Fullstack Rust

In these lines: fn post_error(err: JsonPayloadError, req: &HttpRequest) -> Error { let extns = req.extensions(); let state = extns.get::<web::Data<AppState>>().unwrap(); The unwrap() causes a panic. The state did not get returned from the extension. This is with the unmodified sample code from v4

Why do the messages need to be cloned on page 70(v4) to be passed in a struct to web::Json(). web::Json should not need mutable access because it only serialises it's input and the mutex is locked during this serialisation so why the need for copying?