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use core::pin::Pin;
use futures_core::task::{Context, Poll};
use futures_core::Stream;
use pin_project_lite::pin_project;
pin_project! {
/// Stream for the [poll_immediate](poll_immediate()) function.
///
/// It will never return [Poll::Pending](core::task::Poll::Pending)
#[derive(Debug, Clone)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct PollImmediate<S> {
#[pin]
stream: Option<S>
}
}
impl<T, S> Stream for PollImmediate<S>
where
S: Stream<Item = T>,
{
type Item = Poll<T>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let mut this = self.project();
let stream = match this.stream.as_mut().as_pin_mut() {
// inner is gone, so we can continue to signal that the stream is closed.
None => return Poll::Ready(None),
Some(inner) => inner,
};
match stream.poll_next(cx) {
Poll::Ready(Some(t)) => Poll::Ready(Some(Poll::Ready(t))),
Poll::Ready(None) => {
this.stream.set(None);
Poll::Ready(None)
}
Poll::Pending => Poll::Ready(Some(Poll::Pending)),
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.stream.as_ref().map_or((0, Some(0)), Stream::size_hint)
}
}
impl<S: Stream> super::FusedStream for PollImmediate<S> {
fn is_terminated(&self) -> bool {
self.stream.is_none()
}
}
/// Creates a new stream that always immediately returns [Poll::Ready](core::task::Poll::Ready) when awaiting it.
///
/// This is useful when immediacy is more important than waiting for the next item to be ready.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::stream::{self, StreamExt};
/// use futures::task::Poll;
///
/// let mut r = stream::poll_immediate(Box::pin(stream::iter(1_u32..3)));
/// assert_eq!(r.next().await, Some(Poll::Ready(1)));
/// assert_eq!(r.next().await, Some(Poll::Ready(2)));
/// assert_eq!(r.next().await, None);
///
/// let mut p = stream::poll_immediate(Box::pin(stream::once(async {
/// futures::pending!();
/// 42_u8
/// })));
/// assert_eq!(p.next().await, Some(Poll::Pending));
/// assert_eq!(p.next().await, Some(Poll::Ready(42)));
/// assert_eq!(p.next().await, None);
/// # });
/// ```
pub fn poll_immediate<S: Stream>(s: S) -> PollImmediate<S> {
super::assert_stream::<Poll<S::Item>, PollImmediate<S>>(PollImmediate { stream: Some(s) })
}