Independent Packaging

This is how to use cargo2nix to package Rust software indpenendently of its hosting repository.

Introduction

It's not always desireable or best to nixify projects by embedding nix files into their repositories. This is in fact avoided in nixpkgs. Instead, you can ship a pre-made Cargo.nix and a default.nix that locates and provides the external source to produce a build. Rust Analyzer is packaged here as a demonstration.

First clone the target repository at the commit you wish to generate a build for:

git clone --depth 1 --branch 2020-11-30 [email protected]:rust-analyzer/rust-analyzer.git 

Inside the clone, generate / update the lock file and then create a Cargo.nix

cargo generate-lockfile
cargo2nix -f

Copy just the generated Cargo.nix to the repository you wish to host the new package. Now we need to make a default.nix to consume the Cargo.nix (also called packageFun). The main difference is that we will also pass a workspaceSrc argument to makePackageSet'.

  rustPkgs = pkgs.rustBuilder.makePackageSet' {
    inherit rustChannel;
    packageFun = import ./Cargo.nix;
    
    workspaceSrc = pkgs.fetchFromGitHub {
      owner = "rust-analyzer";
      repo = "rust-analyzer";
      rev = "ac30710ada112984c9cf79c4af39ad666d000171";
      sha256 = "1ycnl9y7vhv7yd8w21904cyfik2y7jjzpb17xkdpiazw5riyyzh3";
    };
    
    # You can also use local paths for local development with a checked out copy
    # workspaceSrc = ~/vendor/rust-analyzer;
  };

In Rust Analyzer's case, we also need to pass a custom optional localPatterns argument because several of its crates are not in locations picked up by the default.

  rustPkgs = pkgs.rustBuilder.makePackageSet' {
    # ... the other arguments
  
    localPatterns = [ ''^(src|tests|crates|xtask|assets|templates)(/.*)?'' ''[^/]*\.(rs|toml)$'' ];
  };

Nix will by default build packages that contain some extra outputs (which cargo2nix utilizes to coordinate between dependencies) that might be necessary when being used as a buildDependency but will collide when attempting to install finished software into a profile. We can expose just the bin output by selecting it explicitly in the final expression of our default.nix:

#... previously expressed rustPkgs
in {
  rust-analyzer = (rustPkgs.workspace.rust-analyzer {}).bin;
}

You can test that this package builds like so:

nix-build -A rust-analyzer

You will now see a clean binary output at result-bin/bin/rust-analyzer