nix: build with Mold linker on x86_64-linux

Signed-off-by: NotAShelf <[email protected]>
Change-Id: I771c36297577aba189058a2183ec2b4a6a6a6964

Author: NotAShelf

README.md

@@ -43,7 +43,7 @@ on your system: it is pretty _[fast](#benchmarks)_...
 - Fast
 - Really fast
 - Minimal dependencies
-- Tiny binary (~370kb)
+- Tiny binary (~370kb [^1])
 - Actually really fast
 - Cool NixOS logo (other, inferior, distros are not supported)
 - Reliable detection of following info:
@@ -60,6 +60,9 @@ on your system: it is pretty _[fast](#benchmarks)_...
 - Did I mention fast?
 - Respects [`NO_COLOR` spec](https://no-color.org/)
 
+[^1]: With the Mold linker, which is enabled by default in the Flake package,
+    the binary size is roughly 350kb. That's nearly 20kb reduction in size :)
+
 ## Motivation
 
 Fastfetch, as its name probably hinted, is a very fast fetch tool written in C.

flake.nix

@@ -10,9 +10,12 @@
     forEachSystem = nixpkgs.lib.genAttrs systems;
     pkgsForEach = nixpkgs.legacyPackages;
   in {
-    packages = forEachSystem (system: {
+    packages = forEachSystem (system: let
+      pkgs = pkgsForEach.${system};
+    in {
       default = self.packages.${system}.microfetch;
-      microfetch = pkgsForEach.${system}.callPackage ./nix/package.nix {};
+      microfetch = pkgs.callPackage ./nix/package.nix {};
+      microfetch-mold = pkgs.callPackage ./nix/package.nix {useMold = true;};
     });
 
     devShells = forEachSystem (system: {

nix/package.nix

@@ -1,14 +1,22 @@
 {
   lib,
-  rustPlatform,
+  stdenv,
   stdenvAdapters,
+  rustPlatform,
   llvm,
+  useMold ? stdenv.isLinux && !stdenv.hostPlatform.isAarch,
 }: let
   toml = (lib.importTOML ../Cargo.toml).package;
   pname = toml.name;
   inherit (toml) version;
+
+  # Select stdenv based on useMold flag
+  stdenv =
+    if useMold
+    then stdenvAdapters.useMoldLinker llvm.stdenv
+    else llvm.stdenv;
 in
-  rustPlatform.buildRustPackage.override {stdenv = stdenvAdapters.useMoldLinker llvm.stdenv;} {
+  rustPlatform.buildRustPackage.override {inherit stdenv;} {
     inherit pname version;
     src = let
       fs = lib.fileset;
@@ -26,7 +34,14 @@ in
 
     cargoLock.lockFile = ../Cargo.lock;
     enableParallelBuilding = true;
-    env.RUSTFLAGS = "-C link-arg=-fuse-ld=mold";
+    buildNoDefaultFeatures = true;
+    doCheck = false;
+
+    # Only set RUSTFLAGS for mold if useMold is enabled
+    env = lib.optionalAttrs useMold {
+      CARGO_LINKER = "clang";
+      RUSTFLAGS = "-C link-arg=-fuse-ld=mold";
+    };
 
     meta = {
       description = "Microscopic fetch script in Rust, for NixOS systems";