message: tracing-env-logger example: Remove useless mut to silence nightly.
diff: @@ -82,7 +82,7 @@ fn echo(req: Request<Body>) -> Instrumented<BoxFut> {
  // future, waiting on concatenating the full body, so that
  // it can be reversed. Only then can we return a `Response`.
  (&Method::POST, "/echo/reversed") => {
  - let mut span = span!(Level::TRACE, "response", response_kind = "reversed");
  + let span = span!(Level::TRACE, "response", response_kind = "reversed");
  let _enter = span.enter();
  let reversed = req.into_body().concat2().map(move |chunk| {
  let body = chunk.iter().rev().cloned().collect::<Vec<u8>>();
  