A stream of values produced asynchronously.
Future<Output = T> is an asynchronous version of
Stream<Item = T> is an asynchronous version of
Iterator<Item = T>. A stream
represents a sequence of value-producing events that occur asynchronously to
The trait is modeled after
Future, but allows
poll_next to be called
even after a value has been produced, yielding
None once the stream has
been fully exhausted.
Attempt to pull out the next value of this stream, registering the
current task for wakeup if the value is not yet available, and returning
None if the stream is exhausted.
There are several possible return values, each indicating a distinct stream state:
Poll::Pendingmeans that this stream'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 stream has successfully produced a value,
val, and may produce further values on subsequent
Poll::Ready(None)means that the stream has terminated, and
poll_nextshould not be invoked again.
Once a stream has finished (returned
poll_next), calling its
poll_next method again may panic, block forever, or cause other kinds of
Stream 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 stream's state.
If this is difficult to guard against then the
fuse adapter can be used
to ensure that
poll_next always returns
Ready(None) in subsequent
Returns the bounds on the remaining length of the stream.
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 a stream implementation yields the declared number of elements. A buggy stream 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 stream, 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
) which is correct for any