pub trait Stream {
    type Item;

    fn poll_next(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>
    ) -> Poll<Option<Self::Item>>; fn size_hint(&self) -> (usize, Option<usize>) { ... } }
Expand description

A stream of values produced asynchronously.

If Future<Output = T> is an asynchronous version of T, then 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 caller.

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.

Required Associated Types

Values yielded by the stream.

Required Methods

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.

Return value

There are several possible return values, each indicating a distinct stream state:

  • Poll::Pending means 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_next calls.

  • Poll::Ready(None) means that the stream has terminated, and poll_next should not be invoked again.

Panics

Once a stream 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 Stream 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 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 calls.

Provided Methods

Returns the bounds on the remaining length of the stream.

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 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 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 stream.

Implementations on Foreign Types

Implementors

impl<T> Stream for Receiver<T>

impl<T> Stream for UnboundedReceiver<T>

impl<F> Stream for FlattenStream<F>where
    Flatten<F, <F as Future>::Output>: Stream,
    F: Future,

impl<F> Stream for IntoStream<F>where
    Once<F>: Stream,

impl<Fut> Stream for TryFlattenStream<Fut>where
    TryFlatten<Fut, Fut::Ok>: Stream,
    Fut: TryFuture,

impl<T, F> Stream for PollImmediate<F>where
    F: Future<Output = T>,

impl<A, B> Stream for Either<A, B>where
    A: Stream,
    B: Stream<Item = A::Item>,

impl<St1, St2> Stream for Chain<St1, St2>where
    St1: Stream,
    St2: Stream<Item = St1::Item>,

impl<St> Stream for Cycle<St>where
    St: Clone + Stream,

impl<St: Stream> Stream for Enumerate<St>

impl<St, Fut, F> Stream for Filter<St, Fut, F>where
    St: Stream,
    F: for<'a> FnMut1<&'a St::Item, Output = Fut>,
    Fut: Future<Output = bool>,

impl<St, Fut, F, T> Stream for FilterMap<St, Fut, F>where
    St: Stream,
    F: FnMut1<St::Item, Output = Fut>,
    Fut: Future<Output = Option<T>>,

impl<St> Stream for Flatten<St>where
    Flatten<St, St::Item>: Stream,
    St: Stream,

impl<S: Stream> Stream for Fuse<S>

impl<St, F> Stream for Inspect<St, F>where
    Map<St, InspectFn<F>>: Stream,

impl<St, F> Stream for Map<St, F>where
    St: Stream,
    F: FnMut1<St::Item>,

impl<St, U, F> Stream for FlatMap<St, U, F>where
    Flatten<Map<St, F>, U>: Stream,

impl<S: Stream> Stream for Peekable<S>

impl<St: Stream> Stream for Skip<St>

impl<St, Fut, F> Stream for SkipWhile<St, Fut, F>where
    St: Stream,
    F: FnMut(&St::Item) -> Fut,
    Fut: Future<Output = bool>,

impl<St> Stream for Take<St>where
    St: Stream,

impl<St, Fut, F> Stream for TakeWhile<St, Fut, F>where
    St: Stream,
    F: FnMut(&St::Item) -> Fut,
    Fut: Future<Output = bool>,

impl<St, Fut> Stream for TakeUntil<St, Fut>where
    St: Stream,
    Fut: Future,

impl<St, Fut, F> Stream for Then<St, Fut, F>where
    St: Stream,
    F: FnMut(St::Item) -> Fut,
    Fut: Future,

impl<St1, St2> Stream for Zip<St1, St2>where
    St1: Stream,
    St2: Stream,

impl<St: Stream> Stream for Chunks<St>

impl<St: Stream> Stream for ReadyChunks<St>

impl<B, St, S, Fut, F> Stream for Scan<St, S, Fut, F>where
    St: Stream,
    F: FnMut(&mut S, St::Item) -> Fut,
    Fut: Future<Output = Option<B>>,

impl<St> Stream for BufferUnordered<St>where
    St: Stream,
    St::Item: Future,

impl<St> Stream for Buffered<St>where
    St: Stream,
    St::Item: Future,

impl<St: Stream + UnwindSafe> Stream for CatchUnwind<St>

impl<St, Fut, F> Stream for AndThen<St, Fut, F>where
    St: TryStream,
    F: FnMut(St::Ok) -> Fut,
    Fut: TryFuture<Error = St::Error>,

impl<St, E> Stream for ErrInto<St, E>where
    MapErr<St, IntoFn<E>>: Stream,

impl<St, F> Stream for InspectOk<St, F>where
    Inspect<IntoStream<St>, InspectOkFn<F>>: Stream,

impl<St, F> Stream for InspectErr<St, F>where
    Inspect<IntoStream<St>, InspectErrFn<F>>: Stream,

impl<St: TryStream> Stream for IntoStream<St>

impl<St, F> Stream for MapOk<St, F>where
    Map<IntoStream<St>, MapOkFn<F>>: Stream,

impl<St, F> Stream for MapErr<St, F>where
    Map<IntoStream<St>, MapErrFn<F>>: Stream,

impl<St, Fut, F> Stream for OrElse<St, Fut, F>where
    St: TryStream,
    F: FnMut(St::Error) -> Fut,
    Fut: TryFuture<Ok = St::Ok>,

impl<St, Fut, F> Stream for TryFilter<St, Fut, F>where
    St: TryStream,
    Fut: Future<Output = bool>,
    F: FnMut(&St::Ok) -> Fut,

impl<St, Fut, F, T> Stream for TryFilterMap<St, Fut, F>where
    St: TryStream,
    Fut: TryFuture<Ok = Option<T>, Error = St::Error>,
    F: FnMut(St::Ok) -> Fut,

impl<St> Stream for TryFlatten<St>where
    St: TryStream,
    St::Ok: TryStream,
    <St::Ok as TryStream>::Error: From<St::Error>,

impl<St: TryStream> Stream for TryChunks<St>

impl<T, F, Fut, Item> Stream for TryUnfold<T, F, Fut>where
    F: FnMut(T) -> Fut,
    Fut: TryFuture<Ok = Option<(Item, T)>>,

impl<St, Fut, F> Stream for TrySkipWhile<St, Fut, F>where
    St: TryStream,
    F: FnMut(&St::Ok) -> Fut,
    Fut: TryFuture<Ok = bool, Error = St::Error>,

impl<St, Fut, F> Stream for TryTakeWhile<St, Fut, F>where
    St: TryStream,
    F: FnMut(&St::Ok) -> Fut,
    Fut: TryFuture<Ok = bool, Error = St::Error>,

impl<St> Stream for TryBufferUnordered<St>where
    St: TryStream,
    St::Ok: TryFuture<Error = St::Error>,

impl<St> Stream for TryBuffered<St>where
    St: TryStream,
    St::Ok: TryFuture<Error = St::Error>,

impl<I> Stream for Iter<I>where
    I: Iterator,

impl<T> Stream for Repeat<T>where
    T: Clone,

impl<A, F: FnMut() -> A> Stream for RepeatWith<F>

impl<T> Stream for Empty<T>

impl<Fut: Future> Stream for Once<Fut>

impl<T> Stream for Pending<T>

impl<T, F> Stream for PollFn<F>where
    F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,

impl<T, S> Stream for PollImmediate<S>where
    S: Stream<Item = T>,

impl<St1, St2> Stream for Select<St1, St2>where
    St1: Stream,
    St2: Stream<Item = St1::Item>,

impl<St1, St2, Clos, State> Stream for SelectWithStrategy<St1, St2, Clos, State>where
    St1: Stream,
    St2: Stream<Item = St1::Item>,
    Clos: FnMut(&mut State) -> PollNext,

impl<T, F, Fut, Item> Stream for Unfold<T, F, Fut>where
    F: FnMut(T) -> Fut,
    Fut: Future<Output = Option<(Item, T)>>,

impl<Fut: Future> Stream for FuturesOrdered<Fut>

impl<Fut: Future> Stream for FuturesUnordered<Fut>

impl<St: Stream + Unpin> Stream for SelectAll<St>

impl<St> Stream for Abortable<St>where
    St: Stream,

impl Stream for Body