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//! Definition of the `Option` (optional step) combinator

use core::pin::Pin;
use futures_core::future::{Future, FusedFuture};
use futures_core::task::{Context, Poll};
use pin_project::pin_project;

/// A future representing a value which may or may not be present.
///
/// Created by the [`From`] implementation for [`Option`](std::option::Option).
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::OptionFuture;
///
/// let mut a: OptionFuture<_> = Some(async { 123 }).into();
/// assert_eq!(a.await, Some(123));
///
/// a = None.into();
/// assert_eq!(a.await, None);
/// # });
/// ```
#[pin_project]
#[derive(Debug, Clone)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct OptionFuture<F>(#[pin] Option<F>);

impl<F: Future> Future for OptionFuture<F> {
    type Output = Option<F::Output>;

    fn poll(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Self::Output> {
        match self.project().0.as_pin_mut() {
            Some(x) => x.poll(cx).map(Some),
            None => Poll::Ready(None),
        }
    }
}

impl<F: FusedFuture> FusedFuture for OptionFuture<F> {
    fn is_terminated(&self) -> bool {
        match &self.0 {
            Some(x) => x.is_terminated(),
            None => true,
        }
    }
}

impl<T> From<Option<T>> for OptionFuture<T> {
    fn from(option: Option<T>) -> Self {
        OptionFuture(option)
    }
}