MessageResult borrow server in future select!

[tchamelot]

I came back to an old project and updated this crate which I still find realy nice ;) (from 0.4.2 to 0.5.1).

I noticed that you changed the behavior of Server::recv to return MessageResult which seems to be a nice upgrade compared to handling the buffer on the user side. However, now the MessageResult borrows the Server for its (the MessageResult) whole lifetime because of the internal pool. Therefore, it is not possible to Server::send while a MessageResult is returned by select!. It took me a while to identify this in my code which was looking like

enum Incoming<'a> {
    Webrtc(MessageResult<'a>),
    Channel(...),
}
loop {
    let received = tokio::select! {
        msg = server.recv() => msg.map(|msg| Incoming::Webrtc(msg)),
        msg = receiver.recv() => msg.map(|msg| Incoming::Channel(msg)).ok_or(...),
    };
    match received {
        // ...
    }
    server.send(&some_bytes, MessageType::Binary, &client).await.unwrap()
} // loop end

the compiler gave an error looking like

    |
    |                   msg = server.recv() => msg.map(|msg| Incoming::Webrtc(msg)),
    |                         ------------------ first mutable borrow occurs here
...
    |               server.send(&some_bytes, MessageType::Binary, &client)
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ second mutable borrow occurs here
...
    |           } // loop end
    |           - first borrow might be used here, when `received` is dropped and runs the destructor for type `Result<RtcServer::run::{closure#0}::Incoming<'_>, std::io::Error>`

For me the solution was to parse the MessageResult and to return a different owned variant such as

enum Incoming {
    Webrtc(MyCustomOwnedPacket, SocketAddr),
    ...
}

loop {
    let received = tokio::select! {
        msg = server.recv() => msg.map(|msg| {
            let content = MyCustomOwnedPacket::from(msg.message.as_ref());
            Incoming::Webrtc(content, message.remote_addr)
        }),
        ...
    };
   ...
    server.send(&some_bytes, MessageType::Binary, &client).await.unwrap()
} // loop end

I am not asking for any changes, just putting this here because somebody might run into the same issue ;)