fn close_channel(&self) {
        // There's no need to park this sender, its dropping,
        // and we don't want to check for capacity, so skip
        // that stuff from `do_send`.

        self.inner.set_closed();
        self.inner.recv_task.wake();
    }
Closes this channel from the sender side, preventing any new messages.
futures-channel/src/mpsc/mod.rs