Rust + Nix Notes

Some nix + rust related notes after a weekend hacking around with bindgen, they might be helpful for the next person who wanders down this road.

Bindgen Dependency Linking

I’m continually impressed with the amount of documentation the rust eco-system maintains in order to help onboard new developers and bindgen is no exception. Following the book you’ll walk through a set of quick start tutorials and eventually setup a build.rs file.

Notice at the top build script you have to specify a search path for the library you wish to link against.

fn main() {
    // Tell cargo to look for shared libraries in the specified directory
    println!("cargo:rustc-link-search=/path/to/lib");

    //.... 

How will this work on nix then? Well some kind soul has worked it out for us already. Bindgen has been wrapped so it injects the needed search paths for us:

exec -a "$0" @unwrapped@/bin/bindgen "$@" $sep $cxxflags @cincludes@ $NIX_CFLAGS_COMPILE

ref: https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/tools/rust/bindgen/wrapper.sh

Bindgen accepts arbitrary arguments that will be forwarded directly to the compiler, so the wrapper passes NIX_CFLAGS_COMPILE which should have all the include paths for any dependencies within this nix build or shell. Hurrah!

Rust Package Src Changes

I wanted to use the latest master of bindgen to test the new macro generation changes but how does one override the src of a rust package?

For any normal package, you can just use the derivation’s overrideAttrs to change the src attr but for rust packages there’s an implicit 2nd step: updating the cargo hash! When you change the src, you may also be changing the dependent crates so the build-rust-package derivation will rightfully complain.

You will have to ‘backwards engineer’ the hash via cargoDeps attr that the derivation exposes:

final: prev: {
  rust-bindgen-unwrapped = prev.rust-bindgen-unwrapped.overrideAttrs rec { 
    version = "main";
    src = prev.fetchFromGitHub {
      owner  = "rust-lang";
      repo   = "rust-bindgen";
      rev    = "66b65517b5568e122e9ce5902dd4868aa2b43d25";
      sha256 = "sha256-aXF6nR3DpeH3o05uyhaa3s8fJF6JUGs/J9bvQz0LGSs=";
    };
    cargoDeps = prev.rustPlatform.fetchCargoTarball({ 
      inherit src; 
      hash = "sha256-Pqnx+9Oa9ypRQDdhwIQ8XlPm8WAeg4CvEr7/sFyMWCI=";
    });
  };
}