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use futures_core::{Poll, Async, Stream, IntoFuture, Future};
use futures_core::task;
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Once<F>(Option<F>);
pub fn once<F: IntoFuture>(item: F) -> Once<F::Future> {
Once(Some(item.into_future()))
}
impl<F: Future> Stream for Once<F> {
type Item = F::Item;
type Error = F::Error;
fn poll_next(&mut self, cx: &mut task::Context) -> Poll<Option<F::Item>, F::Error> {
if let Some(mut f) = self.0.take() {
match f.poll(cx)? {
Async::Ready(x) => Ok(Async::Ready(Some(x))),
Async::Pending => {
self.0 = Some(f);
Ok(Async::Pending)
}
}
} else {
Ok(Async::Ready(None))
}
}
}