Trait tracing::stdlib::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>) { ... }
}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§
Required Methods§
sourcefn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Option<Self::Item>>
🔬This is a nightly-only experimental API. (async_iterator #79024)
fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Option<Self::Item>>
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::Pendingmeans 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 subsequentpoll_nextcalls. -
Poll::Ready(None)means that the async iterator has terminated, andpoll_nextshould 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§
sourcefn size_hint(&self) -> (usize, Option<usize>)
🔬This is a nightly-only experimental API. (async_iterator #79024)
fn size_hint(&self) -> (usize, Option<usize>)
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.