* Added an operator `~' to select paths within a derivation. E.g.,

{stdenv, bash}: derivation {
      builder = bash ~ /bin/sh;
      args = ["-e" "-x" ./builder.sh];
      ...
    }

  Here the attribute `builder' will evaluate to, e.g.,
  `/nix/store/1234abcd...-bash-2.0.1/bin/sh'.

src/libexpr/eval.cc

@@ -190,6 +190,7 @@ Expr evalExpr2(EvalState & state, Expr e)
     if (atMatch(m, e) >> cons &&
         (cons == "Str" ||
          cons == "Path" ||
+         cons == "SubPath" ||
          cons == "Uri" ||
          cons == "Null" ||
          cons == "Int" ||