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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_lite::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);
        /// # });
        /// ```
        #[derive(Debug, Clone)]
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        pub struct OptionFuture<F> {
            #[pin]
            inner: 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().inner.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.inner {
                Some(x) => x.is_terminated(),
                None => true,
            }
        }
    }

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