let moz_overlay = import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz);
nixpkgs = import <nixpkgs> { overlays = [ moz_overlay ]; }; in with 
nixpkgs; stdenv.mkDerivation {
  name = "moz_overlay_shell";
  buildInputs = [ nixpkgs.latest.rustChannels.nightly.rust ];
}