pin.nix

args:
import (builtins.fetchTarball {
  name = "my-pinned-nixpkgs";
  url = https://github.com/nixos/nixpkgs/archive/f2b3bbe44e4f7e220ecb1308d1acdac5e6a0f8ba.tar.gz;
  sha256 = "139ai0dp4yrr9p0wsgckb9g3ykzf47my0z3jfbc8ypfw7fzf6f5c";
})
(args // {
  overlays = [
    (self: super: { 
      pkgsinfo = super.writeShellScriptBin "pkgsinfo" ''
        #!/usr/bin/env bash
          echo ${super.path}
      '';
    })
  ];
})

test.nix

with (import <nixpkgs> {});

stdenv.mkDerivation {
  #NIX_DEBUG=2;
  name = "testing";

  prePatch = ''
    env | grep foo
    env | grep bar
    exit 1
  '';

  foo = "foo";
  bar = lib.optional false "bar";

  src = ./test.nix;
  unpackPhase = ":";
}