Building Rust Projects with Nix

I’ve been working on some Rust personal projects and wanted to build and deploy them with Nix. I was already using a Flake for the development shell, so I figured I’d work out how to build with Nix. It took me a little longer than I’d like to admit, but looking back it’s fairly simple, so I thought I’d explain how in a blog post.

We’re going to use naersk, which is pretty much a drop-in replacement for the built-in rustPlatform.buildRustPackage, but with the added bonus of being able to avoid recompiling every dependency when you rebuild your own crate.

Build file setup

Here’s the file I came up with.

{ naersk', pkgs }:

naersk'.buildPackage {
  pname = "my_package";
  version = "0.1.0";

  src = pkgs.lib.cleanSource ../backend;

  cargoLock = ../backend/Cargo.lock;

  nativeBuildInputs = [ pkgs.pkg-config ];
  buildInputs = [ pkgs.openssl pkgs.sqlite ];

  RUSTFLAGS = [ "--cfg" "sqlx_macros_offline" ];
}

I’ve left a few extra items in for demo purposes. We pass in naersk', which comes from the flake as naersk' = pkgs.callPackage naersk {};. We set a few configurations, including where the source comes from, and point to the Cargo lockfile (I believe it will get auto-detected if it’s in the root). I am using sqlx to build, so I can use RUSTFLAGS to set some necessary flags for compiling my setup. And that’s basically it.

In the flake

Naersk is included as an input to the flake, from GitHub. We get naersk' with pkgs.CallPackage, like so:

naersk' = pkgs.callPackage naersk {};

I include the build file in the outputs of my flake like so:

backend = import ./nix/backend.nix { inherit pkgs naersk'; };

And then I can set build with nix build .#backend by setting packages.backend = backend;.

Conclusion

Perhaps there is a cleaner way to write this, but I’m pleased with its functionality. Using naersk means I don’t have to recompile every crate my project depends on, only my own code, which saves a lot of time when rebuilding. As always, let me know if you have any comments, feedback, or questions.