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use core::pin::Pin;
use futures_core::future::Future;
use futures_core::stream::Stream;
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<Fut::Output>> {
let val = if let Some(f) = self.as_mut().future().as_pin_mut() {
ready!(f.poll(cx))
} else {
return Poll::Ready(None)
};
self.future().set(None);
Poll::Ready(Some(val))
}
}