# Trait std::async_iter::AsyncIterator

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    pub trait AsyncIterator {
        type Item;

        // Required method
        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>
        ) -> Poll<Option<Self::Item>>;

        // Provided method
        fn size_hint(&self) -> (usize, Option<usize>) { ... }
    }

🔬This is a nightly-only experimental API. (`async_iterator` #79024)
Expand description

A trait 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`.

## Required Associated Types§

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#### type Item

🔬This is a nightly-only experimental API. (`async_iterator` #79024)

The type of items yielded by the async iterator.

## Required Methods§

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#### fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_> ) -> Poll<Option<Self::Item>>

🔬This is a nightly-only experimental API. (`async_iterator` #79024)

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.

## Provided Methods§

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#### fn size_hint(&self) -> (usize, Option<usize>)

🔬This is a nightly-only experimental API. (`async_iterator` #79024)

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 `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 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 `(0, None)` which is correct for any async iterator.

## Implementors§

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### impl<I> AsyncIterator for FromIter<I>where I: Iterator,

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#### type Item = <I as Iterator>::Item

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### impl<P> AsyncIterator for Pin<P>where P: DerefMut, <P as Deref>::Target: AsyncIterator,

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#### type Item = <<P as Deref>::Target as AsyncIterator>::Item

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### impl<S> AsyncIterator for &mut S where S: AsyncIterator + Unpin + ?Sized,

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#### type Item = <S as AsyncIterator>::Item

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### impl<S> AsyncIterator for Box<S>where S: AsyncIterator + Unpin + ?Sized,

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#### type Item = <S as AsyncIterator>::Item

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### impl<S> AsyncIterator for AssertUnwindSafe<S>where S: AsyncIterator,

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#### type Item = <S as AsyncIterator>::Item