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

    /// Future for the [`lazy`] function.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    pub struct Lazy<F> {
        f: Option<F>
    }

    // safe because we never generate `Pin<&mut F>`
    impl<F> Unpin for Lazy<F> {}

    /// Creates a new future that allows delayed execution of a closure.
    ///
    /// The provided closure is only run once the future is polled.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::future;
    ///
    /// let a = future::lazy(|_| 1);
    /// assert_eq!(a.await, 1);
    ///
    /// let b = future::lazy(|_| -> i32 {
    ///     panic!("oh no!")
    /// });
    /// drop(b); // closure is never run
    /// # });
    /// ```
    pub fn lazy<F, R>(f: F) -> Lazy<F>
        where F: FnOnce(&mut Context<'_>) -> R,
    {
        assert_future::<R, _>(Lazy { f: Some(f) })
    }

    impl<F, R> FusedFuture for Lazy<F>
        where F: FnOnce(&mut Context<'_>) -> R,
    {
        fn is_terminated(&self) -> bool { self.f.is_none() }
    }

    impl<F, R> Future for Lazy<F>
        where F: FnOnce(&mut Context<'_>) -> R,
    {
        type Output = R;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<R> {
            Poll::Ready((self.f.take().expect("Lazy polled after completion"))(cx))
        }
    }