Add build-time-fetch-tree experimental feature

Author: edolstra

src/libfetchers/builtin.cc

@@ -12,6 +12,8 @@ namespace nix {
 
 static void builtinFetchTree(const BuiltinBuilderContext & ctx)
 {
+    experimentalFeatureSettings.require(Xp::BuildTimeFetchTree);
+
     auto out = get(ctx.drv.outputs, "out");
     if (!out)
         throw Error("'builtin:fetch-tree' requires an 'out' output");

src/libflake/flake.cc

@@ -133,6 +133,8 @@ static FlakeInput parseFlakeInput(
             } else if (attr.name == sBuildTime) {
                 expectType(state, nBool, *attr.value, attr.pos);
                 input.buildTime = attr.value->boolean();
+                if (input.buildTime)
+                    experimentalFeatureSettings.require(Xp::BuildTimeFetchTree);
             } else if (attr.name == sInputs) {
                 input.overrides =
                     parseFlakeInputs(state, attr.value, attr.pos, lockRootAttrPath, flakeDir, false).first;

src/libutil/experimental-features.cc

@@ -16,15 +16,15 @@ struct ExperimentalFeatureDetails
 
 /**
  * If two different PRs both add an experimental feature, and we just
- * used a number for this, we *woudln't* get merge conflict and the
+ * used a number for this, we *wouldn't* get merge conflict and the
  * counter will be incremented once instead of twice, causing a build
  * failure.
  *
  * By instead defining this instead as 1 + the bottom experimental
  * feature, we either have no issue at all if few features are not added
  * at the end of the list, or a proper merge conflict if they are.
  */
-constexpr size_t numXpFeatures = 1 + static_cast<size_t>(Xp::BLAKE3Hashes);
+constexpr size_t numXpFeatures = 1 + static_cast<size_t>(Xp::BuildTimeFetchTree);
 
 constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails = {{
     {
@@ -304,6 +304,14 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
         )",
         .trackingUrl = "",
     },
+    {
+        .tag = Xp::BuildTimeFetchTree,
+        .name = "build-time-fetch-tree",
+        .description = R"(
+            Enable the built-in derivation `builtin:fetch-tree`, as well as the flake input attribute `buildTime`.
+        )",
+        .trackingUrl = "",
+    },
 }};
 
 static_assert(

src/libutil/include/nix/util/experimental-features.hh

@@ -36,6 +36,7 @@ enum struct ExperimentalFeature {
     PipeOperators,
     ExternalBuilders,
     BLAKE3Hashes,
+    BuildTimeFetchTree,
 };
 
 extern std::set<std::string> stabilizedFeatures;

tests/functional/flakes/build-time-flake-inputs.sh

@@ -35,27 +35,27 @@ EOF
 
 cp "${config_nix}" "$repo/"
 git -C "$repo" add flake.nix config.nix
-nix flake lock "$repo"
+nix flake lock --extra-experimental-features build-time-fetch-tree "$repo"
 git -C "$repo" add flake.lock
 git -C "$repo" commit -a -m foo
 
 clearStore
 
-nix build --out-link "$TEST_ROOT/result" -L "$repo"
+nix build --extra-experimental-features build-time-fetch-tree --out-link "$TEST_ROOT/result" -L "$repo"
 [[ $(cat "$TEST_ROOT/result") = world ]]
 
 echo utrecht > "$lazy/who"
 git -C "$lazy" commit -a -m foo
 
-nix flake update --flake "$repo"
+nix flake update --extra-experimental-features build-time-fetch-tree --flake "$repo"
 
 clearStore
 
-nix build --out-link "$TEST_ROOT/result" -L "$repo"
+nix build --extra-experimental-features build-time-fetch-tree --out-link "$TEST_ROOT/result" -L "$repo"
 [[ $(cat "$TEST_ROOT/result") = utrecht ]]
 
 rm -rf "$lazy"
 
 clearStore
 
-expectStderr 1 nix build --out-link "$TEST_ROOT/result" -L "$repo" | grepQuiet "Cannot build.*source.drv"
+expectStderr 1 nix build --extra-experimental-features build-time-fetch-tree --out-link "$TEST_ROOT/result" -L "$repo" | grepQuiet "Cannot build.*source.drv"