fn flush(&mut self) -> Flush<'_, Self, Item>
    where
        Self: Unpin,
    {
        assert_future::<Result<(), Self::Error>, _>(Flush::new(self))
    }
Flush the sink, processing all pending items.

This adapter is intended to be used when you want to stop sending to the sink
until all current requests are processed.
futures-util/src/sink/mod.rs