flake: link host code with mold on Linux; DECISIONS: mold, and no LTO

mold applies to the host target only, in the dev shell, the crane builds,
and the packaged binary (proc macros and build scripts of plugins). A
rebuild of the host test binary after a change takes 0.9 s (15 s with
GNU ld). Measured LTO: no gain worth 4 more minutes per release build.

flake.nix

@@ -25,7 +25,17 @@
         # the native linker, even when the host itself is started with a bare
         # PATH. Keep this same runtime path in the packaged host and exercise
         # the installed binary below with PATH deliberately empty.
-        runtimePath = pkgs.lib.makeBinPath [ rust pkgs.stdenv.cc ];
+        # mold links host code much faster than GNU ld. Only on Linux, and only
+        # for the host target: plugins for wasm32-wasip2 link with rust-lld. Nix
+        # brings mold along, so a build without Nix keeps the default linker.
+        mold = pkgs.lib.optionals pkgs.stdenv.hostPlatform.isLinux [ pkgs.mold ];
+        hostTarget = pkgs.lib.toUpper (builtins.replaceStrings [ "-" ] [ "_" ]
+          pkgs.stdenv.hostPlatform.rust.rustcTarget);
+        moldVar = "CARGO_TARGET_${hostTarget}_RUSTFLAGS";
+        moldEnv = pkgs.lib.optionalAttrs pkgs.stdenv.hostPlatform.isLinux {
+          ${moldVar} = "-C link-arg=-fuse-ld=mold";
+        };
+        runtimePath = pkgs.lib.makeBinPath ([ rust pkgs.stdenv.cc ] ++ mold);
         # A WASI clang with wasi-libc, so that build scripts (the `cc` crate)
         # can compile C for wasm32-wasip2. Its headers are WASI 0.1: C code
         # that needs sockets does not build.
@@ -39,7 +49,8 @@
           pname = "sausage";
           version = "0.1.0";
           src = ./.;
-        };
+          nativeBuildInputs = mold;
+        } // moldEnv;
         # The test packages for `checks.tests`.
         testPackages = "--workspace";
         cargoArtifacts = craneLib.buildDepsOnly (common // {
@@ -55,13 +66,14 @@
           doNotRemoveReferencesToRustToolchain = true;
           cargoExtraArgs = "--locked -p sausage";
           doCheck = false;
-          nativeBuildInputs = [ pkgs.makeWrapper ];
+          nativeBuildInputs = [ pkgs.makeWrapper ] ++ mold;
           # The native `cargo` plugin runs cargo at runtime. Build scripts need a C linker,
           # and C code for WASI needs the WASI clang.
           postInstall = ''
             wrapProgram $out/bin/sausage --prefix PATH : ${runtimePath} \
               --set-default CC_wasm32_wasip2 ${wasiCcEnv.CC_wasm32_wasip2} \
-              --set-default AR_wasm32_wasip2 ${wasiCcEnv.AR_wasm32_wasip2}
+              --set-default AR_wasm32_wasip2 ${wasiCcEnv.AR_wasm32_wasip2} \
+              ${pkgs.lib.optionalString pkgs.stdenv.hostPlatform.isLinux "--set-default ${moldVar} '-C link-arg=-fuse-ld=mold'"}
           '';
           meta.mainProgram = "sausage";
         });
@@ -101,7 +113,7 @@
           # Nothing builds on the test run's target folder: don't store it.
           doInstallCargoArtifacts = false;
           # The SDK tests run sample plugins with the wasmtime CLI.
-          nativeBuildInputs = [ pkgs.wasmtime ];
+          nativeBuildInputs = [ pkgs.wasmtime ] ++ mold;
         });
         # Exercise the installed host exactly as the loop launches it: the
         # caller has no Cargo on PATH, so only the packaged runtime toolchain
@@ -191,8 +203,8 @@
         apps.default = { type = "app"; program = "${sausage}/bin/sausage"; };
         apps.loop = { type = "app"; program = "${loop}/bin/sausage-loop"; };
 
-        devShells.default = pkgs.mkShell (wasiCcEnv // {
-          packages = [ rust pkgs.wasmtime pkgs.wasm-tools ];
+        devShells.default = pkgs.mkShell (wasiCcEnv // moldEnv // {
+          packages = [ rust pkgs.wasmtime pkgs.wasm-tools ] ++ mold;
           shellHook = ''
             export CARGO_HOME="$PWD/.cargo-home"
           '';