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use core::pin::Pin;
use futures_core::future::Future;
use futures_core::stream::{Stream, FusedStream};
use futures_core::task::{Context, Poll};
use pin_utils::unsafe_pinned;
pub fn once<Fut: Future>(future: Fut) -> Once<Fut> {
Once { future: Some(future) }
}
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Once<Fut> {
future: Option<Fut>
}
impl<Fut: Unpin> Unpin for Once<Fut> {}
impl<Fut> Once<Fut> {
unsafe_pinned!(future: Option<Fut>);
}
impl<Fut: Future> Stream for Once<Fut> {
type Item = Fut::Output;
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let v = match self.as_mut().future().as_pin_mut() {
Some(fut) => ready!(fut.poll(cx)),
None => return Poll::Ready(None),
};
self.as_mut().future().set(None);
Poll::Ready(Some(v))
}
fn size_hint(&self) -> (usize, Option<usize>) {
if self.future.is_some() {
(1, Some(1))
} else {
(0, Some(0))
}
}
}
impl<Fut: Future> FusedStream for Once<Fut> {
fn is_terminated(&self) -> bool {
self.future.is_none()
}
}