Trait core::async_iter::AsyncIterator

source ·
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§


type Item

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

The type of items yielded by the async iterator.

Required Methods§


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.


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§


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.