Merge pull request #1498 from cachix/rust-flags

rust: set empty flags if empty

src/modules/languages/rust.nix

@@ -35,7 +35,7 @@ in
       defaultText = lib.literalExpression ''[ ]'';
       description = ''
         List of extra [targets](https://github.com/nix-community/fenix#supported-platforms-and-targets)
-        to install. Defaults to only the native target. 
+        to install. Defaults to only the native target.
       '';
     };
 
@@ -128,6 +128,7 @@ in
         env =
           let
             moldFlags = lib.optionalString cfg.mold.enable "-C link-arg=-fuse-ld=mold";
+            optionalEnv = cond: str: if cond then str else null;
           in
           {
             # RUST_SRC_PATH is necessary when rust-src is not at the same location as
@@ -136,8 +137,8 @@ in
               if cfg.toolchain ? rust-src
               then "${cfg.toolchain.rust-src}/lib/rustlib/src/rust/library"
               else pkgs.rustPlatform.rustLibSrc;
-            RUSTFLAGS = "${moldFlags} ${cfg.rustflags}";
-            RUSTDOCFLAGS = "${moldFlags}";
+            RUSTFLAGS = optionalEnv (moldFlags != "" || cfg.rustflags != "") (lib.concatStringsSep " " (lib.filter (x: x != "") [moldFlags cfg.rustflags]));
+            RUSTDOCFLAGS = optionalEnv (moldFlags != "") moldFlags;
             CFLAGS = lib.optionalString pkgs.stdenv.isDarwin "-iframework ${config.devenv.profile}/Library/Frameworks";
           };
 

tests/rust/devenv.nix

@@ -0,0 +1,10 @@
+{
+  languages.rust.enable = true;
+  languages.rust.mold.enable = false;
+  enterTest = ''
+    if [ -n "$RUSTFLAGS" ]; then
+      echo "RUSTFLAGS is set, but it should not be"
+      exit 1
+    fi
+  '';
+}