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    use super::assert_stream;
    use core::fmt;
    use core::pin::Pin;
    use futures_core::future::TryFuture;
    use futures_core::ready;
    use futures_core::stream::Stream;
    use futures_core::task::{Context, Poll};
    use pin_project_lite::pin_project;

    /// Creates a `TryStream` from a seed and a closure returning a `TryFuture`.
    ///
    /// This function is the dual for the `TryStream::try_fold()` adapter: while
    /// `TryStream::try_fold()` reduces a `TryStream` to one single value,
    /// `try_unfold()` creates a `TryStream` from a seed value.
    ///
    /// `try_unfold()` will call the provided closure with the provided seed, then
    /// wait for the returned `TryFuture` to complete with `(a, b)`. It will then
    /// yield the value `a`, and use `b` as the next internal state.
    ///
    /// If the closure returns `None` instead of `Some(TryFuture)`, then the
    /// `try_unfold()` will stop producing items and return `Poll::Ready(None)` in
    /// future calls to `poll()`.
    ///
    /// In case of error generated by the returned `TryFuture`, the error will be
    /// returned by the `TryStream`. The `TryStream` will then yield
    /// `Poll::Ready(None)` in future calls to `poll()`.
    ///
    /// This function can typically be used when wanting to go from the "world of
    /// futures" to the "world of streams": the provided closure can build a
    /// `TryFuture` using other library functions working on futures, and
    /// `try_unfold()` will turn it into a `TryStream` by repeating the operation.
    ///
    /// # Example
    ///
    /// ```
    /// # #[derive(Debug, PartialEq)]
    /// # struct SomeError;
    /// # futures::executor::block_on(async {
    /// use futures::stream::{self, TryStreamExt};
    ///
    /// let stream = stream::try_unfold(0, |state| async move {
    ///     if state < 0 {
    ///         return Err(SomeError);
    ///     }
    ///
    ///     if state <= 2 {
    ///         let next_state = state + 1;
    ///         let yielded = state * 2;
    ///         Ok(Some((yielded, next_state)))
    ///     } else {
    ///         Ok(None)
    ///     }
    /// });
    ///
    /// let result: Result<Vec<i32>, _> = stream.try_collect().await;
    /// assert_eq!(result, Ok(vec![0, 2, 4]));
    /// # });
    /// ```
    pub fn try_unfold<T, F, Fut, Item>(init: T, f: F) -> TryUnfold<T, F, Fut>
    where
        F: FnMut(T) -> Fut,
        Fut: TryFuture<Ok = Option<(Item, T)>>,
    {
        assert_stream::<Result<Item, Fut::Error>, _>(TryUnfold {
            f,
            state: Some(init),
            fut: None,
        })
    }

    pin_project! {
        /// Stream for the [`try_unfold`] function.
        #[must_use = "streams do nothing unless polled"]
        pub struct TryUnfold<T, F, Fut> {
            f: F,
            state: Option<T>,
            #[pin]
            fut: Option<Fut>,
        }
    }

    impl<T, F, Fut> fmt::Debug for TryUnfold<T, F, Fut>
    where
        T: fmt::Debug,
        Fut: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("TryUnfold")
                .field("state", &self.state)
                .field("fut", &self.fut)
                .finish()
        }
    }

    impl<T, F, Fut, Item> Stream for TryUnfold<T, F, Fut>
    where
        F: FnMut(T) -> Fut,
        Fut: TryFuture<Ok = Option<(Item, T)>>,
    {
        type Item = Result<Item, Fut::Error>;

        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            let mut this = self.project();

            if let Some(state) = this.state.take() {
                this.fut.set(Some((this.f)(state)));
            }

            match this.fut.as_mut().as_pin_mut() {
                None => {
                    // The future previously errored
                    Poll::Ready(None)
                }
                Some(future) => {
                    let step = ready!(future.try_poll(cx));
                    this.fut.set(None);

                    match step {
                        Ok(Some((item, next_state))) => {
                            *this.state = Some(next_state);
                            Poll::Ready(Some(Ok(item)))
                        }
                        Ok(None) => Poll::Ready(None),
                        Err(e) => Poll::Ready(Some(Err(e))),
                    }
                }
            }
        }
    }