rust-incremental-compilation.nix

{ sources ? import ./sources.nix # using niv
}:
let
  pkgs = import sources.nixpkgs {};

  rust-pkgs = import "${sources.nixpkgs-mozilla}/rust-overlay.nix" (pkgs // rust-pkgs) pkgs;

  naersk = pkgs.callPackage sources.naersk {
    cargo = rust-pkgs.latest.rustChannels.nightly.cargo;
    rustc = rust-pkgs.latest.rustChannels.stable.rust;
  };

  program = naersk.buildPackage {
    src = ../.;
    nativeBuildInputs = [ pkgs.pkg-config ];
    buildInputs = [ pkgs.openssl ];
  };

  docker-program = pkgs.dockerTools.buildLayeredImage {
    name = "program";
    tag = program.version;
    contents = [ program pkgs.cacert ];
  };

  # --- here is the interesting part --
  # we copy the binary built by cargo during development
  # this was built with cargo and dependencies provided by devShell
  # if it wasn't, the docker image would not function
  # the reason for all this is to be able to quickly bake docker images via nix
  # and shoot them with skopeo
  program-incremental = pkgs.runCommand "program-0.1.0" {} ''
    mkdir -p $out/bin
    cp ${../target/debug/program} $out/bin/program
  '';
 
  docker-program-incremental = pkgs.dockerTools.buildLayeredImage {
    name = "program";
    tag = "dev";
    contents = [ program-incremental pkgs.openssl pkgs.cacert ];
  };

in
{
  inherit
    program
    docker-program

    program-incremental
    docker-program-incremental
    ;

  devShell = pkgs.mkShell {
    buildInputs = [
      pkgs.nixpkgs-fmt
      pkgs.skopeo
      rust-pkgs.latest.rustChannels.stable.rust
      pkgs.openssl pkgs.pkg-config
    ];
  };

  ci = {
    inherit
      program
      docker-program
      ;
  };
}

The -incremental builds should only be used for development but mainly because they're debug builds, it should be easy to create a version where release binaries are copied over instead, assuming they remain somewhat "nix-pure" (all dependencies used were injected in the dev shell)
What would be best is producing program and program-image in CI and using those for deployments