pub trait Stream {
    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>) { ... }
}
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§

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type Item

Values yielded by the stream.

Required Methods§

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fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_> ) -> Poll<Option<Self::Item>>

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§

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fn size_hint(&self) -> (usize, Option<usize>)

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§

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impl<P> Stream for Pin<P>where P: DerefMut + Unpin, P::Target: Stream,

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type Item = <<P as Deref>::Target as Stream>::Item

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fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_> ) -> Poll<Option<Self::Item>>

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fn size_hint(&self) -> (usize, Option<usize>)

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impl<S: Stream> Stream for AssertUnwindSafe<S>

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type Item = <S as Stream>::Item

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fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_> ) -> Poll<Option<S::Item>>

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fn size_hint(&self) -> (usize, Option<usize>)

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impl<S: ?Sized + Stream + Unpin> Stream for &mut S

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type Item = <S as Stream>::Item

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fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_> ) -> Poll<Option<Self::Item>>

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fn size_hint(&self) -> (usize, Option<usize>)

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impl<S: ?Sized + Stream + Unpin> Stream for Box<S>

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type Item = <S as Stream>::Item

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fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_> ) -> Poll<Option<Self::Item>>

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fn size_hint(&self) -> (usize, Option<usize>)

Implementors§

impl<T, U> Stream for Framed<T, U>where T: AsyncRead, U: Decoder,

impl<S> Stream for Payload<S>where S: Stream<Item = Result<Bytes, PayloadError>>,

impl Stream for Payload

impl<S> Stream for Decoder<S>where S: Stream<Item = Result<Bytes, PayloadError>>,

impl Stream for Payload

impl Stream for Payload

impl<T> Stream for Readlines<T>where T: HttpMessage, T::Stream: Stream<Item = Result<Bytes, PayloadError>> + Unpin,

impl<T> Stream for UnboundedReceiver<T>

impl<T> Stream for Receiver<T>

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

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

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

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

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> Stream for Skip<St>

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

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

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

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

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

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

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

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

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

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

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

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

impl<St, Fut, F> Stream for TryFilter<St, Fut, F>where St: TryStream, Fut: Future<Output = bool>, 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, F> Stream for MapErr<St, F>where Map<IntoStream<St>, MapErrFn<F>>: Stream,

impl<St, U, F> Stream for FlatMapUnordered<St, U, F>where FlattenUnordered<Map<St, F>>: Stream, St: Stream, U: Stream + Unpin, F: FnMut(St::Item) -> U,

impl<T> Stream for Pending<T>

impl<S, Item, E> Stream for SinkErrInto<S, Item, E>where S: Sink<Item> + Stream, S::Error: Into<E>,

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

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

impl<S: Stream> Stream for SplitStream<S>

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: Stream> Stream for Chunks<St>

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

impl<S, Item, U, Fut, F> Stream for With<S, Item, U, Fut, F>where S: Stream + Sink<Item>, F: FnMut(U) -> Fut, Fut: Future,

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

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

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

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 Once<Fut>

impl<R: AsyncBufRead> Stream for Lines<R>

impl<St: Stream + Unpin> Stream for SelectAll<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<St1, St2> Stream for Zip<St1, St2>where St1: Stream, St2: Stream,

impl<S: Stream, F> Stream for SinkMapErr<S, F>

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

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

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

impl<S: Stream> Stream for Fuse<S>

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

impl<S, Item> Stream for Buffer<S, Item>where S: Sink<Item> + Stream,

impl<S, Item, U, St, F> Stream for WithFlatMap<S, Item, U, St, F>where S: Stream + Sink<Item>, F: FnMut(U) -> St, St: Stream<Item = Result<Item, S::Error>>,

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

impl<T> Stream for Empty<T>

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

impl<St> Stream for TryFlattenUnordered<St>where FlattenUnorderedWithFlowController<NestedTryStreamIntoEitherTryStream<St>, PropagateBaseStreamError<St>>: Stream, St: TryStream, St::Ok: TryStream + Unpin, <St::Ok as TryStream>::Error: From<St::Error>,

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

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

impl<Fut, Si> Stream for FlattenSink<Fut, Si>where TryFlatten<Fut, Si>: Stream,

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

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

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

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

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

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

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

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

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

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

impl<T> Stream for Receiver<T>

impl Stream for RowStream

impl<L, R> Stream for Either<L, R>where L: Stream, R: Stream<Item = L::Item>,

impl<T, D> Stream for FramedRead<T, D>where T: AsyncRead, D: Decoder,

impl<T, D> Stream for FramedWrite<T, D>where T: Stream,

impl<R: AsyncRead> Stream for ReaderStream<R>

impl<T, U> Stream for Framed<T, U>where T: AsyncRead, U: Decoder,