message: simplify path joining code a bit
diff: @@ -265,15 +265,7 @@ pub fn phase_rustc(mut args: impl Iterator<Item = String>, phase: RustcPhase) {
  );
  let output =
  String::from_utf8(output.stdout).expect("rustc returned non-UTF-8 filename");
  - output
  - .lines()
  - .filter(|l| !l.is_empty())
  - .map(|l| {
  - let mut p = path.clone();
  - p.push(l);
  - p
  - })
  - .collect()
  + output.lines().filter(|l| !l.is_empty()).map(|l| path.join(l)).collect()
  }
  }
  