fn poll_flush_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>
    where
        Self: Unpin,
    {
        Pin::new(self).poll_flush(cx)
    }
A convenience method for calling [`Sink::poll_flush`] on [`Unpin`]
sink types.
futures-util/src/sink/mod.rs