >> ''${!outputDoc}/nix-support/hydra-build-products
  '';

+  # So the check output gets links for DLLs in the out output.
+  preFixup = lib.optionalString (stdenv.hostPlatform.isWindows && builtins.elem "check" finalAttrs.outputs) ''
+    ln -s "$check/lib/"*.dll "$check/bin"
+    ln -s "$out/bin/"*.dll "$check/bin"
+  '';
+
  doInstallCheck = attrs.doInstallCheck;

  installCheckFlags = "sysconfdir=$(out)/etc";
@@ -343,15 +352,22 @@ in {

  # Work around weird bug where it doesn't think there is a Makefile.
  installCheckPhase = if (!doBuild && doInstallCheck) then ''
+    runHook preInstallCheck
    mkdir -p src/nix-channel
    make installcheck -j$NIX_BUILD_CORES -l$NIX_BUILD_CORES
  '' else null;

  # Needed for tests if we are not doing a build, but testing existing
  # built Nix.
-  preInstallCheck = lib.optionalString (! doBuild) ''
-    mkdir -p src/nix-channel
-  '';
+  preInstallCheck =
+    lib.optionalString (! doBuild) ''
+      mkdir -p src/nix-channel
+    ''
+    # See https://github.com/NixOS/nix/issues/2523
+    # Occurs often in tests since https://github.com/NixOS/nix/pull/9900
+    + lib.optionalString stdenv.hostPlatform.isDarwin ''
+      export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES
+    '';

  separateDebugInfo = !stdenv.hostPlatform.isStatic;

@@ -259,7 +259,7 @@ hashPath(char * algo, int base32, char * path)
            auto [accessor, canonPath] = PosixSourceAccessor::createAtRoot(path);
            Hash h = hashPath(
                accessor, canonPath,
-                FileIngestionMethod::Recursive, parseHashAlgo(algo)).first;
+                FileIngestionMethod::Recursive, parseHashAlgo(algo));
            auto s = h.to_string(base32 ? HashFormat::Nix32 : HashFormat::Base16, false);
            XPUSHs(sv_2mortal(newSVpv(s.c_str(), 0)));
        } catch (Error & e) {
@@ -28,7 +28,7 @@ else
 end

 # Set $NIX_SSL_CERT_FILE so that Nixpkgs applications like curl work.
-if test -n "$NIX_SSH_CERT_FILE"
+if test -n "$NIX_SSL_CERT_FILE"
  : # Allow users to override the NIX_SSL_CERT_FILE
 else if test -e /etc/ssl/certs/ca-certificates.crt # NixOS, Ubuntu, Debian, Gentoo, Arch
  set --export NIX_SSL_CERT_FILE /etc/ssl/certs/ca-certificates.crt
@@ -44,7 +44,7 @@ else if test -e "$NIX_LINK/etc/ca-bundle.crt" # old cacert in Nix profile
  set --export NIX_SSL_CERT_FILE "$NIX_LINK/etc/ca-bundle.crt"
 else
  # Fall back to what is in the nix profiles, favouring whatever is defined last.
-  for i in $NIX_PROFILES
+  for i in (string split ' ' $NIX_PROFILES)
    if test -e "$i/etc/ssl/certs/ca-bundle.crt"
      set --export NIX_SSL_CERT_FILE "$i/etc/ssl/certs/ca-bundle.crt"
    end
@@ -202,7 +202,7 @@ static int main_build_remote(int argc, char * * argv)
                        else
                            drvstr = "<unknown>";

-                        auto error = HintFmt(errorText);
+                        auto error = HintFmt::fromFormatString(errorText);
                        error
                            % drvstr
                            % neededSystem
@@ -20,7 +20,7 @@ MixEvalArgs::MixEvalArgs()
        .description = "Pass the value *expr* as the argument *name* to Nix functions.",
        .category = category,
        .labels = {"name", "expr"},
-        .handler = {[&](std::string name, std::string expr) { autoArgs[name] = 'E' + expr; }}
+        .handler = {[&](std::string name, std::string expr) { autoArgs.insert_or_assign(name, AutoArg{AutoArgExpr(expr)}); }}
    });

    addFlag({
@@ -28,7 +28,24 @@ MixEvalArgs::MixEvalArgs()
        .description = "Pass the string *string* as the argument *name* to Nix functions.",
        .category = category,
        .labels = {"name", "string"},
-        .handler = {[&](std::string name, std::string s) { autoArgs[name] = 'S' + s; }},
+        .handler = {[&](std::string name, std::string s) { autoArgs.insert_or_assign(name, AutoArg{AutoArgString(s)}); }},
+    });
+
+    addFlag({
+        .longName = "arg-from-file",
+        .description = "Pass the contents of file *path* as the argument *name* to Nix functions.",
+        .category = category,
+        .labels = {"name", "path"},
+        .handler = {[&](std::string name, std::string path) { autoArgs.insert_or_assign(name, AutoArg{AutoArgFile(path)}); }},
+        .completer = completePath
+    });
+
+    addFlag({
+        .longName = "arg-from-stdin",
+        .description = "Pass the contents of stdin as the argument *name* to Nix functions.",
+        .category = category,
+        .labels = {"name"},
+        .handler = {[&](std::string name) { autoArgs.insert_or_assign(name, AutoArg{AutoArgStdin{}}); }},
    });

    addFlag({
@@ -154,13 +171,23 @@ MixEvalArgs::MixEvalArgs()
 Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
 {
    auto res = state.buildBindings(autoArgs.size());
-    for (auto & i : autoArgs) {
+    for (auto & [name, arg] : autoArgs) {
        auto v = state.allocValue();
-        if (i.second[0] == 'E')
-            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), state.rootPath(".")));
-        else
-            v->mkString(((std::string_view) i.second).substr(1));
-        res.insert(state.symbols.create(i.first), v);
+        std::visit(overloaded {
+            [&](const AutoArgExpr & arg) {
+                state.mkThunk_(*v, state.parseExprFromString(arg.expr, state.rootPath(".")));
+            },
+            [&](const AutoArgString & arg) {
+                v->mkString(arg.s);
+            },
+            [&](const AutoArgFile & arg) {
+                v->mkString(readFile(arg.path));
+            },
+            [&](const AutoArgStdin & arg) {
+                v->mkString(readFile(STDIN_FILENO));
+            }
+        }, arg);
+        res.insert(state.symbols.create(name), v);
    }
    return res.finish();
 }
@@ -6,6 +6,8 @@
 #include "common-args.hh"
 #include "search-path.hh"

+#include <filesystem>
+
 namespace nix {

 class Store;
@@ -26,7 +28,14 @@ struct MixEvalArgs : virtual Args, virtual MixRepair
    std::optional<std::string> evalStoreUrl;

 private:
-    std::map<std::string, std::string> autoArgs;
+    struct AutoArgExpr { std::string expr; };
+    struct AutoArgString { std::string s; };
+    struct AutoArgFile { std::filesystem::path path; };
+    struct AutoArgStdin { };
+
+    using AutoArg = std::variant<AutoArgExpr, AutoArgString, AutoArgFile, AutoArgStdin>;
+
+    std::map<std::string, AutoArg> autoArgs;
 };

 SourcePath lookupFileArg(EvalState & state, std::string_view s, const Path * baseDir = nullptr);
@@ -21,6 +21,7 @@
 #include "url.hh"
 #include "registry.hh"
 #include "build-result.hh"
+#include "fs-input-accessor.hh"

 #include <regex>
 #include <queue>
@@ -146,7 +147,7 @@ MixFlakeOptions::MixFlakeOptions()
        .category = category,
        .labels = {"flake-lock-path"},
        .handler = {[&](std::string lockFilePath) {
-            lockFlags.referenceLockFilePath = lockFilePath;
+            lockFlags.referenceLockFilePath = getUnfilteredRootPath(CanonPath(absPath(lockFilePath)));
        }},
        .completer = completePath
    });
@@ -442,10 +443,10 @@ ref<eval_cache::EvalCache> openEvalCache(
    EvalState & state,
    std::shared_ptr<flake::LockedFlake> lockedFlake)
 {
-    auto fingerprint = lockedFlake->getFingerprint();
+    auto fingerprint = lockedFlake->getFingerprint(state.store);
    return make_ref<nix::eval_cache::EvalCache>(
        evalSettings.useEvalCache && evalSettings.pureEval
-            ? std::optional { std::cref(fingerprint) }
+            ? fingerprint
            : std::nullopt,
        state,
        [&state, lockedFlake]()
@@ -6,7 +6,7 @@ libcmd_DIR := $(d)

 libcmd_SOURCES := $(wildcard $(d)/*.cc)

-libcmd_CXXFLAGS += -I src/libutil -I src/libstore -I src/libexpr -I src/libmain -I src/libfetchers
+libcmd_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers) $(INCLUDE_libexpr) -I src/libmain

 libcmd_LDFLAGS = $(EDITLINE_LIBS) $(LOWDOWN_LIBS) $(THREAD_LDFLAGS)

@@ -50,7 +50,7 @@ std::string renderMarkdownToTerminal(std::string_view markdown)
    if (!rndr_res)
        throw Error("allocation error while rendering Markdown");

-    return filterANSIEscapes(std::string(buf->data, buf->size), !shouldANSI());
+    return filterANSIEscapes(std::string(buf->data, buf->size), !isTTY());
 #else
    return std::string(markdown);
 #endif
@@ -0,0 +1,186 @@
+#include <cstdio>
+
+#ifdef USE_READLINE
+#include <readline/history.h>
+#include <readline/readline.h>
+#else
+// editline < 1.15.2 don't wrap their API for C++ usage
+// (added in https://github.com/troglobit/editline/commit/91398ceb3427b730995357e9d120539fb9bb7461).
+// This results in linker errors due to to name-mangling of editline C symbols.
+// For compatibility with these versions, we wrap the API here
+// (wrapping multiple times on newer versions is no problem).
+extern "C" {
+#include <editline.h>
+}
+#endif
+
+#include "signals.hh"
+#include "finally.hh"
+#include "repl-interacter.hh"
+#include "file-system.hh"
+#include "libcmd/repl.hh"
+
+namespace nix {
+
+namespace {
+// Used to communicate to NixRepl::getLine whether a signal occurred in ::readline.
+volatile sig_atomic_t g_signal_received = 0;
+
+void sigintHandler(int signo)
+{
+    g_signal_received = signo;
+}
+};
+
+static detail::ReplCompleterMixin * curRepl; // ugly
+
+static char * completionCallback(char * s, int * match)
+{
+    auto possible = curRepl->completePrefix(s);
+    if (possible.size() == 1) {
+        *match = 1;
+        auto * res = strdup(possible.begin()->c_str() + strlen(s));
+        if (!res)
+            throw Error("allocation failure");
+        return res;
+    } else if (possible.size() > 1) {
+        auto checkAllHaveSameAt = [&](size_t pos) {
+            auto & first = *possible.begin();
+            for (auto & p : possible) {
+                if (p.size() <= pos || p[pos] != first[pos])
+                    return false;
+            }
+            return true;
+        };
+        size_t start = strlen(s);
+        size_t len = 0;
+        while (checkAllHaveSameAt(start + len))
+            ++len;
+        if (len > 0) {
+            *match = 1;
+            auto * res = strdup(std::string(*possible.begin(), start, len).c_str());
+            if (!res)
+                throw Error("allocation failure");
+            return res;
+        }
+    }
+
+    *match = 0;
+    return nullptr;
+}
+
+static int listPossibleCallback(char * s, char *** avp)
+{
+    auto possible = curRepl->completePrefix(s);
+
+    if (possible.size() > (INT_MAX / sizeof(char *)))
+        throw Error("too many completions");
+
+    int ac = 0;
+    char ** vp = nullptr;
+
+    auto check = [&](auto * p) {
+        if (!p) {
+            if (vp) {
+                while (--ac >= 0)
+                    free(vp[ac]);
+                free(vp);
+            }
+            throw Error("allocation failure");
+        }
+        return p;
+    };
+
+    vp = check((char **) malloc(possible.size() * sizeof(char *)));
+
+    for (auto & p : possible)
+        vp[ac++] = check(strdup(p.c_str()));
+
+    *avp = vp;
+
+    return ac;
+}
+
+ReadlineLikeInteracter::Guard ReadlineLikeInteracter::init(detail::ReplCompleterMixin * repl)
+{
+    // Allow nix-repl specific settings in .inputrc
+    rl_readline_name = "nix-repl";
+    try {
+        createDirs(dirOf(historyFile));
+    } catch (SystemError & e) {
+        logWarning(e.info());
+    }
+#ifndef USE_READLINE
+    el_hist_size = 1000;
+#endif
+    read_history(historyFile.c_str());
+    auto oldRepl = curRepl;
+    curRepl = repl;
+    Guard restoreRepl([oldRepl] { curRepl = oldRepl; });
+#ifndef USE_READLINE
+    rl_set_complete_func(completionCallback);
+    rl_set_list_possib_func(listPossibleCallback);
+#endif
+    return restoreRepl;
+}
+
+static constexpr const char * promptForType(ReplPromptType promptType)
+{
+    switch (promptType) {
+    case ReplPromptType::ReplPrompt:
+        return "nix-repl> ";
+    case ReplPromptType::ContinuationPrompt:
+        return "          ";
+    }
+    assert(false);
+}
+
+bool ReadlineLikeInteracter::getLine(std::string & input, ReplPromptType promptType)
+{
+    struct sigaction act, old;
+    sigset_t savedSignalMask, set;
+
+    auto setupSignals = [&]() {
+        act.sa_handler = sigintHandler;
+        sigfillset(&act.sa_mask);
+        act.sa_flags = 0;
+        if (sigaction(SIGINT, &act, &old))
+            throw SysError("installing handler for SIGINT");
+
+        sigemptyset(&set);
+        sigaddset(&set, SIGINT);
+        if (sigprocmask(SIG_UNBLOCK, &set, &savedSignalMask))
+            throw SysError("unblocking SIGINT");
+    };
+    auto restoreSignals = [&]() {
+        if (sigprocmask(SIG_SETMASK, &savedSignalMask, nullptr))
+            throw SysError("restoring signals");
+
+        if (sigaction(SIGINT, &old, 0))
+            throw SysError("restoring handler for SIGINT");
+    };
+
+    setupSignals();
+    char * s = readline(promptForType(promptType));
+    Finally doFree([&]() { free(s); });
+    restoreSignals();
+
+    if (g_signal_received) {
+        g_signal_received = 0;
+        input.clear();
+        return true;
+    }
+
+    if (!s)
+        return false;
+    input += s;
+    input += '\n';
+    return true;
+}
+
+ReadlineLikeInteracter::~ReadlineLikeInteracter()
+{
+    write_history(historyFile.c_str());
+}
+
+};
@@ -0,0 +1,48 @@
+#pragma once
+/// @file
+
+#include "finally.hh"
+#include "types.hh"
+#include <functional>
+#include <string>
+
+namespace nix {
+
+namespace detail {
+/** Provides the completion hooks for the repl, without exposing its complete
+ * internals. */
+struct ReplCompleterMixin {
+    virtual StringSet completePrefix(const std::string & prefix) = 0;
+};
+};
+
+enum class ReplPromptType {
+    ReplPrompt,
+    ContinuationPrompt,
+};
+
+class ReplInteracter
+{
+public:
+    using Guard = Finally<std::function<void()>>;
+
+    virtual Guard init(detail::ReplCompleterMixin * repl) = 0;
+    /** Returns a boolean of whether the interacter got EOF */
+    virtual bool getLine(std::string & input, ReplPromptType promptType) = 0;
+    virtual ~ReplInteracter(){};
+};
+
+class ReadlineLikeInteracter : public virtual ReplInteracter
+{
+    std::string historyFile;
+public:
+    ReadlineLikeInteracter(std::string historyFile)
+        : historyFile(historyFile)
+    {
+    }
+    virtual Guard init(detail::ReplCompleterMixin * repl) override;
+    virtual bool getLine(std::string & input, ReplPromptType promptType) override;
+    virtual ~ReadlineLikeInteracter() override;
+};
+
+};
@@ -3,32 +3,17 @@
 #include <cstring>
 #include <climits>

-#include <setjmp.h>
-
-#ifdef USE_READLINE
-#include <readline/history.h>
-#include <readline/readline.h>
-#else
-// editline < 1.15.2 don't wrap their API for C++ usage
-// (added in https://github.com/troglobit/editline/commit/91398ceb3427b730995357e9d120539fb9bb7461).
-// This results in linker errors due to to name-mangling of editline C symbols.
-// For compatibility with these versions, we wrap the API here
-// (wrapping multiple times on newer versions is no problem).
-extern "C" {
-#include <editline.h>
-}
-#endif
-
+#include "libcmd/repl-interacter.hh"
 #include "repl.hh"

 #include "ansicolor.hh"
-#include "signals.hh"
 #include "shared.hh"
 #include "eval.hh"
 #include "eval-cache.hh"
 #include "eval-inline.hh"
 #include "eval-settings.hh"
 #include "attr-path.hh"
+#include "signals.hh"
 #include "store-api.hh"
 #include "log-store.hh"
 #include "common-eval-args.hh"
@@ -38,7 +23,6 @@ extern "C" {
 #include "flake/flake.hh"
 #include "flake/lockfile.hh"
 #include "users.hh"
-#include "terminal.hh"
 #include "editor-for.hh"
 #include "finally.hh"
 #include "markdown.hh"
@@ -75,6 +59,7 @@ enum class ProcessLineResult {

 struct NixRepl
    : AbstractNixRepl
+    , detail::ReplCompleterMixin
    #if HAVE_BOEHMGC
    , gc
    #endif
@@ -90,17 +75,16 @@ struct NixRepl
    int displ;
    StringSet varNames;

-    const Path historyFile;
+    std::unique_ptr<ReplInteracter> interacter;

    NixRepl(const SearchPath & searchPath, nix::ref<Store> store,ref<EvalState> state,
            std::function<AnnotatedValues()> getValues);
-    virtual ~NixRepl();
+    virtual ~NixRepl() = default;

    ReplExitStatus mainLoop() override;
    void initEnv() override;

-    StringSet completePrefix(const std::string & prefix);
-    bool getLine(std::string & input, const std::string & prompt);
+    virtual StringSet completePrefix(const std::string & prefix) override;
    StorePath getDerivationPath(Value & v);
    ProcessLineResult processLine(std::string line);

@@ -123,7 +107,8 @@ struct NixRepl
            .force = true,
            .derivationPaths = true,
            .maxDepth = maxDepth,
-            .prettyIndent = 2
+            .prettyIndent = 2,
+            .errors = ErrorPrintBehavior::ThrowTopLevel,
        });
    }
 };
@@ -143,16 +128,10 @@ NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalS
    , debugTraceIndex(0)
    , getValues(getValues)
    , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
-    , historyFile(getDataDir() + "/nix/repl-history")
+    , interacter(make_unique<ReadlineLikeInteracter>(getDataDir() + "/nix/repl-history"))
 {
 }

-
-NixRepl::~NixRepl()
-{
-    write_history(historyFile.c_str());
-}
-
 void runNix(Path program, const Strings & args,
    const std::optional<std::string> & input = {})
 {
@@ -169,79 +148,6 @@ void runNix(Path program, const Strings & args,
    return;
 }

-static NixRepl * curRepl; // ugly
-
-static char * completionCallback(char * s, int *match) {
-  auto possible = curRepl->completePrefix(s);
-  if (possible.size() == 1) {
-    *match = 1;
-    auto *res = strdup(possible.begin()->c_str() + strlen(s));
-    if (!res) throw Error("allocation failure");
-    return res;
-  } else if (possible.size() > 1) {
-    auto checkAllHaveSameAt = [&](size_t pos) {
-      auto &first = *possible.begin();
-      for (auto &p : possible) {
-        if (p.size() <= pos || p[pos] != first[pos])
-          return false;
-      }
-      return true;
-    };
-    size_t start = strlen(s);
-    size_t len = 0;
-    while (checkAllHaveSameAt(start + len)) ++len;
-    if (len > 0) {
-      *match = 1;
-      auto *res = strdup(std::string(*possible.begin(), start, len).c_str());
-      if (!res) throw Error("allocation failure");
-      return res;
-    }
-  }
-
-  *match = 0;
-  return nullptr;
-}
-
-static int listPossibleCallback(char *s, char ***avp) {
-  auto possible = curRepl->completePrefix(s);
-
-  if (possible.size() > (INT_MAX / sizeof(char*)))
-    throw Error("too many completions");
-
-  int ac = 0;
-  char **vp = nullptr;
-
-  auto check = [&](auto *p) {
-    if (!p) {
-      if (vp) {
-        while (--ac >= 0)
-          free(vp[ac]);
-        free(vp);
-      }
-      throw Error("allocation failure");
-    }
-    return p;
-  };
-
-  vp = check((char **)malloc(possible.size() * sizeof(char*)));
-
-  for (auto & p : possible)
-    vp[ac++] = check(strdup(p.c_str()));
-
-  *avp = vp;
-
-  return ac;
-}
-
-namespace {
-    // Used to communicate to NixRepl::getLine whether a signal occurred in ::readline.
-    volatile sig_atomic_t g_signal_received = 0;
-
-    void sigintHandler(int signo) {
-        g_signal_received = signo;
-    }
-}
-
 static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positions, const DebugTrace & dt)
 {
    if (dt.isError)
@@ -281,24 +187,7 @@ ReplExitStatus NixRepl::mainLoop()

    loadFiles();

-    // Allow nix-repl specific settings in .inputrc
-    rl_readline_name = "nix-repl";
-    try {
-        createDirs(dirOf(historyFile));
-    } catch (SystemError & e) {
-        logWarning(e.info());
-    }
-#ifndef USE_READLINE
-    el_hist_size = 1000;
-#endif
-    read_history(historyFile.c_str());
-    auto oldRepl = curRepl;
-    curRepl = this;
-    Finally restoreRepl([&] { curRepl = oldRepl; });
-#ifndef USE_READLINE
-    rl_set_complete_func(completionCallback);
-    rl_set_list_possib_func(listPossibleCallback);
-#endif
+    auto _guard = interacter->init(static_cast<detail::ReplCompleterMixin *>(this));

    std::string input;

@@ -307,7 +196,7 @@ ReplExitStatus NixRepl::mainLoop()
        logger->pause();
        // When continuing input from previous lines, don't print a prompt, just align to the same
        // number of chars as the prompt.
-        if (!getLine(input, input.empty() ? "nix-repl> " : "          ")) {
+        if (!interacter->getLine(input, input.empty() ? ReplPromptType::ReplPrompt : ReplPromptType::ContinuationPrompt)) {
            // Ctrl-D should exit the debugger.
            state->debugStop = false;
            logger->cout("");
@@ -336,13 +225,7 @@ ReplExitStatus NixRepl::mainLoop()
              printMsg(lvlError, e.msg());
            }
        } catch (EvalError & e) {
-            // in debugger mode, an EvalError should trigger another repl session.
-            // when that session returns the exception will land here.  No need to show it again;
-            // show the error for this repl session instead.
-            if (state->debugRepl && !state->debugTraces.empty())
-                showDebugTrace(std::cout, state->positions, state->debugTraces.front());
-            else
-                printMsg(lvlError, e.msg());
+            printMsg(lvlError, e.msg());
        } catch (Error & e) {
            printMsg(lvlError, e.msg());
        } catch (Interrupted & e) {
@@ -356,52 +239,6 @@ ReplExitStatus NixRepl::mainLoop()
    }
 }

-
-bool NixRepl::getLine(std::string & input, const std::string & prompt)
-{
-    struct sigaction act, old;
-    sigset_t savedSignalMask, set;
-
-    auto setupSignals = [&]() {
-        act.sa_handler = sigintHandler;
-        sigfillset(&act.sa_mask);
-        act.sa_flags = 0;
-        if (sigaction(SIGINT, &act, &old))
-            throw SysError("installing handler for SIGINT");
-
-        sigemptyset(&set);
-        sigaddset(&set, SIGINT);
-        if (sigprocmask(SIG_UNBLOCK, &set, &savedSignalMask))
-            throw SysError("unblocking SIGINT");
-    };
-    auto restoreSignals = [&]() {
-        if (sigprocmask(SIG_SETMASK, &savedSignalMask, nullptr))
-            throw SysError("restoring signals");
-
-        if (sigaction(SIGINT, &old, 0))
-            throw SysError("restoring handler for SIGINT");
-    };
-
-    setupSignals();
-    Finally resetTerminal([&]() { rl_deprep_terminal(); });
-    char * s = readline(prompt.c_str());
-    Finally doFree([&]() { free(s); });
-    restoreSignals();
-
-    if (g_signal_received) {
-        g_signal_received = 0;
-        input.clear();
-        return true;
-    }
-
-    if (!s)
-      return false;
-    input += s;
-    input += '\n';
-    return true;
-}
-
-
 StringSet NixRepl::completePrefix(const std::string & prefix)
 {
    StringSet completions;
@@ -549,6 +386,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
             << "  :l, :load <path>             Load Nix expression and add it to scope\n"
             << "  :lf, :load-flake <ref>       Load Nix flake and add it to scope\n"
             << "  :p, :print <expr>            Evaluate and print expression recursively\n"
+             << "                               Strings are printed directly, without escaping.\n"
             << "  :q, :quit                    Exit nix-repl\n"
             << "  :r, :reload                  Reload all files\n"
             << "  :sh <expr>                   Build dependencies of derivation, then start\n"
@@ -756,7 +594,11 @@ ProcessLineResult NixRepl::processLine(std::string line)
    else if (command == ":p" || command == ":print") {
        Value v;
        evalString(arg, v);
-        printValue(std::cout, v);
+        if (v.type() == nString) {
+            std::cout << v.string_view();
+        } else {
+            printValue(std::cout, v);
+        }
        std::cout << std::endl;
    }

@@ -3,11 +3,6 @@

 #include "eval.hh"

-#if HAVE_BOEHMGC
-#define GC_INCLUDE_NEW
-#include <gc/gc_cpp.h>
-#endif
-
 namespace nix {

 struct AbstractNixRepl
@@ -581,7 +581,7 @@ std::string AttrCursor::getString()
    auto & v = forceValue();

    if (v.type() != nString && v.type() != nPath)
-        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr()).debugThrow();
+        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr(), showType(v)).debugThrow();

    return v.type() == nString ? v.c_str() : v.path().to_string();
 }
@@ -630,7 +630,7 @@ string_t AttrCursor::getStringWithContext()
    else if (v.type() == nPath)
        return {v.path().to_string(), {}};
    else
-        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr()).debugThrow();
+        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr(), showType(v)).debugThrow();
 }

 bool AttrCursor::getBool()
@@ -28,15 +28,7 @@ template<class T>
 EvalErrorBuilder<T> & EvalErrorBuilder<T>::withTrace(PosIdx pos, const std::string_view text)
 {
    error.err.traces.push_front(
-        Trace{.pos = error.state.positions[pos], .hint = HintFmt(std::string(text)), .frame = false});
-    return *this;
-}
-
-template<class T>
-EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrameTrace(PosIdx pos, const std::string_view text)
-{
-    error.err.traces.push_front(
-        Trace{.pos = error.state.positions[pos], .hint = HintFmt(std::string(text)), .frame = true});
+        Trace{.pos = error.state.positions[pos], .hint = HintFmt(std::string(text))});
    return *this;
 }

@@ -63,9 +55,9 @@ EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrame(const Env & env, const Expr
 }

 template<class T>
-EvalErrorBuilder<T> & EvalErrorBuilder<T>::addTrace(PosIdx pos, HintFmt hint, bool frame)
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::addTrace(PosIdx pos, HintFmt hint)
 {
-    error.addTrace(error.state.positions[pos], hint, frame);
+    error.addTrace(error.state.positions[pos], hint);
    return *this;
 }

@@ -89,7 +89,7 @@ public:

    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withFrame(const Env & e, const Expr & ex);

-    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & addTrace(PosIdx pos, HintFmt hint, bool frame = false);
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & addTrace(PosIdx pos, HintFmt hint);

    template<typename... Args>
    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> &
@@ -21,11 +21,24 @@ struct EvalSettings : Config
    Setting<Strings> nixPath{
        this, getDefaultNixPath(), "nix-path",
        R"(
-          List of directories to be searched for `<...>` file references
+          List of search paths to use for [lookup path](@docroot@/language/constructs/lookup-path.md) resolution.
+          This setting determines the value of
+          [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath) and can be used with [`builtins.findFile`](@docroot@/language/builtin-constants.md#builtins-findFile).

-          In particular, outside of [pure evaluation mode](#conf-pure-eval), this determines the value of
-          [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath).
-        )"};
+          The default value is
+
+          ```
+          $HOME/.nix-defexpr/channels
+          nixpkgs=$NIX_STATE_DIR/profiles/per-user/root/channels/nixpkgs
+          $NIX_STATE_DIR/profiles/per-user/root/channels
+          ```
+
+          It can be overridden with the [`NIX_PATH` environment variable](@docroot@/command-ref/env-common.md#env-NIX_PATH) or the [`-I` command line option](@docroot@/command-ref/opt-common.md#opt-I).
+
+          > **Note**
+          >
+          > If [pure evaluation](#conf-pure-eval) is enabled, `nixPath` evaluates to the empty list `[ ]`.
+        )", {}, false};

    Setting<std::string> currentSystem{
        this, "", "eval-system",
@@ -55,8 +68,6 @@ struct EvalSettings : Config
          [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath),
          or to URIs outside of
          [`allowed-uris`](@docroot@/command-ref/conf-file.md#conf-allowed-uris).
-
-          Also the default value for [`nix-path`](#conf-nix-path) is ignored, such that only explicitly set search path entries are taken into account.
        )"};

    Setting<bool> pureEval{this, false, "pure-eval",
@@ -65,9 +76,10 @@ struct EvalSettings : Config

          - Restrict file system and network access to files specified by cryptographic hash
          - Disable impure constants:
-            - [`bultins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem)
+            - [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem)
            - [`builtins.currentTime`](@docroot@/language/builtin-constants.md#builtins-currentTime)
            - [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath)
+            - [`builtins.storePath`](@docroot@/language/builtin-constants.md#builtins-storePath)
        )"
        };

@@ -435,7 +435,14 @@ EvalState::EvalState(

    static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");

-    vEmptyList.mkList(0);
+    vEmptyList.mkList(buildList(0));
+    vNull.mkNull();
+    vTrue.mkBool(true);
+    vFalse.mkBool(false);
+    vStringRegular.mkString("regular");
+    vStringDirectory.mkString("directory");
+    vStringSymlink.mkString("symlink");
+    vStringUnknown.mkString("unknown");

    /* Initialise the Nix expression search path. */
    if (!evalSettings.pureEval) {
@@ -762,10 +769,24 @@ std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const Stati
    return vm;
 }

+/**
+ * Sets `inDebugger` to true on construction and false on destruction.
+ */
+class DebuggerGuard {
+    bool & inDebugger;
+public:
+    DebuggerGuard(bool & inDebugger) : inDebugger(inDebugger) {
+        inDebugger = true;
+    }
+    ~DebuggerGuard() {
+        inDebugger = false;
+    }
+};
+
 void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr & expr)
 {
-    // double check we've got the debugRepl function pointer.
-    if (!debugRepl)
+    // Make sure we have a debugger to run and we're not already in a debugger.
+    if (!debugRepl || inDebugger)
        return;

    auto dts =
@@ -792,6 +813,7 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
    auto se = getStaticEnv(expr);
    if (se) {
        auto vm = mapStaticEnvBindings(symbols, *se.get(), env);
+        DebuggerGuard _guard(inDebugger);
        auto exitStatus = (debugRepl)(ref<EvalState>(shared_from_this()), *vm);
        switch (exitStatus) {
            case ReplExitStatus::QuitAll:
@@ -806,14 +828,16 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
    }
 }

-void EvalState::addErrorTrace(Error & e, const char * s, const std::string & s2) const
+template<typename... Args>
+void EvalState::addErrorTrace(Error & e, const Args & ... formatArgs) const
 {
-    e.addTrace(nullptr, s, s2);
+    e.addTrace(nullptr, HintFmt(formatArgs...));
 }

-void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2, bool frame) const
+template<typename... Args>
+void EvalState::addErrorTrace(Error & e, const PosIdx pos, const Args & ... formatArgs) const
 {
-    e.addTrace(positions[pos], HintFmt(s, s2), frame);
+    e.addTrace(positions[pos], HintFmt(formatArgs...));
 }

 template<typename... Args>
@@ -906,14 +930,16 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
    }
 }

-void EvalState::mkList(Value & v, size_t size)
+ListBuilder::ListBuilder(EvalState & state, size_t size)
+    : size(size)
+    , elems(size <= 2 ? inlineElems : (Value * *) allocBytes(size * sizeof(Value *)))
 {
-    v.mkList(size);
-    if (size > 2)
-        v.bigList.elems = (Value * *) allocBytes(size * sizeof(Value *));
-    nrListElems += size;
+    state.nrListElems += size;
 }

+Value * EvalState::getBool(bool b) {
+    return b ? &vTrue : &vFalse;
+}

 unsigned long nrThunks = 0;

@@ -932,12 +958,11 @@ void EvalState::mkThunk_(Value & v, Expr * expr)

 void EvalState::mkPos(Value & v, PosIdx p)
 {
-    auto pos = positions[p];
-    if (auto path = std::get_if<SourcePath>(&pos.origin)) {
+    auto origin = positions.originOf(p);
+    if (auto path = std::get_if<SourcePath>(&origin)) {
        auto attrs = buildBindings(3);
        attrs.alloc(sFile).mkString(path->path.abs());
-        attrs.alloc(sLine).mkInt(pos.line);
-        attrs.alloc(sColumn).mkInt(pos.column);
+        makePositionThunks(*this, p, attrs.alloc(sLine), attrs.alloc(sColumn));
        v.mkAttrs(attrs);
    } else
        v.mkNull();
@@ -1196,6 +1221,18 @@ void ExprPath::eval(EvalState & state, Env & env, Value & v)
 }


+Env * ExprAttrs::buildInheritFromEnv(EvalState & state, Env & up)
+{
+    Env & inheritEnv = state.allocEnv(inheritFromExprs->size());
+    inheritEnv.up = &up;
+
+    Displacement displ = 0;
+    for (auto from : *inheritFromExprs)
+        inheritEnv.values[displ++] = from->maybeThunk(state, up);
+
+    return &inheritEnv;
+}
+
 void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
 {
    v.mkAttrs(state.buildBindings(attrs.size() + dynamicAttrs.size()).finish());
@@ -1207,6 +1244,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
        Env & env2(state.allocEnv(attrs.size()));
        env2.up = &env;
        dynamicEnv = &env2;
+        Env * inheritEnv = inheritFromExprs ? buildInheritFromEnv(state, env2) : nullptr;

        AttrDefs::iterator overrides = attrs.find(state.sOverrides);
        bool hasOverrides = overrides != attrs.end();
@@ -1217,11 +1255,11 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
        Displacement displ = 0;
        for (auto & i : attrs) {
            Value * vAttr;
-            if (hasOverrides && !i.second.inherited) {
+            if (hasOverrides && i.second.kind != AttrDef::Kind::Inherited) {
                vAttr = state.allocValue();
-                mkThunk(*vAttr, env2, i.second.e);
+                mkThunk(*vAttr, *i.second.chooseByKind(&env2, &env, inheritEnv), i.second.e);
            } else
-                vAttr = i.second.e->maybeThunk(state, i.second.inherited ? env : env2);
+                vAttr = i.second.e->maybeThunk(state, *i.second.chooseByKind(&env2, &env, inheritEnv));
            env2.values[displ++] = vAttr;
            v.attrs->push_back(Attr(i.first, vAttr, i.second.pos));
        }
@@ -1253,9 +1291,15 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
        }
    }

-    else
-        for (auto & i : attrs)
-            v.attrs->push_back(Attr(i.first, i.second.e->maybeThunk(state, env), i.second.pos));
+    else {
+        Env * inheritEnv = inheritFromExprs ? buildInheritFromEnv(state, env) : nullptr;
+        for (auto & i : attrs) {
+            v.attrs->push_back(Attr(
+                    i.first,
+                    i.second.e->maybeThunk(state, *i.second.chooseByKind(&env, &env, inheritEnv)),
+                    i.second.pos));
+        }
+    }

    /* Dynamic attrs apply *after* rec and __overrides. */
    for (auto & i : dynamicAttrs) {
@@ -1287,12 +1331,17 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
    Env & env2(state.allocEnv(attrs->attrs.size()));
    env2.up = &env;

+    Env * inheritEnv = attrs->inheritFromExprs ? attrs->buildInheritFromEnv(state, env2) : nullptr;
+
    /* The recursive attributes are evaluated in the new environment,
       while the inherited attributes are evaluated in the original
       environment. */
    Displacement displ = 0;
-    for (auto & i : attrs->attrs)
-        env2.values[displ++] = i.second.e->maybeThunk(state, i.second.inherited ? env : env2);
+    for (auto & i : attrs->attrs) {
+        env2.values[displ++] = i.second.e->maybeThunk(
+            state,
+            *i.second.chooseByKind(&env2, &env, inheritEnv));
+    }

    auto dts = state.debugRepl
        ? makeDebugTraceStacker(
@@ -1313,9 +1362,10 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)

 void ExprList::eval(EvalState & state, Env & env, Value & v)
 {
-    state.mkList(v, elems.size());
-    for (auto [n, v2] : enumerate(v.listItems()))
-        const_cast<Value * &>(v2) = elems[n]->maybeThunk(state, env);
+    auto list = state.buildList(elems.size());
+    for (const auto & [n, v2] : enumerate(list))
+        v2 = elems[n]->maybeThunk(state, env);
+    v.mkList(list);
 }


@@ -1369,7 +1419,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                state,
                *this,
                env,
-                state.positions[pos2],
+                state.positions[getPos()],
                "while evaluating the attribute '%1%'",
                showAttrPath(state, env, attrPath))
            : nullptr;
@@ -1587,9 +1637,8 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                        "while calling %s",
                        lambda.name
                        ? concatStrings("'", symbols[lambda.name], "'")
-                        : "anonymous lambda",
-                        true);
-                    if (pos) addErrorTrace(e, pos, "from call site%s", "", true);
+                        : "anonymous lambda");
+                    if (pos) addErrorTrace(e, pos, "from call site");
                }
                throw;
            }
@@ -1616,7 +1665,8 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                try {
                    fn->fun(*this, vCur.determinePos(noPos), args, vCur);
                } catch (Error & e) {
-                    addErrorTrace(e, pos, "while calling the '%1%' builtin", fn->name);
+                    if (fn->addTrace)
+                        addErrorTrace(e, pos, "while calling the '%1%' builtin", fn->name);
                    throw;
                }

@@ -1664,7 +1714,8 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                    //    so the debugger allows to inspect the wrong parameters passed to the builtin.
                    fn->fun(*this, vCur.determinePos(noPos), vArgs, vCur);
                } catch (Error & e) {
-                    addErrorTrace(e, pos, "while calling the '%1%' builtin", fn->name);
+                    if (fn->addTrace)
+                        addErrorTrace(e, pos, "while calling the '%1%' builtin", fn->name);
                    throw;
                }

@@ -1906,7 +1957,7 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
 }


-void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx)
+void EvalState::concatLists(Value & v, size_t nrLists, Value * const * lists, const PosIdx pos, std::string_view errorCtx)
 {
    nrListConcats++;

@@ -1924,14 +1975,15 @@ void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const Po
        return;
    }

-    mkList(v, len);
-    auto out = v.listElems();
+    auto list = buildList(len);
+    auto out = list.elems;
    for (size_t n = 0, pos = 0; n < nrLists; ++n) {
        auto l = lists[n]->listSize();
        if (l)
            memcpy(out + pos, lists[n]->listElems(), l * sizeof(Value *));
        pos += l;
    }
+    v.mkList(list);
 }


@@ -2737,9 +2789,12 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<Sta

 Expr * EvalState::parseExprFromString(std::string s_, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
-    auto s = make_ref<std::string>(std::move(s_));
-    s->append("\0\0", 2);
-    return parse(s->data(), s->size(), Pos::String{.source = s}, basePath, staticEnv);
+    // NOTE this method (and parseStdin) must take care to *fully copy* their input
+    // into their respective Pos::Origin until the parser stops overwriting its input
+    // data.
+    auto s = make_ref<std::string>(s_);
+    s_.append("\0\0", 2);
+    return parse(s_.data(), s_.size(), Pos::String{.source = s}, basePath, staticEnv);
 }


@@ -2751,12 +2806,15 @@ Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath

 Expr * EvalState::parseStdin()
 {
+    // NOTE this method (and parseExprFromString) must take care to *fully copy* their
+    // input into their respective Pos::Origin until the parser stops overwriting its
+    // input data.
    //Activity act(*logger, lvlTalkative, "parsing standard input");
    auto buffer = drainFD(0);
    // drainFD should have left some extra space for terminators
    buffer.append("\0\0", 2);
-    auto s = make_ref<std::string>(std::move(buffer));
-    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, rootPath("."), staticBaseEnv);
+    auto s = make_ref<std::string>(buffer);
+    return parse(buffer.data(), buffer.size(), Pos::Stdin{.source = s}, rootPath("."), staticBaseEnv);
 }


@@ -69,6 +69,13 @@ struct PrimOp
     */
    const char * doc = nullptr;

+    /**
+     * Add a trace item, `while calling the '<name>' builtin`
+     *
+     * This is used to remove the redundant item for `builtins.addErrorContext`.
+     */
+    bool addTrace = true;
+
    /**
     * Implementation of the primop.
     */
@@ -153,6 +160,7 @@ struct DebugTrace {
    bool isError;
 };

+
 class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
@@ -185,6 +193,36 @@ public:
     */
    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 for the root filesystem.
     */
@@ -222,6 +260,7 @@ public:
     */
    ReplExitStatus (* debugRepl)(ref<EvalState> es, const ValMap & extraEnv);
    bool debugStop;
+    bool inDebugger = false;
    int trylevel;
    std::list<DebugTrace> debugTraces;
    std::map<const Expr*, const std::shared_ptr<const StaticEnv>> exprEnvs;
@@ -432,10 +471,12 @@ public:
    std::string_view forceString(Value & v, NixStringContext & context, const PosIdx pos, std::string_view errorCtx);
    std::string_view forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx);

+    template<typename... Args>
    [[gnu::noinline]]
-    void addErrorTrace(Error & e, const char * s, const std::string & s2) const;
+    void addErrorTrace(Error & e, const Args & ... formatArgs) const;
+    template<typename... Args>
    [[gnu::noinline]]
-    void addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2, bool frame = false) const;
+    void addErrorTrace(Error & e, const PosIdx pos, const Args & ... formatArgs) const;

 public:
    /**
@@ -611,7 +652,16 @@ public:
        return BindingsBuilder(*this, allocBindings(capacity));
    }

-    void mkList(Value & v, size_t length);
+    ListBuilder buildList(size_t size)
+    {
+        return ListBuilder(*this, size);
+    }
+
+    /**
+     * Return a boolean `Value *` without allocating.
+     */
+    Value *getBool(bool b);
+
    void mkThunk_(Value & v, Expr * expr);
    void mkPos(Value & v, PosIdx pos);

@@ -658,7 +708,7 @@ public:
        const SingleDerivedPath & p,
        Value & v);

-    void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
+    void concatLists(Value & v, size_t nrLists, Value * const * lists, const PosIdx pos, std::string_view errorCtx);

    /**
     * Print statistics, if enabled.
@@ -752,6 +802,7 @@ private:
    friend void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v);

    friend struct Value;
+    friend class ListBuilder;
 };

 struct DebugTraceStacker {
@@ -139,7 +139,7 @@ static FlakeInput parseFlakeInput(EvalState & state,
                        attrs.emplace(state.symbols[attr.name], Explicit<bool> { attr.value->boolean });
                        break;
                    case nInt:
-                        attrs.emplace(state.symbols[attr.name], (long unsigned int)attr.value->integer);
+                        attrs.emplace(state.symbols[attr.name], (long unsigned int) attr.value->integer);
                        break;
                    default:
                        if (attr.name == state.symbols.create("publicKeys")) {
@@ -202,43 +202,27 @@ static std::map<FlakeId, FlakeInput> parseFlakeInputs(
    return inputs;
 }

-static Flake getFlake(
+static Flake readFlake(
    EvalState & state,
    const FlakeRef & originalRef,
-    bool allowLookup,
-    FlakeCache & flakeCache,
-    InputPath lockRootPath)
+    const FlakeRef & resolvedRef,
+    const FlakeRef & lockedRef,
+    const SourcePath & rootDir,
+    const InputPath & lockRootPath)
 {
-    auto [storePath, resolvedRef, lockedRef] = fetchOrSubstituteTree(
-        state, originalRef, allowLookup, flakeCache);
+    auto flakePath = rootDir / CanonPath(resolvedRef.subdir) / "flake.nix";

-    // We need to guard against symlink attacks, but before we start doing
-    // filesystem operations we should make sure there's a flake.nix in the
-    // first place.
-    auto unsafeFlakeDir = state.store->toRealPath(storePath) + "/" + lockedRef.subdir;
-    auto unsafeFlakeFile = unsafeFlakeDir + "/flake.nix";
-    if (!pathExists(unsafeFlakeFile))
-        throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);
-
-    // Guard against symlink attacks.
-    auto flakeDir = canonPath(unsafeFlakeDir, true);
-    auto flakeFile = canonPath(flakeDir + "/flake.nix", true);
-    if (!isInDir(flakeFile, state.store->toRealPath(storePath)))
-        throw Error("'flake.nix' file of flake '%s' escapes from '%s'",
-            lockedRef, state.store->printStorePath(storePath));
+    // NOTE evalFile forces vInfo to be an attrset because mustBeTrivial is true.
+    Value vInfo;
+    state.evalFile(flakePath, vInfo, true);

    Flake flake {
        .originalRef = originalRef,
        .resolvedRef = resolvedRef,
        .lockedRef = lockedRef,
-        .storePath = storePath,
+        .path = flakePath,
    };

-    Value vInfo;
-    state.evalFile(state.rootPath(CanonPath(flakeFile)), vInfo, true); // FIXME: symlink attack
-
-    expectType(state, nAttrs, vInfo, state.positions.add({state.rootPath(CanonPath(flakeFile))}, 1, 1));
-
    if (auto description = vInfo.attrs->get(state.sDescription)) {
        expectType(state, nString, *description->value, description->pos);
        flake.description = description->value->c_str();
@@ -247,7 +231,7 @@ static Flake getFlake(
    auto sInputs = state.symbols.create("inputs");

    if (auto inputs = vInfo.attrs->get(sInputs))
-        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakeDir, lockRootPath);
+        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakePath.parent().path.abs(), lockRootPath); // FIXME

    auto sOutputs = state.symbols.create("outputs");

@@ -264,7 +248,7 @@ static Flake getFlake(
        }

    } else
-        throw Error("flake '%s' lacks attribute 'outputs'", lockedRef);
+        throw Error("flake '%s' lacks attribute 'outputs'", resolvedRef);

    auto sNixConfig = state.symbols.create("nixConfig");

@@ -281,7 +265,7 @@ static Flake getFlake(
                NixStringContext emptyContext = {};
                flake.config.settings.emplace(
                    state.symbols[setting.name],
-                    state.coerceToString(setting.pos, *setting.value, emptyContext, "", false, true, true) .toOwned());
+                    state.coerceToString(setting.pos, *setting.value, emptyContext, "", false, true, true).toOwned());
            }
            else if (setting.value->type() == nInt)
                flake.config.settings.emplace(
@@ -313,12 +297,25 @@ static Flake getFlake(
            attr.name != sOutputs &&
            attr.name != sNixConfig)
            throw Error("flake '%s' has an unsupported attribute '%s', at %s",
-                lockedRef, state.symbols[attr.name], state.positions[attr.pos]);
+                resolvedRef, state.symbols[attr.name], state.positions[attr.pos]);
    }

    return flake;
 }

+static Flake getFlake(
+    EvalState & state,
+    const FlakeRef & originalRef,
+    bool allowLookup,
+    FlakeCache & flakeCache,
+    InputPath lockRootPath)
+{
+    auto [storePath, resolvedRef, lockedRef] = fetchOrSubstituteTree(
+        state, originalRef, allowLookup, flakeCache);
+
+    return readFlake(state, originalRef, resolvedRef, lockedRef, state.rootPath(state.store->toRealPath(storePath)), lockRootPath);
+}
+
 Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup, FlakeCache & flakeCache)
 {
    return getFlake(state, originalRef, allowLookup, flakeCache, {});
@@ -330,6 +327,13 @@ Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup
    return getFlake(state, originalRef, allowLookup, flakeCache);
 }

+static LockFile readLockFile(const SourcePath & lockFilePath)
+{
+    return lockFilePath.pathExists()
+        ? LockFile(lockFilePath.readFile(), fmt("%s", lockFilePath))
+        : LockFile();
+}
+
 /* Compute an in-memory lock file for the specified top-level flake,
   and optionally write it to file, if the flake is writable. */
 LockedFlake lockFlake(
@@ -355,17 +359,16 @@ LockedFlake lockFlake(
            throw Error("reference lock file was provided, but the `allow-dirty` setting is set to false");
        }

-        // FIXME: symlink attack
-        auto oldLockFile = LockFile::read(
+        auto oldLockFile = readLockFile(
            lockFlags.referenceLockFilePath.value_or(
-                state.store->toRealPath(flake.storePath) + "/" + flake.lockedRef.subdir + "/flake.lock"));
+                flake.lockFilePath()));

        debug("old lock file: %s", oldLockFile);

        std::map<InputPath, FlakeInput> overrides;
        std::set<InputPath> explicitCliOverrides;
        std::set<InputPath> overridesUsed, updatesUsed;
-        std::map<ref<Node>, StorePath> nodePaths;
+        std::map<ref<Node>, SourcePath> nodePaths;

        for (auto & i : lockFlags.inputOverrides) {
            overrides.insert_or_assign(i.first, FlakeInput { .ref = i.second });
@@ -538,7 +541,7 @@ LockedFlake lockFlake(

                        if (mustRefetch) {
                            auto inputFlake = getFlake(state, oldLock->lockedRef, false, flakeCache, inputPath);
-                            nodePaths.emplace(childNode, inputFlake.storePath);
+                            nodePaths.emplace(childNode, inputFlake.path.parent());
                            computeLocks(inputFlake.inputs, childNode, inputPath, oldLock, lockRootPath, parentPath, false);
                        } else {
                            computeLocks(fakeInputs, childNode, inputPath, oldLock, lockRootPath, parentPath, true);
@@ -587,13 +590,12 @@ LockedFlake lockFlake(
                               flake. Also, unless we already have this flake
                               in the top-level lock file, use this flake's
                               own lock file. */
-                            nodePaths.emplace(childNode, inputFlake.storePath);
+                            nodePaths.emplace(childNode, inputFlake.path.parent());
                            computeLocks(
                                inputFlake.inputs, childNode, inputPath,
                                oldLock
                                ? std::dynamic_pointer_cast<const Node>(oldLock)
-                                : LockFile::read(
-                                    state.store->toRealPath(inputFlake.storePath) + "/" + inputFlake.lockedRef.subdir + "/flake.lock").root.get_ptr(),
+                                : readLockFile(inputFlake.lockFilePath()).root.get_ptr(),
                                oldLock ? lockRootPath : inputPath,
                                localPath,
                                false);
@@ -605,7 +607,7 @@ LockedFlake lockFlake(

                            auto childNode = make_ref<LockedNode>(lockedRef, ref, false);

-                            nodePaths.emplace(childNode, storePath);
+                            nodePaths.emplace(childNode, state.rootPath(state.store->toRealPath(storePath)));

                            node->inputs.insert_or_assign(id, childNode);
                        }
@@ -619,9 +621,9 @@ LockedFlake lockFlake(
        };

        // Bring in the current ref for relative path resolution if we have it
-        auto parentPath = canonPath(state.store->toRealPath(flake.storePath) + "/" + flake.lockedRef.subdir, true);
+        auto parentPath = flake.path.parent().path.abs();

-        nodePaths.emplace(newLockFile.root, flake.storePath);
+        nodePaths.emplace(newLockFile.root, flake.path.parent());

        computeLocks(
            flake.inputs,
@@ -746,13 +748,15 @@ void callFlake(EvalState & state,

    auto overrides = state.buildBindings(lockedFlake.nodePaths.size());

-    for (auto & [node, storePath] : lockedFlake.nodePaths) {
+    for (auto & [node, sourcePath] : lockedFlake.nodePaths) {
        auto override = state.buildBindings(2);

        auto & vSourceInfo = override.alloc(state.symbols.create("sourceInfo"));

        auto lockedNode = node.dynamic_pointer_cast<const LockedNode>();

+        auto [storePath, subdir] = state.store->toStorePath(sourcePath.path.abs());
+
        emitTreeAttrs(
            state,
            storePath,
@@ -766,7 +770,7 @@ void callFlake(EvalState & state,

        override
            .alloc(state.symbols.create("dir"))
-            .mkString(lockedNode ? lockedNode->lockedRef.subdir : lockedFlake.flake.lockedRef.subdir);
+            .mkString(CanonPath(subdir).rel());

        overrides.alloc(state.symbols.create(key->second)).mkAttrs(override);
    }
@@ -921,18 +925,17 @@ static RegisterPrimOp r4({

 }

-Fingerprint LockedFlake::getFingerprint() const
+std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store) const
 {
+    if (lockFile.isUnlocked()) return std::nullopt;
+
+    auto fingerprint = flake.lockedRef.input.getFingerprint(store);
+    if (!fingerprint) return std::nullopt;
+
    // FIXME: as an optimization, if the flake contains a lock file
    // and we haven't changed it, then it's sufficient to use
    // flake.sourceInfo.storePath for the fingerprint.
-    return hashString(HashAlgorithm::SHA256,
-        fmt("%s;%s;%d;%d;%s",
-            flake.storePath.to_string(),
-            flake.lockedRef.subdir,
-            flake.lockedRef.input.getRevCount().value_or(0),
-            flake.lockedRef.input.getLastModified().value_or(0),
-            lockFile));
+    return hashString(HashAlgorithm::SHA256, fmt("%s;%s;%s", *fingerprint, flake.lockedRef.subdir, lockFile));
 }

 Flake::~Flake() { }
@@ -77,18 +77,27 @@ struct Flake
     * the specific local store result of invoking the fetcher
     */
    FlakeRef lockedRef;
+    /**
+     * The path of `flake.nix`.
+     */
+    SourcePath path;
    /**
     * pretend that 'lockedRef' is dirty
     */
    bool forceDirty = false;
    std::optional<std::string> description;
-    StorePath storePath;
    FlakeInputs inputs;
    /**
     * 'nixConfig' attribute
     */
    ConfigFile config;
+
    ~Flake();
+
+    SourcePath lockFilePath()
+    {
+        return path.parent() / "flake.lock";
+    }
 };

 Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool allowLookup);
@@ -104,13 +113,13 @@ struct LockedFlake
    LockFile lockFile;

    /**
-     * Store paths of nodes that have been fetched in
+     * Source tree accessors for nodes that have been fetched in
     * lockFlake(); in particular, the root node and the overriden
     * inputs.
     */
-    std::map<ref<Node>, StorePath> nodePaths;
+    std::map<ref<Node>, SourcePath> nodePaths;

-    Fingerprint getFingerprint() const;
+    std::optional<Fingerprint> getFingerprint(ref<Store> store) const;
 };

 struct LockFlags
@@ -165,7 +174,7 @@ struct LockFlags
    /**
     * The path to a lock file to read instead of the `flake.lock` file in the top-level flake
     */
-    std::optional<std::string> referenceLockFilePath;
+    std::optional<SourcePath> referenceLockFilePath;

    /**
     * The path to a lock file to write to instead of the `flake.lock` file in the top-level flake
@@ -102,6 +102,19 @@ std::pair<FlakeRef, std::string> parsePathFlakeRefWithFragment(

        if (isFlake) {

+            if (!S_ISDIR(lstat(path).st_mode)) {
+                if (baseNameOf(path) == "flake.nix") {
+                    // Be gentle with people who accidentally write `/foo/bar/flake.nix` instead of `/foo/bar`
+                    warn(
+                        "Path '%s' should point at the directory containing the 'flake.nix' file, not the file itself. "
+                        "Pretending that you meant '%s'"
+                        , path, dirOf(path));
+                    path = dirOf(path);
+                } else {
+                    throw BadURL("path '%s' is not a flake (because it's not a directory)", path);
+                }
+            }
+
            if (!allowMissing && !pathExists(path + "/flake.nix")){
                notice("path '%s' does not contain a 'flake.nix', searching up",path);

@@ -124,9 +137,6 @@ std::pair<FlakeRef, std::string> parsePathFlakeRefWithFragment(
                    throw BadURL("could not find a flake.nix file");
            }

-            if (!S_ISDIR(lstat(path).st_mode))
-                throw BadURL("path '%s' is not a flake (because it's not a directory)", path);
-
            if (!allowMissing && !pathExists(path + "/flake.nix"))
                throw BadURL("path '%s' is not a flake (because it doesn't contain a 'flake.nix' file)", path);

@@ -274,7 +284,7 @@ FlakeRef FlakeRef::fromAttrs(const fetchers::Attrs & attrs)

 std::pair<StorePath, FlakeRef> FlakeRef::fetchTree(ref<Store> store) const
 {
-    auto [storePath, lockedInput] = input.fetch(store);
+    auto [storePath, lockedInput] = input.fetchToStore(store);
    return {std::move(storePath), FlakeRef(std::move(lockedInput), subdir)};
 }

@@ -84,8 +84,10 @@ std::shared_ptr<Node> LockFile::findInput(const InputPath & path)
    return doFind(root, path, visited);
 }

-LockFile::LockFile(const nlohmann::json & json, const Path & path)
+LockFile::LockFile(std::string_view contents, std::string_view path)
 {
+    auto json = nlohmann::json::parse(contents);
+
    auto version = json.value("version", 0);
    if (version < 5 || version > 7)
        throw Error("lock file '%s' has unsupported version %d", path, version);
@@ -203,12 +205,6 @@ std::pair<std::string, LockFile::KeyMap> LockFile::to_string() const
    return {json.dump(2), std::move(nodeKeys)};
 }

-LockFile LockFile::read(const Path & path)
-{
-    if (!pathExists(path)) return LockFile();
-    return LockFile(nlohmann::json::parse(readFile(path)), path);
-}
-
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
 {
    stream << lockFile.toJSON().first.dump(2);
@@ -55,7 +55,7 @@ struct LockFile
    ref<Node> root = make_ref<Node>();

    LockFile() {};
-    LockFile(const nlohmann::json & json, const Path & path);
+    LockFile(std::string_view contents, std::string_view path);

    typedef std::map<ref<const Node>, std::string> KeyMap;

@@ -63,8 +63,6 @@ struct LockFile

    std::pair<std::string, KeyMap> to_string() const;

-    static LockFile read(const Path & path);
-
    /**
     * Check whether this lock file has any unlocked inputs. If so,
     * return one.
@@ -5,13 +5,12 @@
 namespace nix {

 static const std::string attributeNamePattern("[a-zA-Z0-9_-]+");
-static const std::regex lastAttributeRegex("(?:" + attributeNamePattern + "\\.)*(?!default)(" + attributeNamePattern +")(\\^.*)?");
+static const std::regex lastAttributeRegex("^((?:" + attributeNamePattern + "\\.)*)(" + attributeNamePattern +")(\\^.*)?$");
 static const std::string pathSegmentPattern("[a-zA-Z0-9_-]+");
 static const std::regex lastPathSegmentRegex(".*/(" + pathSegmentPattern +")");
 static const std::regex secondPathSegmentRegex("(?:" + pathSegmentPattern + ")/(" + pathSegmentPattern +")(?:/.*)?");
 static const std::regex gitProviderRegex("github|gitlab|sourcehut");
 static const std::regex gitSchemeRegex("git($|\\+.*)");
-static const std::regex defaultOutputRegex(".*\\.default($|\\^.*)");

 std::optional<std::string> getNameFromURL(const ParsedURL & url)
 {
@@ -22,8 +21,11 @@ std::optional<std::string> getNameFromURL(const ParsedURL & url)
        return url.query.at("dir");

    /* If the fragment isn't a "default" and contains two attribute elements, use the last one */
-    if (std::regex_match(url.fragment, match, lastAttributeRegex))
-        return match.str(1);
+    if (std::regex_match(url.fragment, match, lastAttributeRegex)
+        && match.str(1) != "defaultPackage."
+        && match.str(2) != "default") {
+        return match.str(2);
+    }

    /* If this is a github/gitlab/sourcehut flake, use the repo name */
    if (std::regex_match(url.scheme, gitProviderRegex) && std::regex_match(url.path, match, secondPathSegmentRegex))
@@ -33,10 +35,6 @@ std::optional<std::string> getNameFromURL(const ParsedURL & url)
    if (std::regex_match(url.scheme, gitSchemeRegex) && std::regex_match(url.path, match, lastPathSegmentRegex))
        return match.str(1);

-    /* If everything failed but there is a non-default fragment, use it in full */
-    if (!url.fragment.empty() && !std::regex_match(url.fragment, defaultOutputRegex))
-        return url.fragment;
-
    /* If there is no fragment, take the last element of the path */
    if (std::regex_match(url.path, match, lastPathSegmentRegex))
        return match.str(1);
@@ -57,11 +57,10 @@ class JSONSax : nlohmann::json_sax<json> {
        ValueVector values;
        std::unique_ptr<JSONState> resolve(EvalState & state) override
        {
-            Value & v = parent->value(state);
-            state.mkList(v, values.size());
-            for (size_t n = 0; n < values.size(); ++n) {
-                v.listElems()[n] = values[n];
-            }
+            auto list = state.buildList(values.size());
+            for (const auto & [n, v2] : enumerate(list))
+                v2 = values[n];
+            parent->value(state).mkList(list);
            return std::move(parent);
        }
        void add() override {
@@ -33,33 +33,16 @@ namespace nix {

 static void initLoc(YYLTYPE * loc)
 {
-    loc->first_line = loc->last_line = 1;
-    loc->first_column = loc->last_column = 1;
+    loc->first_line = loc->last_line = 0;
+    loc->first_column = loc->last_column = 0;
 }

 static void adjustLoc(YYLTYPE * loc, const char * s, size_t len)
 {
    loc->stash();

-    loc->first_line = loc->last_line;
    loc->first_column = loc->last_column;
-
-    for (size_t i = 0; i < len; i++) {
-       switch (*s++) {
-       case '\r':
-           if (*s == '\n') { /* cr/lf */
-               i++;
-               s++;
-           }
-           /* fall through */
-       case '\n':
-           ++loc->last_line;
-           loc->last_column = 1;
-           break;
-       default:
-           ++loc->last_column;
-       }
-    }
+    loc->last_column += len;
 }


@@ -94,6 +77,9 @@ static StringToken unescapeStr(SymbolTable & symbols, char * s, size_t length)

 }

+// yacc generates code that uses unannotated fallthrough.
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+
 #define YY_USER_INIT initLoc(yylloc)
 #define YY_USER_ACTION adjustLoc(yylloc, yytext, yyleng);

@@ -11,8 +11,11 @@ libexpr_SOURCES := \
  $(wildcard $(d)/flake/*.cc) \
  $(d)/lexer-tab.cc \
  $(d)/parser-tab.cc
+# Not just for this library itself, but also for downstream libraries using this library

-libexpr_CXXFLAGS += -I src/libutil -I src/libstore -I src/libfetchers -I src/libmain -I src/libexpr
+INCLUDE_libexpr := -I $(d)
+
+libexpr_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers) -I src/libmain $(INCLUDE_libexpr)

 libexpr_LIBS = libutil libstore libfetchers

@@ -70,10 +70,8 @@ void ExprOpHasAttr::show(const SymbolTable & symbols, std::ostream & str) const
    str << ") ? " << showAttrPath(symbols, attrPath) << ")";
 }

-void ExprAttrs::show(const SymbolTable & symbols, std::ostream & str) const
+void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) const
 {
-    if (recursive) str << "rec ";
-    str << "{ ";
    typedef const decltype(attrs)::value_type * Attr;
    std::vector<Attr> sorted;
    for (auto & i : attrs) sorted.push_back(&i);
@@ -81,10 +79,37 @@ void ExprAttrs::show(const SymbolTable & symbols, std::ostream & str) const
        std::string_view sa = symbols[a->first], sb = symbols[b->first];
        return sa < sb;
    });
+    std::vector<Symbol> inherits;
+    std::map<ExprInheritFrom *, std::vector<Symbol>> inheritsFrom;
    for (auto & i : sorted) {
-        if (i->second.inherited)
-            str << "inherit " << symbols[i->first] << " " << "; ";
-        else {
+        switch (i->second.kind) {
+        case AttrDef::Kind::Plain:
+            break;
+        case AttrDef::Kind::Inherited:
+            inherits.push_back(i->first);
+            break;
+        case AttrDef::Kind::InheritedFrom: {
+            auto & select = dynamic_cast<ExprSelect &>(*i->second.e);
+            auto & from = dynamic_cast<ExprInheritFrom &>(*select.e);
+            inheritsFrom[&from].push_back(i->first);
+            break;
+        }
+        }
+    }
+    if (!inherits.empty()) {
+        str << "inherit";
+        for (auto sym : inherits) str << " " << symbols[sym];
+        str << "; ";
+    }
+    for (const auto & [from, syms] : inheritsFrom) {
+        str << "inherit (";
+        (*inheritFromExprs)[from->displ]->show(symbols, str);
+        str << ")";
+        for (auto sym : syms) str << " " << symbols[sym];
+        str << "; ";
+    }
+    for (auto & i : sorted) {
+        if (i->second.kind == AttrDef::Kind::Plain) {
            str << symbols[i->first] << " = ";
            i->second.e->show(symbols, str);
            str << "; ";
@@ -97,6 +122,13 @@ void ExprAttrs::show(const SymbolTable & symbols, std::ostream & str) const
        i.valueExpr->show(symbols, str);
        str << "; ";
    }
+}
+
+void ExprAttrs::show(const SymbolTable & symbols, std::ostream & str) const
+{
+    if (recursive) str << "rec ";
+    str << "{ ";
+    showBindings(symbols, str);
    str << "}";
 }

@@ -117,7 +149,10 @@ void ExprLambda::show(const SymbolTable & symbols, std::ostream & str) const
    if (hasFormals()) {
        str << "{ ";
        bool first = true;
-        for (auto & i : formals->formals) {
+        // the natural Symbol ordering is by creation time, which can lead to the
+        // same expression being printed in two different ways depending on its
+        // context. always use lexicographic ordering to avoid this.
+        for (auto & i : formals->lexicographicOrder(symbols)) {
            if (first) first = false; else str << ", ";
            str << symbols[i.name];
            if (i.def) {
@@ -152,15 +187,7 @@ void ExprCall::show(const SymbolTable & symbols, std::ostream & str) const
 void ExprLet::show(const SymbolTable & symbols, std::ostream & str) const
 {
    str << "(let ";
-    for (auto & i : attrs->attrs)
-        if (i.second.inherited) {
-            str << "inherit " << symbols[i.first] << "; ";
-        }
-        else {
-            str << symbols[i.first] << " = ";
-            i.second.e->show(symbols, str);
-            str << "; ";
-        }
+    attrs->showBindings(symbols, str);
    str << "in ";
    body->show(symbols, str);
    str << ")";
@@ -305,6 +332,12 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
    this->level = withLevel;
 }

+void ExprInheritFrom::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
+{
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+}
+
 void ExprSelect::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
    if (es.debugRepl)
@@ -328,22 +361,47 @@ void ExprOpHasAttr::bindVars(EvalState & es, const std::shared_ptr<const StaticE
            i.expr->bindVars(es, env);
 }

+std::shared_ptr<const StaticEnv> ExprAttrs::bindInheritSources(
+    EvalState & es, const std::shared_ptr<const StaticEnv> & env)
+{
+    if (!inheritFromExprs)
+        return nullptr;
+
+    // the inherit (from) source values are inserted into an env of its own, which
+    // does not introduce any variable names.
+    // analysis must see an empty env, or an env that contains only entries with
+    // otherwise unused names to not interfere with regular names. the parser
+    // has already filled all exprs that access this env with appropriate level
+    // and displacement, and nothing else is allowed to access it. ideally we'd
+    // not even *have* an expr that grabs anything from this env since it's fully
+    // invisible, but the evaluator does not allow for this yet.
+    auto inner = std::make_shared<StaticEnv>(nullptr, env.get(), 0);
+    for (auto from : *inheritFromExprs)
+        from->bindVars(es, env);
+
+    return inner;
+}
+
 void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
    if (es.debugRepl)
        es.exprEnvs.insert(std::make_pair(this, env));

    if (recursive) {
-        auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), recursive ? attrs.size() : 0);
+        auto newEnv = [&] () -> std::shared_ptr<const StaticEnv> {
+            auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs.size());

-        Displacement displ = 0;
-        for (auto & i : attrs)
-            newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+            Displacement displ = 0;
+            for (auto & i : attrs)
+                newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+            return newEnv;
+        }();

        // No need to sort newEnv since attrs is in sorted order.

+        auto inheritFromEnv = bindInheritSources(es, newEnv);
        for (auto & i : attrs)
-            i.second.e->bindVars(es, i.second.inherited ? env : newEnv);
+            i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, inheritFromEnv));

        for (auto & i : dynamicAttrs) {
            i.nameExpr->bindVars(es, newEnv);
@@ -351,8 +409,10 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
        }
    }
    else {
+        auto inheritFromEnv = bindInheritSources(es, env);
+
        for (auto & i : attrs)
-            i.second.e->bindVars(es, env);
+            i.second.e->bindVars(es, i.second.chooseByKind(env, env, inheritFromEnv));

        for (auto & i : dynamicAttrs) {
            i.nameExpr->bindVars(es, env);
@@ -409,16 +469,20 @@ void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &

 void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs->attrs.size());
+    auto newEnv = [&] () -> std::shared_ptr<const StaticEnv> {
+        auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs->attrs.size());

-    Displacement displ = 0;
-    for (auto & i : attrs->attrs)
-        newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+        Displacement displ = 0;
+        for (auto & i : attrs->attrs)
+            newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+        return newEnv;
+    }();

    // No need to sort newEnv since attrs->attrs is in sorted order.

+    auto inheritFromEnv = attrs->bindInheritSources(es, newEnv);
    for (auto & i : attrs->attrs)
-        i.second.e->bindVars(es, i.second.inherited ? env : newEnv);
+        i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, inheritFromEnv));

    if (es.debugRepl)
        es.exprEnvs.insert(std::make_pair(this, newEnv));
@@ -519,6 +583,39 @@ std::string ExprLambda::showNamePos(const EvalState & state) const



+/* Position table. */
+
+Pos PosTable::operator[](PosIdx p) const
+{
+    auto origin = resolve(p);
+    if (!origin)
+        return {};
+
+    const auto offset = origin->offsetOf(p);
+
+    Pos result{0, 0, origin->origin};
+    auto lines = this->lines.lock();
+    auto linesForInput = (*lines)[origin->offset];
+
+    if (linesForInput.empty()) {
+        auto source = result.getSource().value_or("");
+        const char * begin = source.data();
+        for (Pos::LinesIterator it(source), end; it != end; it++)
+            linesForInput.push_back(it->data() - begin);
+        if (linesForInput.empty())
+            linesForInput.push_back(0);
+    }
+    // as above: the first line starts at byte 0 and is always present
+    auto lineStartOffset = std::prev(
+        std::upper_bound(linesForInput.begin(), linesForInput.end(), offset));
+
+    result.line = 1 + (lineStartOffset - linesForInput.begin());
+    result.column = 1 + (offset - *lineStartOffset);
+    return result;
+}
+
+
+
 /* Symbol table. */

 size_t SymbolTable::totalSize() const
@@ -7,7 +7,6 @@
 #include "value.hh"
 #include "symbol-table.hh"
 #include "error.hh"
-#include "chunked-vector.hh"
 #include "position.hh"
 #include "eval-error.hh"
 #include "pos-idx.hh"
@@ -135,6 +134,23 @@ struct ExprVar : Expr
    COMMON_METHODS
 };

+/**
+ * A pseudo-expression for the purpose of evaluating the `from` expression in `inherit (from)` syntax.
+ * Unlike normal variable references, the displacement is set during parsing, and always refers to
+ * `ExprAttrs::inheritFromExprs` (by itself or in `ExprLet`), whose values are put into their own `Env`.
+ */
+struct ExprInheritFrom : ExprVar
+{
+    ExprInheritFrom(PosIdx pos, Displacement displ): ExprVar(pos, {})
+    {
+        this->level = 0;
+        this->displ = displ;
+        this->fromWith = nullptr;
+    }
+
+    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
+};
+
 struct ExprSelect : Expr
 {
    PosIdx pos;
@@ -160,16 +176,40 @@ struct ExprAttrs : Expr
    bool recursive;
    PosIdx pos;
    struct AttrDef {
-        bool inherited;
+        enum class Kind {
+            /** `attr = expr;` */
+            Plain,
+            /** `inherit attr1 attrn;` */
+            Inherited,
+            /** `inherit (expr) attr1 attrn;` */
+            InheritedFrom,
+        };
+
+        Kind kind;
        Expr * e;
        PosIdx pos;
        Displacement displ; // displacement
-        AttrDef(Expr * e, const PosIdx & pos, bool inherited=false)
-            : inherited(inherited), e(e), pos(pos) { };
+        AttrDef(Expr * e, const PosIdx & pos, Kind kind = Kind::Plain)
+            : kind(kind), e(e), pos(pos) { };
        AttrDef() { };
+
+        template<typename T>
+        const T & chooseByKind(const T & plain, const T & inherited, const T & inheritedFrom) const
+        {
+            switch (kind) {
+            case Kind::Plain:
+                return plain;
+            case Kind::Inherited:
+                return inherited;
+            default:
+            case Kind::InheritedFrom:
+                return inheritedFrom;
+            }
+        }
    };
    typedef std::map<Symbol, AttrDef> AttrDefs;
    AttrDefs attrs;
+    std::unique_ptr<std::vector<Expr *>> inheritFromExprs;
    struct DynamicAttrDef {
        Expr * nameExpr, * valueExpr;
        PosIdx pos;
@@ -182,6 +222,11 @@ struct ExprAttrs : Expr
    ExprAttrs() : recursive(false) { };
    PosIdx getPos() const override { return pos; }
    COMMON_METHODS
+
+    std::shared_ptr<const StaticEnv> bindInheritSources(
+        EvalState & es, const std::shared_ptr<const StaticEnv> & env);
+    Env * buildInheritFromEnv(EvalState & state, Env & up);
+    void showBindings(const SymbolTable & symbols, std::ostream & str) const;
 };

 struct ExprList : Expr
@@ -24,20 +24,15 @@ struct ParserLocation
    int last_line, last_column;

    // backup to recover from yyless(0)
-    int stashed_first_line, stashed_first_column;
-    int stashed_last_line, stashed_last_column;
+    int stashed_first_column, stashed_last_column;

    void stash() {
-        stashed_first_line = first_line;
        stashed_first_column = first_column;
-        stashed_last_line = last_line;
        stashed_last_column = last_column;
    }

    void unstash() {
-        first_line = stashed_first_line;
        first_column = stashed_first_column;
-        last_line = stashed_last_line;
        last_column = stashed_last_column;
    }
 };
@@ -89,7 +84,7 @@ inline void ParserState::addAttr(ExprAttrs * attrs, AttrPath && attrPath, Expr *
        if (i->symbol) {
            ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
            if (j != attrs->attrs.end()) {
-                if (!j->second.inherited) {
+                if (j->second.kind != ExprAttrs::AttrDef::Kind::Inherited) {
                    ExprAttrs * attrs2 = dynamic_cast<ExprAttrs *>(j->second.e);
                    if (!attrs2) dupAttr(attrPath, pos, j->second.pos);
                    attrs = attrs2;
@@ -118,13 +113,24 @@ inline void ParserState::addAttr(ExprAttrs * attrs, AttrPath && attrPath, Expr *
            auto ae = dynamic_cast<ExprAttrs *>(e);
            auto jAttrs = dynamic_cast<ExprAttrs *>(j->second.e);
            if (jAttrs && ae) {
+                if (ae->inheritFromExprs && !jAttrs->inheritFromExprs)
+                    jAttrs->inheritFromExprs = std::make_unique<std::vector<Expr *>>();
                for (auto & ad : ae->attrs) {
                    auto j2 = jAttrs->attrs.find(ad.first);
                    if (j2 != jAttrs->attrs.end()) // Attr already defined in iAttrs, error.
                        dupAttr(ad.first, j2->second.pos, ad.second.pos);
                    jAttrs->attrs.emplace(ad.first, ad.second);
+                    if (ad.second.kind == ExprAttrs::AttrDef::Kind::InheritedFrom) {
+                        auto & sel = dynamic_cast<ExprSelect &>(*ad.second.e);
+                        auto & from = dynamic_cast<ExprInheritFrom &>(*sel.e);
+                        from.displ += jAttrs->inheritFromExprs->size();
+                    }
                }
                jAttrs->dynamicAttrs.insert(jAttrs->dynamicAttrs.end(), ae->dynamicAttrs.begin(), ae->dynamicAttrs.end());
+                if (ae->inheritFromExprs) {
+                    jAttrs->inheritFromExprs->insert(jAttrs->inheritFromExprs->end(),
+                        ae->inheritFromExprs->begin(), ae->inheritFromExprs->end());
+                }
            } else {
                dupAttr(attrPath, pos, j->second.pos);
            }
@@ -265,7 +271,7 @@ inline Expr * ParserState::stripIndentation(const PosIdx pos,

 inline PosIdx ParserState::at(const ParserLocation & loc)
 {
-    return positions.add(origin, loc.first_line, loc.first_column);
+    return positions.add(origin, loc.first_column);
 }

 }
@@ -64,6 +64,10 @@ using namespace nix;

 void yyerror(YYLTYPE * loc, yyscan_t scanner, ParserState * state, const char * error)
 {
+    if (std::string_view(error).starts_with("syntax error, unexpected end of file")) {
+        loc->first_column = loc->last_column;
+        loc->first_line = loc->last_line;
+    }
    throw ParseError({
        .msg = HintFmt(error),
        .pos = state->positions[state->at(*loc)]
@@ -87,6 +91,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParserState * state, const char *
  nix::StringToken uri;
  nix::StringToken str;
  std::vector<nix::AttrName> * attrNames;
+  std::vector<std::pair<nix::AttrName, nix::PosIdx>> * inheritAttrs;
  std::vector<std::pair<nix::PosIdx, nix::Expr *>> * string_parts;
  std::vector<std::pair<nix::PosIdx, std::variant<nix::Expr *, nix::StringToken>>> * ind_string_parts;
 }
@@ -97,7 +102,8 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParserState * state, const char *
 %type <attrs> binds
 %type <formals> formals
 %type <formal> formal
-%type <attrNames> attrs attrpath
+%type <attrNames> attrpath
+%type <inheritAttrs> attrs
 %type <string_parts> string_parts_interpolated
 %type <ind_string_parts> ind_string_parts
 %type <e> path_start string_parts string_attr
@@ -309,21 +315,30 @@ binds
  : binds attrpath '=' expr ';' { $$ = $1; state->addAttr($$, std::move(*$2), $4, state->at(@2)); delete $2; }
  | binds INHERIT attrs ';'
    { $$ = $1;
-      for (auto & i : *$3) {
+      for (auto & [i, iPos] : *$3) {
          if ($$->attrs.find(i.symbol) != $$->attrs.end())
-              state->dupAttr(i.symbol, state->at(@3), $$->attrs[i.symbol].pos);
-          auto pos = state->at(@3);
-          $$->attrs.emplace(i.symbol, ExprAttrs::AttrDef(new ExprVar(CUR_POS, i.symbol), pos, true));
+              state->dupAttr(i.symbol, iPos, $$->attrs[i.symbol].pos);
+          $$->attrs.emplace(
+              i.symbol,
+              ExprAttrs::AttrDef(new ExprVar(iPos, i.symbol), iPos, ExprAttrs::AttrDef::Kind::Inherited));
      }
      delete $3;
    }
  | binds INHERIT '(' expr ')' attrs ';'
    { $$ = $1;
-      /* !!! Should ensure sharing of the expression in $4. */
-      for (auto & i : *$6) {
+      if (!$$->inheritFromExprs)
+          $$->inheritFromExprs = std::make_unique<std::vector<Expr *>>();
+      $$->inheritFromExprs->push_back($4);
+      auto from = new nix::ExprInheritFrom(state->at(@4), $$->inheritFromExprs->size() - 1);
+      for (auto & [i, iPos] : *$6) {
          if ($$->attrs.find(i.symbol) != $$->attrs.end())
-              state->dupAttr(i.symbol, state->at(@6), $$->attrs[i.symbol].pos);
-          $$->attrs.emplace(i.symbol, ExprAttrs::AttrDef(new ExprSelect(CUR_POS, $4, i.symbol), state->at(@6)));
+              state->dupAttr(i.symbol, iPos, $$->attrs[i.symbol].pos);
+          $$->attrs.emplace(
+              i.symbol,
+              ExprAttrs::AttrDef(
+                  new ExprSelect(iPos, from, i.symbol),
+                  iPos,
+                  ExprAttrs::AttrDef::Kind::InheritedFrom));
      }
      delete $6;
    }
@@ -331,12 +346,12 @@ binds
  ;

 attrs
-  : attrs attr { $$ = $1; $1->push_back(AttrName(state->symbols.create($2))); }
+  : attrs attr { $$ = $1; $1->emplace_back(AttrName(state->symbols.create($2)), state->at(@2)); }
  | attrs string_attr
    { $$ = $1;
      ExprString * str = dynamic_cast<ExprString *>($2);
      if (str) {
-          $$->push_back(AttrName(state->symbols.create(str->s)));
+          $$->emplace_back(AttrName(state->symbols.create(str->s)), state->at(@2));
          delete str;
      } else
          throw ParseError({
@@ -344,7 +359,7 @@ attrs
              .pos = state->positions[state->at(@2)]
          });
    }
-  | { $$ = new AttrPath; }
+  | { $$ = new std::vector<std::pair<AttrName, PosIdx>>; }
  ;

 attrpath
@@ -423,7 +438,7 @@ Expr * parseExprFromBuf(
        .symbols = symbols,
        .positions = positions,
        .basePath = basePath,
-        .origin = {origin},
+        .origin = positions.addOrigin(origin, length),
        .rootFS = rootFS,
        .s = astSymbols,
    };
@@ -6,6 +6,7 @@ namespace nix {

 class PosIdx
 {
+    friend struct LazyPosAcessors;
    friend class PosTable;

 private:
@@ -7,6 +7,7 @@
 #include "chunked-vector.hh"
 #include "pos-idx.hh"
 #include "position.hh"
+#include "sync.hh"

 namespace nix {

@@ -17,66 +18,69 @@ public:
    {
        friend PosTable;
    private:
-        // must always be invalid by default, add() replaces this with the actual value.
-        // subsequent add() calls use this index as a token to quickly check whether the
-        // current origins.back() can be reused or not.
-        mutable uint32_t idx = std::numeric_limits<uint32_t>::max();
+        uint32_t offset;

-        // Used for searching in PosTable::[].
-        explicit Origin(uint32_t idx)
-            : idx(idx)
-            , origin{std::monostate()}
-        {
-        }
+        Origin(Pos::Origin origin, uint32_t offset, size_t size):
+            offset(offset), origin(origin), size(size)
+        {}

    public:
        const Pos::Origin origin;
+        const size_t size;

-        Origin(Pos::Origin origin)
-            : origin(origin)
+        uint32_t offsetOf(PosIdx p) const
        {
+            return p.id - 1 - offset;
        }
    };

-    struct Offset
-    {
-        uint32_t line, column;
-    };
-
 private:
-    std::vector<Origin> origins;
-    ChunkedVector<Offset, 8192> offsets;
+    using Lines = std::vector<uint32_t>;

-public:
-    PosTable()
-        : offsets(1024)
-    {
-        origins.reserve(1024);
-    }
+    std::map<uint32_t, Origin> origins;
+    mutable Sync<std::map<uint32_t, Lines>> lines;

-    PosIdx add(const Origin & origin, uint32_t line, uint32_t column)
+    const Origin * resolve(PosIdx p) const
    {
-        const auto idx = offsets.add({line, column}).second;
-        if (origins.empty() || origins.back().idx != origin.idx) {
-            origin.idx = idx;
-            origins.push_back(origin);
-        }
-        return PosIdx(idx + 1);
-    }
+        if (p.id == 0)
+            return nullptr;

-    Pos operator[](PosIdx p) const
-    {
-        if (p.id == 0 || p.id > offsets.size())
-            return {};
        const auto idx = p.id - 1;
        /* we want the last key <= idx, so we'll take prev(first key > idx).
-           this is guaranteed to never rewind origin.begin because the first
-           key is always 0. */
-        const auto pastOrigin = std::upper_bound(
-            origins.begin(), origins.end(), Origin(idx), [](const auto & a, const auto & b) { return a.idx < b.idx; });
-        const auto origin = *std::prev(pastOrigin);
-        const auto offset = offsets[idx];
-        return {offset.line, offset.column, origin.origin};
+            this is guaranteed to never rewind origin.begin because the first
+            key is always 0. */
+        const auto pastOrigin = origins.upper_bound(idx);
+        return &std::prev(pastOrigin)->second;
+    }
+
+public:
+    Origin addOrigin(Pos::Origin origin, size_t size)
+    {
+        uint32_t offset = 0;
+        if (auto it = origins.rbegin(); it != origins.rend())
+            offset = it->first + it->second.size;
+        // +1 because all PosIdx are offset by 1 to begin with, and
+        // another +1 to ensure that all origins can point to EOF, eg
+        // on (invalid) empty inputs.
+        if (2 + offset + size < offset)
+            return Origin{origin, offset, 0};
+        return origins.emplace(offset, Origin{origin, offset, size}).first->second;
+    }
+
+    PosIdx add(const Origin & origin, size_t offset)
+    {
+        if (offset > origin.size)
+            return PosIdx();
+        return PosIdx(1 + origin.offset + offset);
+    }
+
+    Pos operator[](PosIdx p) const;
+
+    Pos::Origin originOf(PosIdx p) const
+    {
+        if (auto o = resolve(p))
+            return o->origin;
+        return std::monostate{};
    }
 };

@@ -187,13 +187,13 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
            NixStringContextElem::DrvDeep { .drvPath = *storePath },
        });
        attrs.alloc(state.sName).mkString(drv.env["name"]);
-        auto & outputsVal = attrs.alloc(state.sOutputs);
-        state.mkList(outputsVal, drv.outputs.size());

+        auto list = state.buildList(drv.outputs.size());
        for (const auto & [i, o] : enumerate(drv.outputs)) {
            mkOutputString(state, attrs, *storePath, o);
-            (outputsVal.listElems()[i] = state.allocValue())->mkString(o.first);
+            (list[i] = state.allocValue())->mkString(o.first);
        }
+        attrs.alloc(state.sOutputs).mkList(list);

        auto w = state.allocValue();
        w->mkAttrs(attrs);
@@ -694,10 +694,10 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value * * a
    }

    /* Create the result list. */
-    state.mkList(v, res.size());
-    unsigned int n = 0;
-    for (auto & i : res)
-        v.listElems()[n++] = i;
+    auto list = state.buildList(res.size());
+    for (const auto & [n, i] : enumerate(res))
+        list[n] = i;
+    v.mkList(list);
 }

 static RegisterPrimOp primop_genericClosure(PrimOp {
@@ -705,38 +705,53 @@ static RegisterPrimOp primop_genericClosure(PrimOp {
    .args = {"attrset"},
    .arity = 1,
    .doc = R"(
-      Take an *attrset* with values named `startSet` and `operator` in order to
-      return a *list of attrsets* by starting with the `startSet` and recursively
-      applying the `operator` function to each `item`. The *attrsets* in the
-      `startSet` and the *attrsets* produced by `operator` must contain a value
-      named `key` which is comparable. The result is produced by calling `operator`
-      for each `item` with a value for `key` that has not been called yet including
-      newly produced `item`s. The function terminates when no new `item`s are
-      produced. The resulting *list of attrsets* contains only *attrsets* with a
-      unique key. For example,
+      `builtins.genericClosure` iteratively computes the transitive closure over an arbitrary relation defined by a function.

-      ```
-      builtins.genericClosure {
-        startSet = [ {key = 5;} ];
-        operator = item: [{
-          key = if (item.key / 2 ) * 2 == item.key
-               then item.key / 2
-               else 3 * item.key + 1;
-        }];
-      }
-      ```
-      evaluates to
-      ```
-      [ { key = 5; } { key = 16; } { key = 8; } { key = 4; } { key = 2; } { key = 1; } ]
-      ```
+      It takes *attrset* with two attributes named `startSet` and `operator`, and returns a list of attrbute sets:
+
+      - `startSet`:
+        The initial list of attribute sets.
+
+      - `operator`:
+        A function that takes an attribute set and returns a list of attribute sets.
+        It defines how each item in the current set is processed and expanded into more items.
+
+      Each attribute set in the list `startSet` and the list returned by `operator` must have an attribute `key`, which must support equality comparison.
+      The value of `key` can be one of the following types:

-      `key` can be one of the following types:
      - [Number](@docroot@/language/values.md#type-number)
      - [Boolean](@docroot@/language/values.md#type-boolean)
      - [String](@docroot@/language/values.md#type-string)
      - [Path](@docroot@/language/values.md#type-path)
      - [List](@docroot@/language/values.md#list)

+      The result is produced by calling the `operator` on each `item` that has not been called yet, including newly added items, until no new items are added.
+      Items are compared by their `key` attribute.
+
+      Common usages are:
+
+      - Generating unique collections of items, such as dependency graphs.
+      - Traversing through structures that may contain cycles or loops.
+      - Processing data structures with complex internal relationships.
+
+      > **Example**
+      >
+      > ```nix
+      > builtins.genericClosure {
+      >   startSet = [ {key = 5;} ];
+      >   operator = item: [{
+      >     key = if (item.key / 2 ) * 2 == item.key
+      >          then item.key / 2
+      >          else 3 * item.key + 1;
+      >   }];
+      > }
+      > ```
+      >
+      > evaluates to
+      >
+      > ```nix
+      > [ { key = 5; } { key = 16; } { key = 8; } { key = 4; } { key = 2; } { key = 1; } ]
+      > ```
      )",
    .fun = prim_genericClosure,
 });
@@ -811,7 +826,7 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
        auto message = state.coerceToString(pos, *args[0], context,
                "while evaluating the error message passed to builtins.addErrorContext",
                false, false).toOwned();
-        e.addTrace(nullptr, HintFmt(message), true);
+        e.addTrace(nullptr, HintFmt(message), TracePrint::Always);
        throw;
    }
 }
@@ -819,6 +834,8 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
 static RegisterPrimOp primop_addErrorContext(PrimOp {
    .name = "__addErrorContext",
    .arity = 2,
+    // The normal trace item is redundant
+    .addTrace = false,
    .fun = prim_addErrorContext,
 });

@@ -881,10 +898,11 @@ static void prim_tryEval(EvalState & state, const PosIdx pos, Value * * args, Va
    try {
        state.forceValue(*args[0], pos);
        attrs.insert(state.sValue, args[0]);
-        attrs.alloc("success").mkBool(true);
+        attrs.insert(state.symbols.create("success"), &state.vTrue);
    } catch (AssertionError & e) {
-        attrs.alloc(state.sValue).mkBool(false);
-        attrs.alloc("success").mkBool(false);
+        // `value = false;` is unfortunate but removing it is a breaking change.
+        attrs.insert(state.sValue, &state.vFalse);
+        attrs.insert(state.symbols.create("success"), &state.vFalse);
    }

    // restore the debugRepl pointer if we saved it earlier.
@@ -1075,7 +1093,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
        e.addTrace(nullptr, HintFmt(
                "while evaluating derivation '%s'\n"
                "  whose name attribute is located at %s",
-                drvName, pos), true);
+                drvName, pos));
        throw;
    }
 }
@@ -1123,7 +1141,10 @@ drvName, Bindings * attrs, Value & v)
        auto handleHashMode = [&](const std::string_view s) {
            if (s == "recursive") ingestionMethod = FileIngestionMethod::Recursive;
            else if (s == "flat") ingestionMethod = FileIngestionMethod::Flat;
-            else if (s == "text") {
+            else if (s == "git") {
+                experimentalFeatureSettings.require(Xp::GitHashing);
+                ingestionMethod = FileIngestionMethod::Git;
+            } else if (s == "text") {
                experimentalFeatureSettings.require(Xp::DynamicDerivations);
                ingestionMethod = TextIngestionMethod {};
            } else
@@ -1233,8 +1254,7 @@ drvName, Bindings * attrs, Value & v)

        } catch (Error & e) {
            e.addTrace(state.positions[i->pos],
-                HintFmt("while evaluating attribute '%1%' of derivation '%2%'", key, drvName),
-                true);
+                HintFmt("while evaluating attribute '%1%' of derivation '%2%'", key, drvName));
            throw;
        }
    }
@@ -1550,23 +1570,50 @@ static RegisterPrimOp primop_pathExists({
    .fun = prim_pathExists,
 });

+// Ideally, all trailing slashes should have been removed, but it's been like this for
+// almost a decade as of writing. Changing it will affect reproducibility.
+static std::string_view legacyBaseNameOf(std::string_view path)
+{
+    if (path.empty())
+        return "";
+
+    auto last = path.size() - 1;
+    if (path[last] == '/' && last > 0)
+        last -= 1;
+
+    auto pos = path.rfind('/', last);
+    if (pos == path.npos)
+        pos = 0;
+    else
+        pos += 1;
+
+    return path.substr(pos, last - pos + 1);
+}
+
 /* Return the base name of the given string, i.e., everything
   following the last slash. */
 static void prim_baseNameOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
    NixStringContext context;
-    v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context,
+    v.mkString(legacyBaseNameOf(*state.coerceToString(pos, *args[0], context,
            "while evaluating the first argument passed to builtins.baseNameOf",
            false, false)), context);
 }

 static RegisterPrimOp primop_baseNameOf({
    .name = "baseNameOf",
-    .args = {"s"},
+    .args = {"x"},
    .doc = R"(
-      Return the *base name* of the string *s*, that is, everything
-      following the final slash in the string. This is similar to the GNU
-      `basename` command.
+      Return the *base name* of either a [path value](@docroot@/language/values.md#type-path) *x* or a string *x*, depending on which type is passed, and according to the following rules.
+
+      For a path value, the *base name* is considered to be the part of the path after the last directory separator, including any file extensions.
+      This is the simple case, as path values don't have trailing slashes.
+
+      When the argument is a string, a more involved logic applies. If the string ends with a `/`, only this one final slash is removed.
+
+      After this, the *base name* is returned as previously described, assuming `/` as the directory separator. (Note that evaluation must be platform independent.)
+
+      This is somewhat similar to the [GNU `basename`](https://www.gnu.org/software/coreutils/manual/html_node/basename-invocation.html) command, but GNU `basename` will strip any number of trailing slashes.
    )",
    .fun = prim_baseNameOf,
 });
@@ -1719,7 +1766,7 @@ static RegisterPrimOp primop_findFile(PrimOp {
      - If the suffix is found inside that directory, then the entry is a match.
        The combined absolute path of the directory (now downloaded if need be) and the suffix is returned.

-      [Lookup path](@docroot@/language/constructs/lookup-path.md) expressions can be [desugared](https://en.wikipedia.org/wiki/Syntactic_sugar) using this and [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath):
+      [Lookup path](@docroot@/language/constructs/lookup-path.md) expressions are [desugared](https://en.wikipedia.org/wiki/Syntactic_sugar) using this and [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath):

      ```nix
      <nixpkgs>
@@ -1758,20 +1805,20 @@ static RegisterPrimOp primop_hashFile({
    .fun = prim_hashFile,
 });

-static std::string_view fileTypeToString(InputAccessor::Type type)
+static Value * fileTypeToString(EvalState & state, InputAccessor::Type type)
 {
    return
-        type == InputAccessor::Type::tRegular ? "regular" :
-        type == InputAccessor::Type::tDirectory ? "directory" :
-        type == InputAccessor::Type::tSymlink ? "symlink" :
-        "unknown";
+        type == InputAccessor::Type::tRegular ? &state.vStringRegular :
+        type == InputAccessor::Type::tDirectory ? &state.vStringDirectory :
+        type == InputAccessor::Type::tSymlink ? &state.vStringSymlink :
+        &state.vStringUnknown;
 }

 static void prim_readFileType(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
    auto path = realisePath(state, pos, *args[0], std::nullopt);
    /* Retrieve the directory entry type and stringize it. */
-    v.mkString(fileTypeToString(path.lstat().type));
+    v = *fileTypeToString(state, path.lstat().type);
 }

 static RegisterPrimOp primop_readFileType({
@@ -1802,8 +1849,8 @@ static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Va
    Value * readFileType = nullptr;

    for (auto & [name, type] : entries) {
-        auto & attr = attrs.alloc(name);
        if (!type) {
+            auto & attr = attrs.alloc(name);
            // Some filesystems or operating systems may not be able to return
            // detailed node info quickly in this case we produce a thunk to
            // query the file type lazily.
@@ -1815,7 +1862,7 @@ static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Va
        } else {
            // This branch of the conditional is much more likely.
            // Here we just stringize the directory entry type.
-            attr.mkString(fileTypeToString(*type));
+            attrs.insert(state.symbols.create(name), fileTypeToString(state, *type));
        }
    }

@@ -2075,7 +2122,7 @@ static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Val
        })
        : ({
            StringSource s { contents };
-            state.store->addToStoreFromDump(s, name, TextIngestionMethod {}, HashAlgorithm::SHA256, refs, state.repair);
+            state.store->addToStoreFromDump(s, name, FileSerialisationMethod::Flat, TextIngestionMethod {}, HashAlgorithm::SHA256, refs, state.repair);
        });

    /* Note: we don't need to add `context' to the context of the
@@ -2176,11 +2223,8 @@ bool EvalState::callPathFilter(
    Value arg1;
    arg1.mkString(pathArg);

-    Value arg2;
    // assert that type is not "unknown"
-    arg2.mkString(fileTypeToString(st.type));
-
-    Value * args []{&arg1, &arg2};
+    Value * args []{&arg1, fileTypeToString(*this, st.type)};
    Value res;
    callFunction(*filterFun, 2, args, res, pos);

@@ -2406,14 +2450,15 @@ static void prim_attrNames(EvalState & state, const PosIdx pos, Value * * args,
 {
    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.attrNames");

-    state.mkList(v, args[0]->attrs->size());
+    auto list = state.buildList(args[0]->attrs->size());

-    size_t n = 0;
-    for (auto & i : *args[0]->attrs)
-        (v.listElems()[n++] = state.allocValue())->mkString(state.symbols[i.name]);
+    for (const auto & [n, i] : enumerate(*args[0]->attrs))
+        (list[n] = state.allocValue())->mkString(state.symbols[i.name]);

-    std::sort(v.listElems(), v.listElems() + n,
+    std::sort(list.begin(), list.end(),
              [](Value * v1, Value * v2) { return strcmp(v1->c_str(), v2->c_str()) < 0; });
+
+    v.mkList(list);
 }

 static RegisterPrimOp primop_attrNames({
@@ -2433,21 +2478,22 @@ static void prim_attrValues(EvalState & state, const PosIdx pos, Value * * args,
 {
    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.attrValues");

-    state.mkList(v, args[0]->attrs->size());
+    auto list = state.buildList(args[0]->attrs->size());

-    unsigned int n = 0;
-    for (auto & i : *args[0]->attrs)
-        v.listElems()[n++] = (Value *) &i;
+    for (const auto & [n, i] : enumerate(*args[0]->attrs))
+        list[n] = (Value *) &i;

-    std::sort(v.listElems(), v.listElems() + n,
+    std::sort(list.begin(), list.end(),
        [&](Value * v1, Value * v2) {
            std::string_view s1 = state.symbols[((Attr *) v1)->name],
                s2 = state.symbols[((Attr *) v2)->name];
            return s1 < s2;
        });

-    for (unsigned int i = 0; i < n; ++i)
-        v.listElems()[i] = ((Attr *) v.listElems()[i])->value;
+    for (auto & v : list)
+        v = ((Attr *) v)->value;
+
+    v.mkList(list);
 }

 static RegisterPrimOp primop_attrValues({
@@ -2507,6 +2553,54 @@ static RegisterPrimOp primop_unsafeGetAttrPos(PrimOp {
    .fun = prim_unsafeGetAttrPos,
 });

+// access to exact position information (ie, line and colum numbers) is deferred
+// due to the cost associated with calculating that information and how rarely
+// it is used in practice. this is achieved by creating thunks to otherwise
+// inaccessible primops that are not exposed as __op or under builtins to turn
+// the internal PosIdx back into a line and column number, respectively. exposing
+// these primops in any way would at best be not useful and at worst create wildly
+// indeterministic eval results depending on parse order of files.
+//
+// in a simpler world this would instead be implemented as another kind of thunk,
+// but each type of thunk has an associated runtime cost in the current evaluator.
+// as with black holes this cost is too high to justify another thunk type to check
+// for in the very hot path that is forceValue.
+static struct LazyPosAcessors {
+    PrimOp primop_lineOfPos{
+        .arity = 1,
+        .fun = [] (EvalState & state, PosIdx pos, Value * * args, Value & v) {
+            v.mkInt(state.positions[PosIdx(args[0]->integer)].line);
+        }
+    };
+    PrimOp primop_columnOfPos{
+        .arity = 1,
+        .fun = [] (EvalState & state, PosIdx pos, Value * * args, Value & v) {
+            v.mkInt(state.positions[PosIdx(args[0]->integer)].column);
+        }
+    };
+
+    Value lineOfPos, columnOfPos;
+
+    LazyPosAcessors()
+    {
+        lineOfPos.mkPrimOp(&primop_lineOfPos);
+        columnOfPos.mkPrimOp(&primop_columnOfPos);
+    }
+
+    void operator()(EvalState & state, const PosIdx pos, Value & line, Value & column)
+    {
+        Value * posV = state.allocValue();
+        posV->mkInt(pos.id);
+        line.mkApp(&lineOfPos, posV);
+        column.mkApp(&columnOfPos, posV);
+    }
+} makeLazyPosAccessors;
+
+void makePositionThunks(EvalState & state, const PosIdx pos, Value & line, Value & column)
+{
+    makeLazyPosAccessors(state, pos, line, column);
+}
+
 /* Dynamic version of the `?' operator. */
 static void prim_hasAttr(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
@@ -2740,9 +2834,10 @@ static void prim_catAttrs(EvalState & state, const PosIdx pos, Value * * args, V
            res[found++] = i->value;
    }

-    state.mkList(v, found);
+    auto list = state.buildList(found);
    for (unsigned int n = 0; n < found; ++n)
-        v.listElems()[n] = res[n];
+        list[n] = res[n];
+    v.mkList(list);
 }

 static RegisterPrimOp primop_catAttrs({
@@ -2779,8 +2874,7 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value * * arg

    auto attrs = state.buildBindings(args[0]->lambda.fun->formals->formals.size());
    for (auto & i : args[0]->lambda.fun->formals->formals)
-        // !!! should optimise booleans (allocate only once)
-        attrs.alloc(i.name, i.pos).mkBool(i.def);
+        attrs.insert(i.name, state.getBool(i.def), i.pos);
    v.mkAttrs(attrs);
 }

@@ -2843,43 +2937,50 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
    // attribute with the merge function application. this way we need not
    // use (slightly slower) temporary storage the GC does not know about.

-    std::map<Symbol, std::pair<size_t, Value * *>> attrsSeen;
+    struct Item
+    {
+        size_t size = 0;
+        size_t pos = 0;
+        std::optional<ListBuilder> list;
+    };
+
+    std::map<Symbol, Item> attrsSeen;

    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.zipAttrsWith");
    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.zipAttrsWith");
-    const auto listSize = args[1]->listSize();
-    const auto listElems = args[1]->listElems();
+    const auto listItems = args[1]->listItems();

-    for (unsigned int n = 0; n < listSize; ++n) {
-        Value * vElem = listElems[n];
+    for (auto & vElem : listItems) {
        state.forceAttrs(*vElem, noPos, "while evaluating a value of the list passed as second argument to builtins.zipAttrsWith");
        for (auto & attr : *vElem->attrs)
-            attrsSeen[attr.name].first++;
+            attrsSeen.try_emplace(attr.name).first->second.size++;
+    }
+
+    for (auto & [sym, elem] : attrsSeen)
+        elem.list.emplace(state.buildList(elem.size));
+
+    for (auto & vElem : listItems) {
+        for (auto & attr : *vElem->attrs) {
+            auto & item = attrsSeen.at(attr.name);
+            (*item.list)[item.pos++] = attr.value;
+        }
    }

    auto attrs = state.buildBindings(attrsSeen.size());
+
    for (auto & [sym, elem] : attrsSeen) {
-        auto & list = attrs.alloc(sym);
-        state.mkList(list, elem.first);
-        elem.second = list.listElems();
-    }
-    v.mkAttrs(attrs.alreadySorted());
-
-    for (unsigned int n = 0; n < listSize; ++n) {
-        Value * vElem = listElems[n];
-        for (auto & attr : *vElem->attrs)
-            *attrsSeen[attr.name].second++ = attr.value;
-    }
-
-    for (auto & attr : *v.attrs) {
        auto name = state.allocValue();
-        name->mkString(state.symbols[attr.name]);
+        name->mkString(state.symbols[sym]);
        auto call1 = state.allocValue();
        call1->mkApp(args[0], name);
        auto call2 = state.allocValue();
-        call2->mkApp(call1, attr.value);
-        attr.value = call2;
+        auto arg = state.allocValue();
+        arg->mkList(*elem.list);
+        call2->mkApp(call1, arg);
+        attrs.insert(sym, call2);
    }
+
+    v.mkAttrs(attrs.alreadySorted());
 }

 static RegisterPrimOp primop_zipAttrsWith({
@@ -2990,9 +3091,10 @@ static void prim_tail(EvalState & state, const PosIdx pos, Value * * args, Value
    if (args[0]->listSize() == 0)
        state.error<EvalError>("'tail' called on an empty list").atPos(pos).debugThrow();

-    state.mkList(v, args[0]->listSize() - 1);
-    for (unsigned int n = 0; n < v.listSize(); ++n)
-        v.listElems()[n] = args[0]->listElems()[n + 1];
+    auto list = state.buildList(args[0]->listSize() - 1);
+    for (const auto & [n, v] : enumerate(list))
+        v = args[0]->listElems()[n + 1];
+    v.mkList(list);
 }

 static RegisterPrimOp primop_tail({
@@ -3023,10 +3125,11 @@ static void prim_map(EvalState & state, const PosIdx pos, Value * * args, Value

    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.map");

-    state.mkList(v, args[1]->listSize());
-    for (unsigned int n = 0; n < v.listSize(); ++n)
-        (v.listElems()[n] = state.allocValue())->mkApp(
+    auto list = state.buildList(args[1]->listSize());
+    for (const auto & [n, v] : enumerate(list))
+        (v = state.allocValue())->mkApp(
            args[0], args[1]->listElems()[n]);
+    v.mkList(list);
 }

 static RegisterPrimOp primop_map({
@@ -3075,8 +3178,9 @@ static void prim_filter(EvalState & state, const PosIdx pos, Value * * args, Val
    if (same)
        v = *args[1];
    else {
-        state.mkList(v, k);
-        for (unsigned int n = 0; n < k; ++n) v.listElems()[n] = vs[n];
+        auto list = state.buildList(k);
+        for (const auto & [n, v] : enumerate(list)) v = vs[n];
+        v.mkList(list);
    }
 }

@@ -3251,12 +3355,13 @@ static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Va
    // as evaluating map without accessing any values makes little sense.
    state.forceFunction(*args[0], noPos, "while evaluating the first argument passed to builtins.genList");

-    state.mkList(v, len);
-    for (unsigned int n = 0; n < (unsigned int) len; ++n) {
+    auto list = state.buildList(len);
+    for (const auto & [n, v] : enumerate(list)) {
        auto arg = state.allocValue();
        arg->mkInt(n);
-        (v.listElems()[n] = state.allocValue())->mkApp(args[0], arg);
+        (v = state.allocValue())->mkApp(args[0], arg);
    }
+    v.mkList(list);
 }

 static RegisterPrimOp primop_genList({
@@ -3290,11 +3395,9 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value

    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.sort");

-    state.mkList(v, len);
-    for (unsigned int n = 0; n < len; ++n) {
-        state.forceValue(*args[1]->listElems()[n], pos);
-        v.listElems()[n] = args[1]->listElems()[n];
-    }
+    auto list = state.buildList(len);
+    for (const auto & [n, v] : enumerate(list))
+        state.forceValue(*(v = args[1]->listElems()[n]), pos);

    auto comparator = [&](Value * a, Value * b) {
        /* Optimization: if the comparator is lessThan, bypass
@@ -3313,7 +3416,9 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
    /* FIXME: std::sort can segfault if the comparator is not a strict
       weak ordering. What to do? std::stable_sort() seems more
       resilient, but no guarantees... */
-    std::stable_sort(v.listElems(), v.listElems() + len, comparator);
+    std::stable_sort(list.begin(), list.end(), comparator);
+
+    v.mkList(list);
 }

 static RegisterPrimOp primop_sort({
@@ -3359,17 +3464,17 @@ static void prim_partition(EvalState & state, const PosIdx pos, Value * * args,

    auto attrs = state.buildBindings(2);

-    auto & vRight = attrs.alloc(state.sRight);
    auto rsize = right.size();
-    state.mkList(vRight, rsize);
+    auto rlist = state.buildList(rsize);
    if (rsize)
-        memcpy(vRight.listElems(), right.data(), sizeof(Value *) * rsize);
+        memcpy(rlist.elems, right.data(), sizeof(Value *) * rsize);
+    attrs.alloc(state.sRight).mkList(rlist);

-    auto & vWrong = attrs.alloc(state.sWrong);
    auto wsize = wrong.size();
-    state.mkList(vWrong, wsize);
+    auto wlist = state.buildList(wsize);
    if (wsize)
-        memcpy(vWrong.listElems(), wrong.data(), sizeof(Value *) * wsize);
+        memcpy(wlist.elems, wrong.data(), sizeof(Value *) * wsize);
+    attrs.alloc(state.sWrong).mkList(wlist);

    v.mkAttrs(attrs);
 }
@@ -3416,10 +3521,10 @@ static void prim_groupBy(EvalState & state, const PosIdx pos, Value * * args, Va
    auto attrs2 = state.buildBindings(attrs.size());

    for (auto & i : attrs) {
-        auto & list = attrs2.alloc(i.first);
        auto size = i.second.size();
-        state.mkList(list, size);
-        memcpy(list.listElems(), i.second.data(), sizeof(Value *) * size);
+        auto list = state.buildList(size);
+        memcpy(list.elems, i.second.data(), sizeof(Value *) * size);
+        attrs2.alloc(i.first).mkList(list);
    }

    v.mkAttrs(attrs2.alreadySorted());
@@ -3466,14 +3571,15 @@ static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args,
        len += lists[n].listSize();
    }

-    state.mkList(v, len);
-    auto out = v.listElems();
+    auto list = state.buildList(len);
+    auto out = list.elems;
    for (unsigned int n = 0, pos = 0; n < nrLists; ++n) {
        auto l = lists[n].listSize();
        if (l)
            memcpy(out + pos, lists[n].listElems(), l * sizeof(Value *));
        pos += l;
    }
+    v.mkList(list);
 }

 static RegisterPrimOp primop_concatMap({
@@ -3755,7 +3861,7 @@ static RegisterPrimOp primop_stringLength({
    .name = "__stringLength",
    .args = {"e"},
    .doc = R"(
-      Return the length of the string *e*. If *e* is not a string,
+      Return the number of bytes of the string *e*. If *e* is not a string,
      evaluation is aborted.
    )",
    .fun = prim_stringLength,
@@ -3921,14 +4027,13 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
        }

        // the first match is the whole string
-        const size_t len = match.size() - 1;
-        state.mkList(v, len);
-        for (size_t i = 0; i < len; ++i) {
-            if (!match[i+1].matched)
-                (v.listElems()[i] = state.allocValue())->mkNull();
+        auto list = state.buildList(match.size() - 1);
+        for (const auto & [i, v2] : enumerate(list))
+            if (!match[i + 1].matched)
+                v2 = &state.vNull;
            else
-                (v.listElems()[i] = state.allocValue())->mkString(match[i + 1].str());
-        }
+                (v2 = state.allocValue())->mkString(match[i + 1].str());
+        v.mkList(list);

    } catch (std::regex_error & e) {
        if (e.code() == std::regex_constants::error_space) {
@@ -3997,11 +4102,12 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)

        // Any matches results are surrounded by non-matching results.
        const size_t len = std::distance(begin, end);
-        state.mkList(v, 2 * len + 1);
+        auto list = state.buildList(2 * len + 1);
        size_t idx = 0;

        if (len == 0) {
-            v.listElems()[idx++] = args[1];
+            list[0] = args[1];
+            v.mkList(list);
            return;
        }

@@ -4010,28 +4116,31 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
            auto match = *i;

            // Add a string for non-matched characters.
-            (v.listElems()[idx++] = state.allocValue())->mkString(match.prefix().str());
+            (list[idx++] = state.allocValue())->mkString(match.prefix().str());

            // Add a list for matched substrings.
            const size_t slen = match.size() - 1;
-            auto elem = v.listElems()[idx++] = state.allocValue();

            // Start at 1, beacause the first match is the whole string.
-            state.mkList(*elem, slen);
-            for (size_t si = 0; si < slen; ++si) {
+            auto list2 = state.buildList(slen);
+            for (const auto & [si, v2] : enumerate(list2)) {
                if (!match[si + 1].matched)
-                    (elem->listElems()[si] = state.allocValue())->mkNull();
+                    v2 = &state.vNull;
                else
-                    (elem->listElems()[si] = state.allocValue())->mkString(match[si + 1].str());
+                    (v2 = state.allocValue())->mkString(match[si + 1].str());
            }

+            (list[idx++] = state.allocValue())->mkList(list2);
+
            // Add a string for non-matched suffix characters.
            if (idx == 2 * len)
-                (v.listElems()[idx++] = state.allocValue())->mkString(match.suffix().str());
+                (list[idx++] = state.allocValue())->mkString(match.suffix().str());
        }

        assert(idx == 2 * len + 1);

+        v.mkList(list);
+
    } catch (std::regex_error & e) {
        if (e.code() == std::regex_constants::error_space) {
            // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
@@ -4251,9 +4360,10 @@ static void prim_splitVersion(EvalState & state, const PosIdx pos, Value * * arg
            break;
        components.emplace_back(component);
    }
-    state.mkList(v, components.size());
+    auto list = state.buildList(components.size());
    for (const auto & [n, component] : enumerate(components))
-        (v.listElems()[n] = state.allocValue())->mkString(std::move(component));
+        (list[n] = state.allocValue())->mkString(std::move(component));
+    v.mkList(list);
 }

 static RegisterPrimOp primop_splitVersion({
@@ -4346,8 +4456,7 @@ void EvalState::createBaseEnv()
        )",
    });

-    v.mkNull();
-    addConstant("null", v, {
+    addConstant("null", &vNull, {
        .type = nNull,
        .doc = R"(
          Primitive value.
@@ -4494,22 +4603,20 @@ void EvalState::createBaseEnv()
    });

    /* Add a value containing the current Nix expression search path. */
-    mkList(v, searchPath.elements.size());
-    int n = 0;
-    for (auto & i : searchPath.elements) {
+    auto list = buildList(searchPath.elements.size());
+    for (const auto & [n, i] : enumerate(searchPath.elements)) {
        auto attrs = buildBindings(2);
        attrs.alloc("path").mkString(i.path.s);
        attrs.alloc("prefix").mkString(i.prefix.s);
-        (v.listElems()[n++] = allocValue())->mkAttrs(attrs);
+        (list[n] = allocValue())->mkAttrs(attrs);
    }
+    v.mkList(list);
    addConstant("__nixPath", v, {
        .type = nList,
        .doc = R"(
-          List of search path entries used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).
+          The value of the [`nix-path` configuration setting](@docroot@/command-ref/conf-file.md#conf-nix-path): a list of search path entries used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).

-          Lookup path expressions can be
-          [desugared](https://en.wikipedia.org/wiki/Syntactic_sugar)
-          using this and
+          Lookup path expressions are [desugared](https://en.wikipedia.org/wiki/Syntactic_sugar) using this and
          [`builtins.findFile`](./builtins.html#builtins-findFile):

          ```nix
@@ -51,4 +51,6 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 */
 void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v);

+void makePositionThunks(EvalState & state, const PosIdx pos, Value & line, Value & column);
+
 }
@@ -137,14 +137,14 @@ static RegisterPrimOp primop_addDrvOutputDependencies({
    .name = "__addDrvOutputDependencies",
    .args = {"s"},
    .doc = R"(
-      Create a copy of the given string where a single consant string context element is turned into a "derivation deep" string context element.
+      Create a copy of the given string where a single constant string context element is turned into a "derivation deep" string context element.

      The store path that is the constant string context element should point to a valid derivation, and end in `.drv`.

      The original string context element must not be empty or have multiple elements, and it must not have any other type of element other than a constant or derivation deep element.
      The latter is supported so this function is idempotent.

-      This is the opposite of [`builtins.unsafeDiscardOutputDependency`](#builtins-addDrvOutputDependencies).
+      This is the opposite of [`builtins.unsafeDiscardOutputDependency`](#builtins-unsafeDiscardOutputDependency).
    )",
    .fun = prim_addDrvOutputDependencies
 });
@@ -207,10 +207,10 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
        if (info.second.allOutputs)
            infoAttrs.alloc(sAllOutputs).mkBool(true);
        if (!info.second.outputs.empty()) {
-            auto & outputsVal = infoAttrs.alloc(state.sOutputs);
-            state.mkList(outputsVal, info.second.outputs.size());
+            auto list = state.buildList(info.second.outputs.size());
            for (const auto & [i, output] : enumerate(info.second.outputs))
-                (outputsVal.listElems()[i] = state.allocValue())->mkString(output);
+                (list[i] = state.allocValue())->mkString(output);
+            infoAttrs.alloc(state.sOutputs).mkList(list);
        }
        attrs.alloc(state.store->printStorePath(info.first)).mkAttrs(infoAttrs);
    }
@@ -246,7 +246,7 @@ static RegisterPrimOp primop_getContext({

 /* Append the given context to a given string.

-   See the commentary above unsafeGetContext for details of the
+   See the commentary above getContext for details of the
   context representation.
 */
 static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
@@ -64,8 +64,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
    if (rev) attrs.insert_or_assign("rev", rev->gitRev());
    auto input = fetchers::Input::fromAttrs(std::move(attrs));

-    // FIXME: use name
-    auto [storePath, input2] = input.fetch(state.store);
+    auto [storePath, input2] = input.fetchToStore(state.store);

    auto attrs2 = state.buildBindings(8);
    state.mkStorePathString(storePath, attrs2.alloc(state.sOutPath));
@@ -182,7 +182,7 @@ static void fetchTree(

    state.checkURI(input.toURLString());

-    auto [storePath, input2] = input.fetch(state.store);
+    auto [storePath, input2] = input.fetchToStore(state.store);

    state.allowPath(storePath);

@@ -38,10 +38,10 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
                {
                    auto array = toml::get<std::vector<toml::value>>(t);

-                    size_t size = array.size();
-                    state.mkList(v, size);
-                    for (size_t i = 0; i < size; ++i)
-                        visit(*(v.listElems()[i] = state.allocValue()), array[i]);
+                    auto list = state.buildList(array.size());
+                    for (const auto & [n, v] : enumerate(list))
+                        visit(*(v = state.allocValue()), array[n]);
+                    v.mkList(list);
                }
                break;;
            case toml::value_t::boolean:
@@ -8,6 +8,29 @@

 namespace nix {

+/**
+ * How errors should be handled when printing values.
+ */
+enum class ErrorPrintBehavior {
+    /**
+     * Print the first line of the error in brackets: `«error: oh no!»`
+     */
+    Print,
+    /**
+     * Throw the error to the code that attempted to print the value, instead
+     * of suppressing it it.
+     */
+    Throw,
+    /**
+     * Only throw the error if encountered at the top level of the expression.
+     *
+     * This will cause expressions like `builtins.throw "uh oh!"` to throw
+     * errors, but will print attribute sets and other nested structures
+     * containing values that error (like `nixpkgs`) normally.
+     */
+    ThrowTopLevel,
+};
+
 /**
 * Options for printing Nix values.
 */
@@ -68,6 +91,11 @@ struct PrintOptions
     */
    size_t prettyIndent = 0;

+    /**
+     * How to handle errors encountered while printing values.
+     */
+    ErrorPrintBehavior errors = ErrorPrintBehavior::Print;
+
    /**
     * True if pretty-printing is enabled.
     */
@@ -86,7 +114,7 @@ static PrintOptions errorPrintOptions = PrintOptions {
    .maxDepth = 10,
    .maxAttrs = 10,
    .maxListItems = 10,
-    .maxStringLength = 1024
+    .maxStringLength = 1024,
 };

 }
@@ -271,25 +271,21 @@ private:

    void printDerivation(Value & v)
    {
-        try {
-            Bindings::iterator i = v.attrs->find(state.sDrvPath);
-            NixStringContext context;
-            std::string storePath;
-            if (i != v.attrs->end())
-                storePath = state.store->printStorePath(state.coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));
+        Bindings::iterator i = v.attrs->find(state.sDrvPath);
+        NixStringContext context;
+        std::string storePath;
+        if (i != v.attrs->end())
+            storePath = state.store->printStorePath(state.coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));

-            if (options.ansiColors)
-                output << ANSI_GREEN;
-            output << "«derivation";
-            if (!storePath.empty()) {
-                output << " " << storePath;
-            }
-            output << "»";
-            if (options.ansiColors)
-                output << ANSI_NORMAL;
-        } catch (Error & e) {
-            printError_(e);
+        if (options.ansiColors)
+            output << ANSI_GREEN;
+        output << "«derivation";
+        if (!storePath.empty()) {
+            output << " " << storePath;
        }
+        output << "»";
+        if (options.ansiColors)
+            output << ANSI_NORMAL;
    }

    bool shouldPrettyPrintAttrs(AttrVec & v)
@@ -510,64 +506,68 @@ private:
        output.flush();
        checkInterrupt();

-        if (options.force) {
-            try {
+        try {
+            if (options.force) {
                state.forceValue(v, v.determinePos(noPos));
-            } catch (Error & e) {
-                printError_(e);
-                return;
            }
-        }

-        switch (v.type()) {
+            switch (v.type()) {

-        case nInt:
-            printInt(v);
-            break;
+            case nInt:
+                printInt(v);
+                break;

-        case nFloat:
-            printFloat(v);
-            break;
+            case nFloat:
+                printFloat(v);
+                break;

-        case nBool:
-            printBool(v);
-            break;
+            case nBool:
+                printBool(v);
+                break;

-        case nString:
-            printString(v);
-            break;
+            case nString:
+                printString(v);
+                break;

-        case nPath:
-            printPath(v);
-            break;
+            case nPath:
+                printPath(v);
+                break;

-        case nNull:
-            printNull();
-            break;
+            case nNull:
+                printNull();
+                break;

-        case nAttrs:
-            printAttrs(v, depth);
-            break;
+            case nAttrs:
+                printAttrs(v, depth);
+                break;

-        case nList:
-            printList(v, depth);
-            break;
+            case nList:
+                printList(v, depth);
+                break;

-        case nFunction:
-            printFunction(v);
-            break;
+            case nFunction:
+                printFunction(v);
+                break;

-        case nThunk:
-            printThunk(v);
-            break;
+            case nThunk:
+                printThunk(v);
+                break;

-        case nExternal:
-            printExternal(v);
-            break;
+            case nExternal:
+                printExternal(v);
+                break;

-        default:
-            printUnknown();
-            break;
+            default:
+                printUnknown();
+                break;
+            }
+        } catch (Error & e) {
+            if (options.errors == ErrorPrintBehavior::Throw
+                || (options.errors == ErrorPrintBehavior::ThrowTopLevel
+                    && depth == 0)) {
+                throw;
+            }
+            printError_(e);
        }
    }

@@ -18,6 +18,7 @@

 namespace nix {

+struct Value;
 class BindingsBuilder;


@@ -134,6 +135,34 @@ class ExternalValueBase
 std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);


+class ListBuilder
+{
+    const size_t size;
+    Value * inlineElems[2] = {nullptr, nullptr};
+public:
+    Value * * elems;
+    ListBuilder(EvalState & state, size_t size);
+
+    ListBuilder(ListBuilder && x)
+        : size(x.size)
+        , inlineElems{x.inlineElems[0], x.inlineElems[1]}
+        , elems(size <= 2 ? inlineElems : x.elems)
+    { }
+
+    Value * & operator [](size_t n)
+    {
+        return elems[n];
+    }
+
+    typedef Value * * iterator;
+
+    iterator begin() { return &elems[0]; }
+    iterator end() { return &elems[size]; }
+
+    friend struct Value;
+};
+
+
 struct Value
 {
 private:
@@ -217,7 +246,7 @@ public:
        Bindings * attrs;
        struct {
            size_t size;
-            Value * * elems;
+            Value * const * elems;
        } bigList;
        Value * smallList[2];
        ClosureThunk thunk;
@@ -323,16 +352,20 @@ public:

    Value & mkAttrs(BindingsBuilder & bindings);

-    inline void mkList(size_t size)
+    void mkList(const ListBuilder & builder)
    {
        clearValue();
-        if (size == 1)
+        if (builder.size == 1) {
+            smallList[0] = builder.inlineElems[0];
            internalType = tList1;
-        else if (size == 2)
+        } else if (builder.size == 2) {
+            smallList[0] = builder.inlineElems[0];
+            smallList[1] = builder.inlineElems[1];
            internalType = tList2;
-        else {
+        } else {
+            bigList.size = builder.size;
+            bigList.elems = builder.elems;
            internalType = tListN;
-            bigList.size = size;
        }
    }

@@ -392,7 +425,7 @@ public:
        return internalType == tList1 || internalType == tList2 || internalType == tListN;
    }

-    Value * * listElems()
+    Value * const * listElems()
    {
        return internalType == tList1 || internalType == tList2 ? smallList : bigList.elems;
    }
@@ -161,7 +161,7 @@ bool Input::contains(const Input & other) const
    return false;
 }

-std::pair<StorePath, Input> Input::fetch(ref<Store> store) const
+std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
 {
    if (!scheme)
        throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
@@ -186,56 +186,85 @@ std::pair<StorePath, Input> Input::fetch(ref<Store> store) const

    auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
        try {
-            return scheme->fetch(store, *this);
+            auto [accessor, final] = getAccessorUnchecked(store);
+
+            auto storePath = nix::fetchToStore(*store, SourcePath(accessor), FetchMode::Copy, final.getName());
+
+            auto narHash = store->queryPathInfo(storePath)->narHash;
+            final.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
+
+            scheme->checkLocks(*this, final);
+
+            return {storePath, final};
        } catch (Error & e) {
            e.addTrace({}, "while fetching the input '%s'", to_string());
            throw;
        }
    }();

-    auto narHash = store->queryPathInfo(storePath)->narHash;
-    input.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
-
-    if (auto prevNarHash = getNarHash()) {
-        if (narHash != *prevNarHash)
-            throw Error((unsigned int) 102, "NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
-                to_string(),
-                store->printStorePath(storePath),
-                prevNarHash->to_string(HashFormat::SRI, true),
-                narHash.to_string(HashFormat::SRI, true));
-    }
-
-    if (auto prevLastModified = getLastModified()) {
-        if (input.getLastModified() != prevLastModified)
-            throw Error("'lastModified' attribute mismatch in input '%s', expected %d",
-                input.to_string(), *prevLastModified);
-    }
-
-    if (auto prevRev = getRev()) {
-        if (input.getRev() != prevRev)
-            throw Error("'rev' attribute mismatch in input '%s', expected %s",
-                input.to_string(), prevRev->gitRev());
-    }
-
-    if (auto prevRevCount = getRevCount()) {
-        if (input.getRevCount() != prevRevCount)
-            throw Error("'revCount' attribute mismatch in input '%s', expected %d",
-                input.to_string(), *prevRevCount);
-    }
-
    return {std::move(storePath), input};
 }

+void InputScheme::checkLocks(const Input & specified, const Input & final) const
+{
+    if (auto prevNarHash = specified.getNarHash()) {
+        if (final.getNarHash() != prevNarHash) {
+            if (final.getNarHash())
+                throw Error((unsigned int) 102, "NAR hash mismatch in input '%s', expected '%s' but got '%s'",
+                    specified.to_string(), prevNarHash->to_string(HashFormat::SRI, true), final.getNarHash()->to_string(HashFormat::SRI, true));
+            else
+                throw Error((unsigned int) 102, "NAR hash mismatch in input '%s', expected '%s' but got none",
+                    specified.to_string(), prevNarHash->to_string(HashFormat::SRI, true));
+        }
+    }
+
+    if (auto prevLastModified = specified.getLastModified()) {
+        if (final.getLastModified() != prevLastModified)
+            throw Error("'lastModified' attribute mismatch in input '%s', expected %d",
+                final.to_string(), *prevLastModified);
+    }
+
+    if (auto prevRev = specified.getRev()) {
+        if (final.getRev() != prevRev)
+            throw Error("'rev' attribute mismatch in input '%s', expected %s",
+                final.to_string(), prevRev->gitRev());
+    }
+
+    if (auto prevRevCount = specified.getRevCount()) {
+        if (final.getRevCount() != prevRevCount)
+            throw Error("'revCount' attribute mismatch in input '%s', expected %d",
+                final.to_string(), *prevRevCount);
+    }
+}
+
 std::pair<ref<InputAccessor>, Input> Input::getAccessor(ref<Store> store) const
 {
    try {
-        return scheme->getAccessor(store, *this);
+        auto [accessor, final] = getAccessorUnchecked(store);
+
+        scheme->checkLocks(*this, final);
+
+        return {accessor, std::move(final)};
    } catch (Error & e) {
        e.addTrace({}, "while fetching the input '%s'", to_string());
        throw;
    }
 }

+std::pair<ref<InputAccessor>, Input> Input::getAccessorUnchecked(ref<Store> store) const
+{
+    // FIXME: cache the accessor
+
+    if (!scheme)
+        throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
+
+    auto [accessor, final] = scheme->getAccessor(store, *this);
+
+    accessor->fingerprint = scheme->getFingerprint(store, final);
+
+    return {accessor, std::move(final)};
+}
+
 Input Input::applyOverrides(
    std::optional<std::string> ref,
    std::optional<Hash> rev) const
@@ -372,18 +401,6 @@ void InputScheme::clone(const Input & input, const Path & destDir) const
    throw Error("do not know how to clone input '%s'", input.to_string());
 }

-std::pair<StorePath, Input> InputScheme::fetch(ref<Store> store, const Input & input)
-{
-    auto [accessor, input2] = getAccessor(store, input);
-    auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy, input2.getName());
-    return {storePath, input2};
-}
-
-std::pair<ref<InputAccessor>, Input> InputScheme::getAccessor(ref<Store> store, const Input & input) const
-{
-    throw UnimplementedError("InputScheme must implement fetch() or getAccessor()");
-}
-
 std::optional<ExperimentalFeature> InputScheme::experimentalFeature() const
 {
    return {};
@@ -80,10 +80,21 @@ public:
     * Fetch the entire input into the Nix store, returning the
     * location in the Nix store and the locked input.
     */
-    std::pair<StorePath, Input> fetch(ref<Store> store) const;
+    std::pair<StorePath, Input> fetchToStore(ref<Store> store) const;

+    /**
+     * Return an InputAccessor that allows access to files in the
+     * input without copying it to the store. Also return a possibly
+     * unlocked input.
+     */
    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store) const;

+private:
+
+    std::pair<ref<InputAccessor>, Input> getAccessorUnchecked(ref<Store> store) const;
+
+public:
+
    Input applyOverrides(
        std::optional<std::string> ref,
        std::optional<Hash> rev) const;
@@ -173,9 +184,7 @@ struct InputScheme
        std::string_view contents,
        std::optional<std::string> commitMsg) const;

-    virtual std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input);
-
-    virtual std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const;
+    virtual std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const = 0;

    /**
     * Is this `InputScheme` part of an experimental feature?
@@ -202,6 +211,14 @@ struct InputScheme
     */
    virtual bool isLocked(const Input & input) const
    { return false; }
+
+    /**
+     * Check the locking attributes in `final` against
+     * `specified`. E.g. if `specified` has a `rev` attribute, then
+     * `final` must have the same `rev` attribute. Throw an exception
+     * if there is a mismatch.
+     */
+    virtual void checkLocks(const Input & specified, const Input & final) const;
 };

 void registerInputScheme(std::shared_ptr<InputScheme> && fetcher);
@@ -21,6 +21,7 @@
 #include <git2/refs.h>
 #include <git2/remote.h>
 #include <git2/repository.h>
+#include <git2/revparse.h>
 #include <git2/status.h>
 #include <git2/submodule.h>
 #include <git2/tree.h>
@@ -199,27 +200,10 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>

    Hash resolveRef(std::string ref) override
    {
-        // Handle revisions used as refs.
-        {
-            git_oid oid;
-            if (git_oid_fromstr(&oid, ref.c_str()) == 0)
-                return toHash(oid);
-        }
-
-        // Resolve short names like 'master'.
-        Reference ref2;
-        if (!git_reference_dwim(Setter(ref2), *this, ref.c_str()))
-            ref = git_reference_name(ref2.get());
-
-        // Resolve full references like 'refs/heads/master'.
-        Reference ref3;
-        if (git_reference_lookup(Setter(ref3), *this, ref.c_str()))
+        Object object;
+        if (git_revparse_single(Setter(object), *this, ref.c_str()))
            throw Error("resolving Git reference '%s': %s", ref, git_error_last()->message);
-
-        auto oid = git_reference_target(ref3.get());
-        if (!oid)
-            throw Error("cannot get OID for Git reference '%s'", git_reference_name(ref3.get()));
-
+        auto oid = git_object_id(object.get());
        return toHash(*oid);
    }

@@ -585,7 +585,7 @@ struct GitInputScheme : InputScheme
                        repoInfo.url
                        );
            } else
-                input.attrs.insert_or_assign("rev", Hash::parseAny(chomp(readFile(localRefFile)), HashAlgorithm::SHA1).gitRev());
+                input.attrs.insert_or_assign("rev", repo->resolveRef(ref).gitRev());

            // cache dir lock is removed at scope end; we will only use read-only operations on specific revisions in the remainder
        }
@@ -761,8 +761,6 @@ struct GitInputScheme : InputScheme
            ? getAccessorFromCommit(store, repoInfo, std::move(input))
            : getAccessorFromWorkdir(store, repoInfo, std::move(input));

-        accessor->fingerprint = final.getFingerprint(store);
-
        return {accessor, std::move(final)};
    }

@@ -98,6 +98,10 @@ struct GitArchiveInputScheme : InputScheme
        if (ref) input.attrs.insert_or_assign("ref", *ref);
        if (host_url) input.attrs.insert_or_assign("host", *host_url);

+        auto narHash = url.query.find("narHash");
+        if (narHash != url.query.end())
+            input.attrs.insert_or_assign("narHash", narHash->second);
+
        return input;
    }

@@ -111,6 +115,7 @@ struct GitArchiveInputScheme : InputScheme
            "narHash",
            "lastModified",
            "host",
+            "treeHash",
        };
    }

@@ -134,10 +139,13 @@ struct GitArchiveInputScheme : InputScheme
        assert(!(ref && rev));
        if (ref) path += "/" + *ref;
        if (rev) path += "/" + rev->to_string(HashFormat::Base16, false);
-        return ParsedURL {
+        auto url = ParsedURL {
            .scheme = std::string { schemeName() },
            .path = path,
        };
+        if (auto narHash = input.getNarHash())
+            url.query.insert_or_assign("narHash", narHash->to_string(HashFormat::SRI, true));
+        return url;
    }

    Input applyOverrides(
@@ -268,15 +276,15 @@ struct GitArchiveInputScheme : InputScheme
    {
        auto [input, tarballInfo] = downloadArchive(store, _input);

+        #if 0
        input.attrs.insert_or_assign("treeHash", tarballInfo.treeHash.gitRev());
+        #endif
        input.attrs.insert_or_assign("lastModified", uint64_t(tarballInfo.lastModified));

        auto accessor = getTarballCache()->getAccessor(tarballInfo.treeHash, false);

        accessor->setPathDisplay("«" + input.to_string() + "»");

-        accessor->fingerprint = input.getFingerprint(store);
-
        return {accessor, input};
    }

@@ -97,7 +97,7 @@ struct IndirectInputScheme : InputScheme
        return input;
    }

-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) override
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const override
    {
        throw Error("indirect input '%s' cannot be fetched directly", input.to_string());
    }
@@ -6,7 +6,11 @@ libfetchers_DIR := $(d)

 libfetchers_SOURCES := $(wildcard $(d)/*.cc)

-libfetchers_CXXFLAGS += -I src/libutil -I src/libstore
+# Not just for this library itself, but also for downstream libraries using this library
+
+INCLUDE_libfetchers := -I $(d)
+
+libfetchers_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers)

 libfetchers_LDFLAGS += $(THREAD_LDFLAGS) $(LIBGIT2_LIBS) -larchive

@@ -6,8 +6,8 @@
 #include "tarfile.hh"
 #include "store-api.hh"
 #include "url-parts.hh"
+#include "fs-input-accessor.hh"
 #include "posix-source-accessor.hh"
-
 #include "fetch-settings.hh"

 #include <sys/time.h>
@@ -161,9 +161,9 @@ struct MercurialInputScheme : InputScheme
        return {isLocal, isLocal ? url.path : url.base};
    }

-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & _input) override
+    StorePath fetchToStore(ref<Store> store, Input & input) const
    {
-        Input input(_input);
+        auto origRev = input.getRev();

        auto name = input.getName();

@@ -218,7 +218,7 @@ struct MercurialInputScheme : InputScheme
                    FileIngestionMethod::Recursive, HashAlgorithm::SHA256, {},
                    filter);

-                return {std::move(storePath), input};
+                return storePath;
            }
        }

@@ -242,13 +242,12 @@ struct MercurialInputScheme : InputScheme
            });
        };

-        auto makeResult = [&](const Attrs & infoAttrs, StorePath && storePath)
-            -> std::pair<StorePath, Input>
+        auto makeResult = [&](const Attrs & infoAttrs, const StorePath & storePath) -> StorePath
        {
            assert(input.getRev());
-            assert(!_input.getRev() || _input.getRev() == input.getRev());
+            assert(!origRev || origRev == input.getRev());
            input.attrs.insert_or_assign("revCount", getIntAttr(infoAttrs, "revCount"));
-            return {std::move(storePath), input};
+            return storePath;
        };

        if (input.getRev()) {
@@ -329,7 +328,7 @@ struct MercurialInputScheme : InputScheme
            {"revCount", (uint64_t) revCount},
        });

-        if (!_input.getRev())
+        if (!origRev)
            getCache()->add(
                *store,
                unlockedAttrs,
@@ -347,6 +346,15 @@ struct MercurialInputScheme : InputScheme
        return makeResult(infoAttrs, std::move(storePath));
    }

+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & _input) const override
+    {
+        Input input(_input);
+
+        auto storePath = fetchToStore(store, input);
+
+        return {makeStorePathAccessor(store, storePath), input};
+    }
+
    bool isLocked(const Input & input) const override
    {
        return (bool) input.getRev();
@@ -1,6 +1,8 @@
 #include "fetchers.hh"
 #include "store-api.hh"
 #include "archive.hh"
+#include "fs-input-accessor.hh"
+#include "posix-source-accessor.hh"

 namespace nix::fetchers {

@@ -87,6 +89,15 @@ struct PathInputScheme : InputScheme
        writeFile((CanonPath(getAbsPath(input)) / path).abs(), contents);
    }

+    std::optional<std::string> isRelative(const Input & input) const
+    {
+        auto path = getStrAttr(input.attrs, "path");
+        if (hasPrefix(path, "/"))
+            return std::nullopt;
+        else
+            return path;
+    }
+
    bool isLocked(const Input & input) const override
    {
        return (bool) input.getNarHash();
@@ -102,7 +113,7 @@ struct PathInputScheme : InputScheme
        throw Error("cannot fetch input '%s' because it uses a relative path", input.to_string());
    }

-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & _input) override
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & _input) const override
    {
        Input input(_input);
        std::string absPath;
@@ -144,7 +155,24 @@ struct PathInputScheme : InputScheme
        }
        input.attrs.insert_or_assign("lastModified", uint64_t(mtime));

-        return {std::move(*storePath), input};
+        return {makeStorePathAccessor(store, *storePath), std::move(input)};
+    }
+
+    std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
+    {
+        if (isRelative(input))
+            return std::nullopt;
+
+        /* If this path is in the Nix store, use the hash of the
+           store object and the subpath. */
+        auto path = getAbsPath(input);
+        try {
+            auto [storePath, subPath] = store->toStorePath(path.abs());
+            auto info = store->queryPathInfo(storePath);
+            return fmt("path:%s:%s", info->narHash.to_string(HashFormat::Base16, false), subPath);
+        } catch (Error &) {
+            return std::nullopt;
+        }
    }

    std::optional<ExperimentalFeature> experimentalFeature() const override
@@ -6,7 +6,7 @@ libmain_DIR := $(d)

 libmain_SOURCES := $(wildcard $(d)/*.cc)

-libmain_CXXFLAGS += -I src/libutil -I src/libstore
+libmain_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore)

 libmain_LDFLAGS += $(OPENSSL_LIBS)

@@ -123,14 +123,18 @@ public:
    }

    void pause() override {
-        state_.lock()->paused = true;
-        writeToStderr("\r\e[K");
+        auto state (state_.lock());
+        state->paused = true;
+        if (state->active)
+            writeToStderr("\r\e[K");
    }

    void resume() override {
-        state_.lock()->paused = false;
-        writeToStderr("\r\e[K");
-        state_.lock()->haveUpdate = true;
+        auto state (state_.lock());
+        state->paused = false;
+        if (state->active)
+            writeToStderr("\r\e[K");
+        state->haveUpdate = true;
        updateCV.notify_one();
    }

@@ -162,9 +166,7 @@ public:
            writeToStderr("\r\e[K" + filterANSIEscapes(s, !isTTY) + ANSI_NORMAL "\n");
            draw(state);
        } else {
-            auto s2 = s + ANSI_NORMAL "\n";
-            if (!isTTY) s2 = filterANSIEscapes(s2, true);
-            writeToStderr(s2);
+            writeToStderr(filterANSIEscapes(s, !isTTY) + "\n");
        }
    }

@@ -519,7 +521,7 @@ public:
    std::optional<char> ask(std::string_view msg) override
    {
        auto state(state_.lock());
-        if (!state->active || !isatty(STDIN_FILENO)) return {};
+        if (!state->active) return {};
        std::cerr << fmt("\r\e[K%s ", msg);
        auto s = trim(readLine(STDIN_FILENO));
        if (s.size() != 1) return {};
@@ -535,7 +537,7 @@ public:

 Logger * makeProgressBar()
 {
-    return new ProgressBar(shouldANSI());
+    return new ProgressBar(isTTY());
 }

 void startProgressBar()
@@ -305,7 +305,8 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource
 StorePath BinaryCacheStore::addToStoreFromDump(
    Source & dump,
    std::string_view name,
-    ContentAddressMethod method,
+    FileSerialisationMethod dumpMethod,
+    ContentAddressMethod hashMethod,
    HashAlgorithm hashAlgo,
    const StorePathSet & references,
    RepairFlag repair)
@@ -313,17 +314,27 @@ StorePath BinaryCacheStore::addToStoreFromDump(
    std::optional<Hash> caHash;
    std::string nar;

+    // Calculating Git hash from NAR stream not yet implemented. May not
+    // be possible to implement in single-pass if the NAR is in an
+    // inconvenient order. Could fetch after uploading, however.
+    if (hashMethod.getFileIngestionMethod() == FileIngestionMethod::Git)
+        unsupported("addToStoreFromDump");
+
    if (auto * dump2p = dynamic_cast<StringSource *>(&dump)) {
        auto & dump2 = *dump2p;
        // Hack, this gives us a "replayable" source so we can compute
        // multiple hashes more easily.
-        caHash = hashString(HashAlgorithm::SHA256, dump2.s);
-        switch (method.getFileIngestionMethod()) {
-        case FileIngestionMethod::Recursive:
+        //
+        // Only calculate if the dump is in the right format, however.
+        if (static_cast<FileIngestionMethod>(dumpMethod) == hashMethod.getFileIngestionMethod())
+            caHash = hashString(HashAlgorithm::SHA256, dump2.s);
+        switch (dumpMethod) {
+        case FileSerialisationMethod::Recursive:
            // The dump is already NAR in this case, just use it.
            nar = dump2.s;
            break;
-        case FileIngestionMethod::Flat:
+        case FileSerialisationMethod::Flat:
+        {
            // The dump is Flat, so we need to convert it to NAR with a
            // single file.
            StringSink s;
@@ -331,10 +342,11 @@ StorePath BinaryCacheStore::addToStoreFromDump(
            nar = std::move(s.s);
            break;
        }
+        }
    } else {
        // Otherwise, we have to do th same hashing as NAR so our single
        // hash will suffice for both purposes.
-        if (method != FileIngestionMethod::Recursive || hashAlgo != HashAlgorithm::SHA256)
+        if (dumpMethod != FileSerialisationMethod::Recursive || hashAlgo != HashAlgorithm::SHA256)
            unsupported("addToStoreFromDump");
    }
    StringSource narDump { nar };
@@ -349,7 +361,7 @@ StorePath BinaryCacheStore::addToStoreFromDump(
            *this,
            name,
            ContentAddressWithReferences::fromParts(
-                method,
+                hashMethod,
                caHash ? *caHash : nar.first,
                {
                    .others = references,
@@ -450,7 +462,7 @@ StorePath BinaryCacheStore::addToStore(
       non-recursive+sha256 so we can just use the default
       implementation of this method in terms of addToStoreFromDump. */

-    auto h = hashPath(accessor, path, method.getFileIngestionMethod(), hashAlgo, filter).first;
+    auto h = hashPath(accessor, path, method.getFileIngestionMethod(), hashAlgo, filter);

    auto source = sinkToSource([&](Sink & sink) {
        accessor.dumpPath(path, sink, filter);
@@ -125,7 +125,8 @@ public:
    StorePath addToStoreFromDump(
        Source & dump,
        std::string_view name,
-        ContentAddressMethod method,
+        FileSerialisationMethod dumpMethod,
+        ContentAddressMethod hashMethod,
        HashAlgorithm hashAlgo,
        const StorePathSet & references,
        RepairFlag repair) override;
@@ -147,7 +148,7 @@ public:

    void narFromPath(const StorePath & path, Sink & sink) override;

-    ref<SourceAccessor> getFSAccessor(bool requireValidPath) override;
+    ref<SourceAccessor> getFSAccessor(bool requireValidPath = true) override;

    void addSignatures(const StorePath & storePath, const StringSet & sigs) override;
@@ -1 +1 @@
 .21.0
 .22.0