bar.nix

with import <nixpkgs> {};

let
  runForText = name: attrs: cmd:
    lib.removeSuffix "\n" (builtins.readFile (runCommand name attrs cmd));

in stdenv.mkDerivation rec {
  name = "foo-${version}";
  version = 
    runForText "version" { buildInputs = [ git ]; } ''
      cd ${src} && git describe --always > $out
    '';

  src = ...;
}