message: doc: invalid code in replace_append docs
  `&str` doesn't implement `IntoIterator`, so this should just
  use `push_str` instead. The invalid code was probably
  copied from the `bytes` module, where it does work.
  PR
diff: @@ -1122,7 +1122,7 @@ pub trait Replacer {
  /// have a match at capture group `0`.
  ///
  /// For example, a no-op replacement would be
  - /// `dst.extend(caps.get(0).unwrap().as_str())`.
  + /// `dst.push_str(caps.get(0).unwrap().as_str())`.
  fn replace_append(&mut self, caps: &Captures, dst: &mut String);
  /// Return a fixed unchanging replacement string.
  