nix-configs/unstable.nix

{ config, pkgs, ...}:

let
  unstable = import
    (builtins.fetchTarball https://github.com/nixos/nixpkgs/tarball/master)
    # reuse the current configuration
    { config = config.nixpkgs.config; };
in

{
  environment.systemPackages = with pkgs; [
    unstable.rustc
    unstable.just
  ];
}