buildRustCrate: add devDependencies parameter for buildTests

Cargo links [dev-dependencies] only when building test or bench
targets. buildRustCrate models normal deps (`dependencies`) and
build-script deps (`buildDependencies`) but has no equivalent: callers
that set `buildTests = true` for a crate with dev-deps must compute the
dev-dep set themselves and splice it into `dependencies` via .override.

Add a `devDependencies` parameter that is appended to the linked
dependency list only when `buildTests = true`. The append happens at the
single point where `dependencies` is normalized, so it threads through
to the --extern flags, the deps directory in configurePhase, the
metadata hash, and propagated build inputs without further changes.

When `buildTests = false` the parameter is dropped, so existing lib/bin
derivations are byte-identical to before — no rebuilds for users that
don't pass it.

Includes a test case in build-rust-crate/test that fails without the
change (`can't find crate for dev_dep`).

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -198,6 +198,13 @@ lib.makeOverridable
       # Rust build dependencies, i.e. other libraries that were built
       # with buildRustCrate and are used by a build script.
       buildDependencies,
+      # Rust dev-dependencies, i.e. other libraries that were built
+      # with buildRustCrate and are linked only when `buildTests = true`.
+      # Mirrors Cargo's `[dev-dependencies]`: ignored for the regular
+      # lib/bin build, appended to `dependencies` for the test build.
+      #
+      # Default: []
+      devDependencies,
       # Specify the "extern" name of a library if it differs from the library target.
       # See above for an extended explanation.
       #
@@ -323,6 +330,7 @@ lib.makeOverridable
       crate = crate_ // (lib.attrByPath [ crate_.crateName ] (attr: { }) crateOverrides crate_);
       dependencies_ = dependencies;
       buildDependencies_ = buildDependencies;
+      devDependencies_ = devDependencies;
       processedAttrs = [
         "src"
         "propagatedBuildInputs"
@@ -334,6 +342,7 @@ lib.makeOverridable
         "libPath"
         "buildDependencies"
         "dependencies"
+        "devDependencies"
         "features"
         "crateRenames"
         "crateName"
@@ -421,7 +430,10 @@ lib.makeOverridable
           ++ (crate.buildInputs or [ ])
           ++ buildInputs_
           ++ completePropagatedBuildInputs;
-        dependencies = map lib.getLib dependencies_;
+        # Dev-dependencies are only linked when building tests, mirroring
+        # Cargo. When buildTests is false this is a no-op, so the metadata
+        # hash and store path of normal lib/bin builds are unchanged.
+        dependencies = map lib.getLib (dependencies_ ++ lib.optionals buildTests_ devDependencies_);
         buildDependencies = map lib.getLib buildDependencies_;
 
         completeDeps = lib.unique (dependencies ++ lib.concatMap (dep: dep.completeDeps) dependencies);
@@ -617,6 +629,7 @@ lib.makeOverridable
     postInstall = crate_.postInstall or "";
     dependencies = crate_.dependencies or [ ];
     buildDependencies = crate_.buildDependencies or [ ];
+    devDependencies = crate_.devDependencies or [ ];
     crateRenames = crate_.crateRenames or { };
     buildTests = crate_.buildTests or false;
   }

pkgs/build-support/rust/build-rust-crate/test/default.nix

@@ -404,6 +404,27 @@ rec {
             "test something ... ok"
           ];
         };
+        rustLibTestsWithDevDependency =
+          let
+            devDep = mkHostCrate {
+              crateName = "dev-dep";
+              src = mkLib "src/lib.rs";
+            };
+          in
+          {
+            src = mkFile "src/lib.rs" ''
+              #[cfg(test)]
+              mod tests {
+                  #[test]
+                  fn uses_dev_dep() {
+                      assert_eq!(dev_dep::test(), 23);
+                  }
+              }
+            '';
+            devDependencies = [ devDep ];
+            buildTests = true;
+            expectedTestOutputs = [ "test tests::uses_dev_dep ... ok" ];
+          };
         rustBinTestsCombined = {
           src = symlinkJoin {
             name = "rust-bin-tests-combined";