async-sink 0.3.1

The asynchronous `Sink` trait for the tokio ecosystem.
Documentation 
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use super::Sink;
use core::future::Future;
use core::pin::Pin;
use core::task::{Context, Poll};

/// Future for the [`feed`](super::SinkExt::feed) method.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct Feed<'a, Si: ?Sized, Item> {
    sink: &'a mut Si,
    item: Option<Item>,
}

// `Feed` is `Unpin` because it only contains a mutable reference to the sink,
// not the sink itself.
impl<Si: ?Sized, Item> Unpin for Feed<'_, Si, Item> {}

impl<'a, Si: Sink<Item> + Unpin + ?Sized, Item> Feed<'a, Si, Item> {
    pub(super) fn new(sink: &'a mut Si, item: Item) -> Self {
        Feed {
            sink,
            item: Some(item),
        }
    }

    pub(super) fn sink_pin_mut(&mut self) -> Pin<&mut Si> {
        Pin::new(self.sink)
    }

    pub(super) fn is_item_pending(&self) -> bool {
        self.item.is_some()
    }
}

impl<Si: Sink<Item> + Unpin + ?Sized, Item> Future for Feed<'_, Si, Item> {
    type Output = Result<(), Si::Error>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = self.get_mut();
        let mut sink = Pin::new(&mut *this.sink);
        match sink.as_mut().poll_ready(cx) {
            Poll::Ready(Ok(())) => {
                let item = this.item.take().expect("polled Feed after completion");
                Poll::Ready(sink.as_mut().start_send(item))
            }
            Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
            Poll::Pending => Poll::Pending,
        }
    }
}