Add reminder to match the error kind once DirectoryNotEmpty is stabilized

src/bootstrap/clean.rs

@@ -75,6 +75,8 @@ fn rm_rf(path: &Path) {
             do_op(path, "remove dir", |p| {
                 fs::remove_dir(p).or_else(|e| {
                     // Check for dir not empty on Windows
+                    // FIXME: Once `ErrorKind::DirectoryNotEmpty` is stabilized,
+                    // match on `e.kind()` instead.
                     #[cfg(windows)]
                     if e.raw_os_error() == Some(145) {
                         return Ok(());