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diff --git a/library/core/src/async_iter/async_iter.rs b/library/core/src/async_iter/async_iter.rs
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+++ b/library/core/src/async_iter/async_iter.rs
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+use crate::ops::DerefMut;
+use crate::pin::Pin;
+use crate::task::{Context, Poll};
+
+/// An interface for dealing with asynchronous iterators.
+///
+/// This is the main async iterator trait. For more about the concept of async iterators
+/// generally, please see the [module-level documentation]. In particular, you
+/// may want to know how to [implement `AsyncIterator`][impl].
+///
+/// [module-level documentation]: index.html
+/// [impl]: index.html#implementing-async-iterator
+#[unstable(feature = "async_iterator", issue = "79024")]
+#[must_use = "async iterators do nothing unless polled"]
+#[doc(alias = "Stream")]
+pub trait AsyncIterator {
+    /// The type of items yielded by the async iterator.
+    type Item;
+
+    /// Attempt to pull out the next value of this async iterator, registering the
+    /// current task for wakeup if the value is not yet available, and returning
+    /// `None` if the async iterator is exhausted.
+    ///
+    /// # Return value
+    ///
+    /// There are several possible return values, each indicating a distinct
+    /// async iterator state:
+    ///
+    /// - `Poll::Pending` means that this async iterator'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 async iterator has successfully
+    /// produced a value, `val`, and may produce further values on subsequent
+    /// `poll_next` calls.
+    ///
+    /// - `Poll::Ready(None)` means that the async iterator has terminated, and
+    /// `poll_next` should not be invoked again.
+    ///
+    /// # Panics
+    ///
+    /// Once an async iterator 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 `AsyncIterator` 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 async iterator's state.
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>;
+
+    /// Returns the bounds on the remaining length of the async iterator.
+    ///
+    /// 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 <code>[Option]<[usize]></code>.
+    /// 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 an async iterator implementation yields the declared
+    /// number of elements. A buggy async iterator 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 async iterator, 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 <code>(0, [None])</code> which is correct for any
+    /// async iterator.
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, None)
+    }
+}
+
+#[unstable(feature = "async_iterator", issue = "79024")]
+impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator 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()
+    }
+}
+
+#[unstable(feature = "async_iterator", issue = "79024")]
+impl<P> AsyncIterator for Pin<P>
+where
+    P: DerefMut,
+    P::Target: AsyncIterator,
+{
+    type Item = <P::Target as AsyncIterator>::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        <P::Target as AsyncIterator>::poll_next(self.as_deref_mut(), cx)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (**self).size_hint()
+    }
+}