Simple guide to using mold on NixOS.

I struggled to get this right but after a couple days I figured it out.

{
  home.file.".cargo/config.toml".text = ''
    [target.'cfg(target_os = "linux")']
    linker = "${pkgs.clang}/bin/clang"
    rustflags = ["-C", "link-arg=-fuse-ld=${pkgs.mold-wrapped}/bin/mold"]
  '';
}

The gotcha was using the pkgs.mold-wrapped instead of pkgs.mold. Unwrapped mold doesn't set the runpath correctly into the built binaries. This is a very NixOS specific issue because of it not being FHS compliant.

With this set you can see the library paths from nix store linked into the binary, use readelf -d ./target/debug/my-binary to verify the runpaths getting set correctly.

You wouldn't need to though, since this works.

Some useful commands when debugging dynamic linking issues:

• readelf -d ./path/to/binary will tell you shared lib dependencies and you should also look for RUNPATHs.
• ldd ./path/to/binary will tell you the shared lib dependencies and their resolved paths.
• objdump -p ./path/to/binary can also be useful.

All of them have some similarities in what they show.

You may also want to look at nix-ld and this nix-ld blog post.