rust: use mold

src/modules/languages/rust.nix

@@ -46,6 +46,12 @@ in
       description = "The rustup toolchain to install.";
     };
 
+    mold.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = pkgs.stdenv.isLinux && pkgs.stdenv.isx86_64 && cfg.targets == [ ];
+      description = "Enable mold as the linker.";
+    };
+
     toolchain = lib.mkOption {
       type = lib.types.submodule ({
         freeformType = lib.types.attrsOf lib.types.package;
@@ -95,23 +101,29 @@ in
         # enable compiler tooling by default to expose things like cc
         languages.c.enable = lib.mkDefault true;
 
-        # RUST_SRC_PATH is necessary when rust-src is not at the same location as
-        # as rustc. This is the case with the rust toolchain from nixpkgs.
-        env.RUST_SRC_PATH =
-          if cfg.toolchain ? rust-src
-          then "${cfg.toolchain.rust-src}/lib/rustlib/src/rust/library"
-          else pkgs.rustPlatform.rustLibSrc;
+
+        env =
+          let
+            darwinFlags = lib.optionalString pkgs.stdenv.isDarwin "-L framework=${config.devenv.profile}/Library/Frameworks";
+            moldFlags = lib.optionalString cfg.mold.enable "-C link-arg=-fuse-ld=${pkgs.mold-wrapped}/bin/ld.mold";
+          in
+          {
+            # RUST_SRC_PATH is necessary when rust-src is not at the same location as
+            # as rustc. This is the case with the rust toolchain from nixpkgs.
+            RUST_SRC_PATH =
+              if cfg.toolchain ? rust-src
+              then "${cfg.toolchain.rust-src}/lib/rustlib/src/rust/library"
+              else pkgs.rustPlatform.rustLibSrc;
+            RUSTFLAGS = "${darwinFlags} ${moldFlags}";
+            RUSTDOCFLAGS = "${darwinFlags} ${moldFlags}";
+            CFLAGS = lib.optionalString pkgs.stdenv.isDarwin "-iframework ${config.devenv.profile}/Library/Frameworks";
+          };
 
         pre-commit.tools.cargo = mkOverrideTools cfg.toolchain.cargo or null;
         pre-commit.tools.rustfmt = mkOverrideTools cfg.toolchain.rustfmt or null;
         pre-commit.tools.clippy = mkOverrideTools cfg.toolchain.clippy or null;
       }
     ))
-    (lib.mkIf (cfg.enable && pkgs.stdenv.isDarwin) {
-      env.RUSTFLAGS = "-L framework=${config.devenv.profile}/Library/Frameworks";
-      env.RUSTDOCFLAGS = "-L framework=${config.devenv.profile}/Library/Frameworks";
-      env.CFLAGS = "-iframework ${config.devenv.profile}/Library/Frameworks";
-    })
     (lib.mkIf (cfg.channel != "nixpkgs") (
       let
         rustPackages = fenix.packages.${pkgs.stdenv.system};