message: Added comments for dead_code exceptions
diff: @@ -553,7 +553,7 @@ enum KeysIteratorState<K, V, S: TrieStore<K, V>> {
  /// Iterate normally
  Ok,
  /// Return the error and stop iterating
  - #[allow(dead_code)]
  + #[allow(dead_code)] // Return variant alone is used in testing.
  ReturnError(S::Error),
  /// Already failed, only return None
  Failed,
  