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    use super::assert_stream;
    use core::pin::Pin;
    use futures_core::stream::{Stream, FusedStream};
    use futures_core::task::{Context, Poll};

    /// An stream that repeats elements of type `A` endlessly by
    /// applying the provided closure `F: FnMut() -> A`.
    ///
    /// This `struct` is created by the [`repeat_with()`] function.
    /// See its documentation for more.
    #[derive(Debug, Clone)]
    #[must_use = "streams do nothing unless polled"]
    pub struct RepeatWith<F> {
        repeater: F,
    }

    impl<A, F: FnMut() -> A> Unpin for RepeatWith<F> {}

    impl<A, F: FnMut() -> A> Stream for RepeatWith<F> {
        type Item = A;

        fn poll_next(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            Poll::Ready(Some((&mut self.repeater)()))
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (usize::max_value(), None)
        }
    }

    impl<A, F: FnMut() -> A> FusedStream for RepeatWith<F>
    {
        fn is_terminated(&self) -> bool {
            false
        }
    }

    /// Creates a new stream that repeats elements of type `A` endlessly by
    /// applying the provided closure, the repeater, `F: FnMut() -> A`.
    ///
    /// The `repeat_with()` function calls the repeater over and over again.
    ///
    /// Infinite stream like `repeat_with()` are often used with adapters like
    /// [`stream.take()`], in order to make them finite.
    ///
    /// If the element type of the stream you need implements [`Clone`], and
    /// it is OK to keep the source element in memory, you should instead use
    /// the [`stream.repeat()`] function.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::stream::{self, StreamExt};
    ///
    /// // let's assume we have some value of a type that is not `Clone`
    /// // or which don't want to have in memory just yet because it is expensive:
    /// #[derive(PartialEq, Debug)]
    /// struct Expensive;
    ///
    /// // a particular value forever:
    /// let mut things = stream::repeat_with(|| Expensive);
    ///
    /// assert_eq!(Some(Expensive), things.next().await);
    /// assert_eq!(Some(Expensive), things.next().await);
    /// assert_eq!(Some(Expensive), things.next().await);
    /// # });
    /// ```
    ///
    /// Using mutation and going finite:
    ///
    /// ```rust
    /// # futures::executor::block_on(async {
    /// use futures::stream::{self, StreamExt};
    ///
    /// // From the zeroth to the third power of two:
    /// let mut curr = 1;
    /// let mut pow2 = stream::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
    ///                     .take(4);
    ///
    /// assert_eq!(Some(1), pow2.next().await);
    /// assert_eq!(Some(2), pow2.next().await);
    /// assert_eq!(Some(4), pow2.next().await);
    /// assert_eq!(Some(8), pow2.next().await);
    ///
    /// // ... and now we're done
    /// assert_eq!(None, pow2.next().await);
    /// # });
    /// ```
    pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
        assert_stream::<A, _>(RepeatWith { repeater })
    }