futures-util 0.3.5

use core::pin::Pin;
use futures_core::future::Future;
use futures_core::stream::{Stream, FusedStream};
use futures_core::task::{Context, Poll};
use pin_project::{pin_project, project};

/// Creates a stream of a single element.
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::stream::{self, StreamExt};
///
/// let stream = stream::once(async { 17 });
/// let collected = stream.collect::<Vec<i32>>().await;
/// assert_eq!(collected, vec![17]);
/// # });
/// ```
pub fn once<Fut: Future>(future: Fut) -> Once<Fut> {
    Once::new(future)
}

/// A stream which emits single element and then EOF.
///
/// This stream will never block and is always ready.
#[pin_project]
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Once<Fut> {
    #[pin]
    future: Option<Fut>
}

impl<Fut> Once<Fut> {
    pub(crate) fn new(future: Fut) -> Self {
        Self { future: Some(future) }
    }
}

impl<Fut: Future> Stream for Once<Fut> {
    type Item = Fut::Output;

    #[project]
    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        #[project]
        let Once { mut future } = self.project();
        let v = match future.as_mut().as_pin_mut() {
            Some(fut) => ready!(fut.poll(cx)),
            None => return Poll::Ready(None),
        };

        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()
    }
}