fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), E>> {
        let mut this = self.project();

        let item = match this.state.as_mut().as_pin_mut() {
            None => return Poll::Ready(Ok(())),
            Some(fut) => ready!(fut.poll(cx))?,
        };
        this.state.set(None);
        this.sink.start_send(item)?;
        Poll::Ready(Ok(()))
    }
Completes the processing of previous item if any.
futures-util/src/sink/with.rs