@aloof-angle-91616: i should have noticed it earlier, but watch out for stuff like:

@rule(SetupPyInvocationEnvironment, [Select(NativeToolchain)])
def get_setup_py_env(native_toolchain):
  joined_path = get_joined_path(
    native_toolchain.path_entries(), os.environ.copy())
  return SetupPyInvocationEnvironment(joined_path)