default.nix

{ pkgs ? (import <nixpkgs> {})
, stdenv ? pkgs.stdenv
, go ? pkgs.go
, git ? pkgs.git
}:

rec {
  readChop = with builtins; path:
    let
      str = readFile path;
      len = stringLength str;
    in substring 0 (sub len 1) str;

  gitref = with builtins; dir:
    let
      head = readChop (toPath "${dir}/.git/HEAD");
    in
      if (pkgs.lib.hasPrefix "ref: " head) then
        readChop (substring 3 (sub (stringLength head) 4) head)
      else
        head;

  crank = stdenv.mkDerivation rec {
    git_version = gitref ./.;
    version = "1.0.0-${git_version}";
    name = "direnv-${version}";
    src = ./.;

    buildInputs = [ go ];
    buildPhase = "make";
    installPhase = ''make install "DESTDIR=$out"'';
    dontStrip = 1;

    meta = {
      homepage = "http://direnv.net";
      description = "path-dependent environments";
      maintainers = [
        stdenv.lib.maintainers.zimbatm
        ];
      license = stdenv.lib.licenses.mit;
      platforms = go.meta.platforms;
    };
  };
}

Produces:

error: string `/nix/store/38xyldzh0nzcy6r2yfkqqwnhg1vyl5fc-direnv/.git/HEAD' cannot refer to other paths, at /Users/zimbatm/code/github.com/zimbatm/direnv/default.nix:10:13