buildRustCrate: Add extraRustcOptsForBuildRs

`extraRustcOpts` should not be used for build.rs, lest it contain
host-platform-specific options during cross builds.

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

@@ -13,7 +13,7 @@
 , crateRenames
 , crateVersion
 , extraLinkFlags
-, extraRustcOpts
+, extraRustcOptsForBuildRs
 , libName
 , libPath
 , release
@@ -24,7 +24,7 @@ let version_ = lib.splitString "-" crateVersion;
     version = lib.splitVersion (lib.head version_);
     rustcOpts = lib.foldl' (opts: opt: opts + " " + opt)
         (if release then "-C opt-level=3" else "-C debuginfo=2")
-        (["-C codegen-units=$NIX_BUILD_CORES -C incremental=no"] ++ extraRustcOpts);
+        (["-C codegen-units=$NIX_BUILD_CORES -C incremental=no"] ++ extraRustcOptsForBuildRs);
     buildDeps = mkRustcDepArgs buildDependencies crateRenames;
     authors = lib.concatStringsSep ":" crateAuthors;
     optLevel = if release then 3 else 0;

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

@@ -157,6 +157,11 @@ in
    # Example: [ "-Z debuginfo=2" ]
    # Default: []
    , extraRustcOpts
+   # A list of extra options to pass to rustc when building a build.rs.
+   #
+   # Example: [ "-Z debuginfo=2" ]
+   # Default: []
+   , extraRustcOptsForBuildRs
    # Whether to enable building tests.
    # Use true to enable.
    # Default: false
@@ -198,6 +203,7 @@ let crate = crate_ // (lib.attrByPath [ crate_.crateName ] (attr: {}) crateOverr
     nativeBuildInputs_ = nativeBuildInputs;
     buildInputs_ = buildInputs;
     extraRustcOpts_ = extraRustcOpts;
+    extraRustcOptsForBuildRs_ = extraRustcOptsForBuildRs;
     buildTests_ = buildTests;
 
     # crate2nix has a hack for the old bash based build script that did split
@@ -276,12 +282,16 @@ stdenv.mkDerivation (rec {
       lib.optionals (crate ? extraRustcOpts) crate.extraRustcOpts
       ++ extraRustcOpts_
       ++ (lib.optional (edition != null) "--edition ${edition}");
+    extraRustcOptsForBuildRs =
+      lib.optionals (crate ? extraRustcOptsForBuildRs) crate.extraRustcOptsForBuildRs
+      ++ extraRustcOptsForBuildRs_
+      ++ (lib.optional (edition != null) "--edition ${edition}");
 
 
     configurePhase = configureCrate {
       inherit crateName buildDependencies completeDeps completeBuildDeps crateDescription
               crateFeatures crateRenames libName build workspace_member release libPath crateVersion
-              extraLinkFlags extraRustcOpts
+              extraLinkFlags extraRustcOptsForBuildRs
               crateAuthors crateHomepage verbose colors;
     };
     buildPhase = buildCrate {
@@ -302,8 +312,9 @@ stdenv.mkDerivation (rec {
   rust = rustc;
   release = crate_.release or true;
   verbose = crate_.verbose or true;
-  extraRustcOpts = [];
   features = [];
+  extraRustcOpts = [];
+  extraRustcOptsForBuildRs = [];
   nativeBuildInputs = [];
   buildInputs = [];
   crateOverrides = defaultCrateOverrides;