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    //! Asynchronous streams.

    use core::ops::DerefMut;
    use core::pin::Pin;
    use core::task::{Context, Poll};

    /// An owned dynamically typed [`Stream`] for use in cases where you can't
    /// statically type your result or need to add some indirection.
    #[cfg(feature = "alloc")]
    pub type BoxStream<'a, T> = Pin<alloc::boxed::Box<dyn Stream<Item = T> + Send + 'a>>;

    /// `BoxStream`, but without the `Send` requirement.
    #[cfg(feature = "alloc")]
    pub type LocalBoxStream<'a, T> = Pin<alloc::boxed::Box<dyn Stream<Item = T> + 'a>>;

    /// A stream of values produced asynchronously.
    ///
    /// If `Future<Output = T>` is an asynchronous version of `T`, then `Stream<Item
    /// = T>` is an asynchronous version of `Iterator<Item = T>`. A stream
    /// represents a sequence of value-producing events that occur asynchronously to
    /// the caller.
    ///
    /// The trait is modeled after `Future`, but allows `poll_next` to be called
    /// even after a value has been produced, yielding `None` once the stream has
    /// been fully exhausted.
    #[must_use = "streams do nothing unless polled"]
    pub trait Stream {
        /// Values yielded by the stream.
        type Item;

        /// Attempt to pull out the next value of this stream, registering the
        /// current task for wakeup if the value is not yet available, and returning
        /// `None` if the stream is exhausted.
        ///
        /// # Return value
        ///
        /// There are several possible return values, each indicating a distinct
        /// stream state:
        ///
        /// - `Poll::Pending` means that this stream's next value is not ready
        /// yet. Implementations will ensure that the current task will be notified
        /// when the next value may be ready.
        ///
        /// - `Poll::Ready(Some(val))` means that the stream has successfully
        /// produced a value, `val`, and may produce further values on subsequent
        /// `poll_next` calls.
        ///
        /// - `Poll::Ready(None)` means that the stream has terminated, and
        /// `poll_next` should not be invoked again.
        ///
        /// # Panics
        ///
        /// Once a stream has finished (returned `Ready(None)` from `poll_next`), calling its
        /// `poll_next` method again may panic, block forever, or cause other kinds of
        /// problems; the `Stream` trait places no requirements on the effects of
        /// such a call. However, as the `poll_next` method is not marked `unsafe`,
        /// Rust's usual rules apply: calls must never cause undefined behavior
        /// (memory corruption, incorrect use of `unsafe` functions, or the like),
        /// regardless of the stream's state.
        ///
        /// If this is difficult to guard against then the [`fuse`] adapter can be used
        /// to ensure that `poll_next` always returns `Ready(None)` in subsequent
        /// calls.
        ///
        /// [`fuse`]: https://docs.rs/futures/0.3/futures/stream/trait.StreamExt.html#method.fuse
        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>>;

        /// Returns the bounds on the remaining length of the stream.
        ///
        /// Specifically, `size_hint()` returns a tuple where the first element
        /// is the lower bound, and the second element is the upper bound.
        ///
        /// The second half of the tuple that is returned is an [`Option`]`<`[`usize`]`>`.
        /// A [`None`] here means that either there is no known upper bound, or the
        /// upper bound is larger than [`usize`].
        ///
        /// # Implementation notes
        ///
        /// It is not enforced that a stream implementation yields the declared
        /// number of elements. A buggy stream may yield less than the lower bound
        /// or more than the upper bound of elements.
        ///
        /// `size_hint()` is primarily intended to be used for optimizations such as
        /// reserving space for the elements of the stream, but must not be
        /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
        /// implementation of `size_hint()` should not lead to memory safety
        /// violations.
        ///
        /// That said, the implementation should provide a correct estimation,
        /// because otherwise it would be a violation of the trait's protocol.
        ///
        /// The default implementation returns `(0, `[`None`]`)` which is correct for any
        /// stream.
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            (0, None)
        }
    }

    impl<S: ?Sized + Stream + Unpin> Stream for &mut S {
        type Item = S::Item;

        fn poll_next(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            S::poll_next(Pin::new(&mut **self), cx)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (**self).size_hint()
        }
    }

    impl<P> Stream for Pin<P>
    where
        P: DerefMut + Unpin,
        P::Target: Stream,
    {
        type Item = <P::Target as Stream>::Item;

        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            self.get_mut().as_mut().poll_next(cx)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (**self).size_hint()
        }
    }

    /// A stream which tracks whether or not the underlying stream
    /// should no longer be polled.
    ///
    /// `is_terminated` will return `true` if a future should no longer be polled.
    /// Usually, this state occurs after `poll_next` (or `try_poll_next`) returned
    /// `Poll::Ready(None)`. However, `is_terminated` may also return `true` if a
    /// stream has become inactive and can no longer make progress and should be
    /// ignored or dropped rather than being polled again.
    pub trait FusedStream: Stream {
        /// Returns `true` if the stream should no longer be polled.
        fn is_terminated(&self) -> bool;
    }

    impl<F: ?Sized + FusedStream + Unpin> FusedStream for &mut F {
        fn is_terminated(&self) -> bool {
            <F as FusedStream>::is_terminated(&**self)
        }
    }

    impl<P> FusedStream for Pin<P>
    where
        P: DerefMut + Unpin,
        P::Target: FusedStream,
    {
        fn is_terminated(&self) -> bool {
            <P::Target as FusedStream>::is_terminated(&**self)
        }
    }

    mod private_try_stream {
        use super::Stream;

        pub trait Sealed {}

        impl<S, T, E> Sealed for S where S: ?Sized + Stream<Item = Result<T, E>> {}
    }

    /// A convenience for streams that return `Result` values that includes
    /// a variety of adapters tailored to such futures.
    pub trait TryStream: Stream + private_try_stream::Sealed {
        /// The type of successful values yielded by this future
        type Ok;

        /// The type of failures yielded by this future
        type Error;

        /// Poll this `TryStream` as if it were a `Stream`.
        ///
        /// This method is a stopgap for a compiler limitation that prevents us from
        /// directly inheriting from the `Stream` trait; in the future it won't be
        /// needed.
        fn try_poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>)
            -> Poll<Option<Result<Self::Ok, Self::Error>>>;
    }

    impl<S, T, E> TryStream for S
        where S: ?Sized + Stream<Item = Result<T, E>>
    {
        type Ok = T;
        type Error = E;

        fn try_poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>)
            -> Poll<Option<Result<Self::Ok, Self::Error>>>
        {
            self.poll_next(cx)
        }
    }

    #[cfg(feature = "alloc")]
    mod if_alloc {
        use alloc::boxed::Box;
        use super::*;

        impl<S: ?Sized + Stream + Unpin> Stream for Box<S> {
            type Item = S::Item;

            fn poll_next(
                mut self: Pin<&mut Self>,
                cx: &mut Context<'_>,
            ) -> Poll<Option<Self::Item>> {
                Pin::new(&mut **self).poll_next(cx)
            }

            fn size_hint(&self) -> (usize, Option<usize>) {
                (**self).size_hint()
            }
        }

        #[cfg(feature = "std")]
        impl<S: Stream> Stream for std::panic::AssertUnwindSafe<S> {
            type Item = S::Item;

            fn poll_next(
                self: Pin<&mut Self>,
                cx: &mut Context<'_>,
            ) -> Poll<Option<S::Item>> {
                unsafe { self.map_unchecked_mut(|x| &mut x.0) }.poll_next(cx)
            }

            fn size_hint(&self) -> (usize, Option<usize>) {
                self.0.size_hint()
            }
        }

        impl<S: ?Sized + FusedStream + Unpin> FusedStream for Box<S> {
            fn is_terminated(&self) -> bool {
                <S as FusedStream>::is_terminated(&**self)
            }
        }
    }