pub fn start_send(&mut self, msg: T) -> Result<(), SendError> {
        self.try_send(msg).map_err(|e| e.err)
    }
Send a message on the channel.

This function should only be called after
[`poll_ready`](Sender::poll_ready) has reported that the channel is
ready to receive a message.
futures-channel/src/mpsc/mod.rs