pub fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
        if let Some(inner) = &mut self.0 {
            inner.try_send(msg)
        } else {
            Err(TrySendError { err: SendError { kind: SendErrorKind::Disconnected }, val: msg })
        }
    }
Attempts to send a message on this `Sender`, returning the message
if there was an error.
futures-channel/src/mpsc/mod.rs