Function serve 

pub async fn serve<S, H>(
    scheduler: &S,
    listener: S::Socket,
    handler: &H,
    options: &Options,
    cancellation: &Cancellation,
) -> Result<Statistics, Error>
where
    S: SocketAccept + SocketSetOption<TcpNoDelay> + SocketStreamReading + SocketStreamWriting + SocketClose,
    S::Socket: 'static,
    H: Handler,

Serve an already-bound listener with bounded concurrent connection futures. No hidden tasks or detached jobs are created. Cancellation stops acceptance, cancels active exchanges, joins retirement, and closes the owned listener. Individual peer/handler failures are counted without stopping other peers.