The type of items yielded by the async iterator.
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.
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 subsequent
Poll::Ready(None)means that the async iterator has terminated, and
poll_nextshould not be invoked again.
Once an async iterator has finished (returned
poll_next), calling its
poll_next method again may panic, block forever, or cause other kinds of
AsyncIterator trait places no requirements on the effects of
such a call. However, as the
poll_next method is not marked
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.
Returns the bounds on the remaining length of the async iterator.
size_hint() returns a tuple where the first element
is the lower bound, and the second element is the upper bound.
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
size_hint() should not lead to memory safety
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