Add release note

This allows doing `builtins.getFlake ./subflake` instead of ugly hacks.

doc/manual/rl-next/getflake-path.md

@@ -0,0 +1,6 @@
+---
+synopsis: "`builtins.getFlake` now supports path values"
+prs: [15290]
+---
+
+`builtins.getFlake` now accepts path values in addition to flakerefs, allowing you to write `builtins.getFlake ./subflake` instead of having to use ugly workarounds to construct a pure flakeref.

src/libflake/flake-primops.cc

@@ -35,35 +35,39 @@ namespace nix::flake::primops {
 PrimOp getFlake(const Settings & settings)
 {
     auto prim_getFlake = [&settings](EvalState & state, const PosIdx pos, Value ** args, Value & v) {
-        std::string flakeRefS(
-            state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.getFlake"));
-        auto flakeRef = nix::parseFlakeRef(state.fetchSettings, flakeRefS, {}, true);
-        if (state.settings.pureEval && !flakeRef.input.isLocked(state.fetchSettings))
-            throw Error(
-                "cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)",
-                flakeRefS,
-                state.positions[pos]);
+        state.forceValue(*args[0], pos);
 
-        callFlake(
-            state,
-            lockFlake(
-                settings,
-                state,
-                flakeRef,
-                LockFlags{
-                    .updateLockFile = false,
-                    .writeLockFile = false,
-                    .useRegistries = !state.settings.pureEval && settings.useRegistries,
-                    .allowUnlocked = !state.settings.pureEval,
-                }),
-            v);
+        LockFlags lockFlags{
+            .updateLockFile = false,
+            .writeLockFile = false,
+            .useRegistries = !state.settings.pureEval && settings.useRegistries,
+            .allowUnlocked = !state.settings.pureEval,
+        };
+
+        if (args[0]->type() == nPath) {
+            auto path = state.realisePath(pos, *args[0]);
+            callFlake(state, lockFlake(settings, state, path, lockFlags), v);
+        } else {
+            NixStringContext context;
+            std::string flakeRefS(
+                state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.getFlake"));
+
+            auto flakeRef = nix::parseFlakeRef(state.fetchSettings, flakeRefS, {}, true);
+            if (state.settings.pureEval && !flakeRef.input.isLocked(state.fetchSettings))
+                throw Error(
+                    "cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)",
+                    flakeRefS,
+                    state.positions[pos]);
+
+            callFlake(state, lockFlake(settings, state, flakeRef, lockFlags), v);
+        }
     };
 
     return PrimOp{
         .name = "__getFlake",
         .args = {"args"},
         .doc = R"(
-          Fetch a flake from a flake reference, and return its output attributes and some metadata. For example:
+          Fetch a flake from a flake reference or a path, and return its output attributes and some metadata. For example:
 
           ```nix
           (builtins.getFlake "nix/55bc52401966fbffa525c574c14f67b00bc4fb3a").packages.x86_64-linux.nix

src/libflake/flake.cc

@@ -416,10 +416,8 @@ static LockFile readLockFile(const fetchers::Settings & fetchSettings, const Sou
                                      : 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(const Settings & settings, EvalState & state, const FlakeRef & topRef, const LockFlags & lockFlags)
+LockedFlake lockFlake(
+    const Settings & settings, EvalState & state, const FlakeRef & topRef, const LockFlags & lockFlags, Flake flake)
 {
     experimentalFeatureSettings.require(Xp::Flakes);
 
@@ -427,8 +425,6 @@ lockFlake(const Settings & settings, EvalState & state, const FlakeRef & topRef,
     auto useRegistriesTop = useRegistries ? fetchers::UseRegistries::All : fetchers::UseRegistries::No;
     auto useRegistriesInputs = useRegistries ? fetchers::UseRegistries::Limited : fetchers::UseRegistries::No;
 
-    auto flake = getFlake(state, topRef, useRegistriesTop, {});
-
     if (lockFlags.applyNixConfig) {
         flake.config.apply(settings);
         state.store->setOptions();
@@ -908,6 +904,22 @@ lockFlake(const Settings & settings, EvalState & state, const FlakeRef & topRef,
     }
 }
 
+LockedFlake
+lockFlake(const Settings & settings, EvalState & state, const FlakeRef & topRef, const LockFlags & lockFlags)
+{
+    auto useRegistries = lockFlags.useRegistries.value_or(settings.useRegistries);
+    auto useRegistriesTop = useRegistries ? fetchers::UseRegistries::All : fetchers::UseRegistries::No;
+    return lockFlake(settings, state, topRef, lockFlags, getFlake(state, topRef, useRegistriesTop, {}));
+}
+
+LockedFlake
+lockFlake(const Settings & settings, EvalState & state, const SourcePath & flakeDir, const LockFlags & lockFlags)
+{
+    /* We need a fake flakeref to put in the `Flake` struct, but it's not used for anything. */
+    auto fakeRef = parseFlakeRef(state.fetchSettings, "flake:get-flake");
+    return lockFlake(settings, state, fakeRef, lockFlags, readFlake(state, fakeRef, fakeRef, fakeRef, flakeDir, {}));
+}
+
 static ref<SourceAccessor> makeInternalFS()
 {
     auto internalFS = make_ref<MemorySourceAccessor>(MemorySourceAccessor{});

src/libflake/include/nix/flake/flake.hh

@@ -214,9 +214,16 @@ struct LockFlags
     std::set<NonEmptyInputAttrPath> inputUpdates;
 };
 
+/*
+ * 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(const Settings & settings, EvalState & state, const FlakeRef & flakeRef, const LockFlags & lockFlags);
 
+LockedFlake
+lockFlake(const Settings & settings, EvalState & state, const SourcePath & flakeDir, const LockFlags & lockFlags);
+
 void callFlake(EvalState & state, const LockedFlake & lockedFlake, Value & v);
 
 /**

tests/functional/flakes/common.sh

@@ -32,6 +32,8 @@ writeSimpleFlake() {
     baseName = builtins.baseNameOf ./.;
 
     root = ./.;
+
+    number = 123;
   };
 }
 EOF

tests/functional/flakes/get-flake.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+
+source ./common.sh
+
+createFlake1
+
+mkdir -p "$flake1Dir/subflake"
+cat > "$flake1Dir/subflake/flake.nix" <<EOF
+{
+  outputs = { self }:
+    let
+      # Bad, legacy way of getting a flake from an input.
+      parentFlake = builtins.getFlake (builtins.flakeRefToString { type = "path"; path = self.sourceInfo.outPath; narHash = self.narHash; });
+      # Better way using a path value.
+      parentFlake2 = builtins.getFlake ./..;
+    in {
+      x = parentFlake.number;
+      y = parentFlake2.number;
+    };
+}
+EOF
+git -C "$flake1Dir" add subflake/flake.nix
+
+[[ $(nix eval "$flake1Dir/subflake#x") = 123 ]]
+
+[[ $(nix eval "$flake1Dir/subflake#y") = 123 ]]

tests/functional/flakes/meson.build

@@ -34,6 +34,7 @@ suites += {
     'source-paths.sh',
     'old-lockfiles.sh',
     'trace-ifd.sh',
+    'get-flake.sh',
   ],
   'workdir' : meson.current_source_dir(),
 }