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 as Deref>::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 Pin<P>>, cx: &mut Context<'_> ) -> Poll<Option<<Pin<P> as Stream>::Item>>

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

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

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type Item = <S as Stream>::Item

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

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

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

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type Item = <S as Stream>::Item

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

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

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

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type Item = <S as Stream>::Item

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

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

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impl<T> Stream for Receiver<T>

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type Item = T

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fn poll_next( self: Pin<&mut Receiver<T>>, cx: &mut Context<'_> ) -> Poll<Option<T>>

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

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impl<T> Stream for UnboundedReceiver<T>

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type Item = T

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fn poll_next( self: Pin<&mut UnboundedReceiver<T>>, cx: &mut Context<'_> ) -> Poll<Option<T>>

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

Implementors§

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

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type Item = <A as Stream>::Item

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

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type Item = A

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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>>,

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type Item = B

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

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type Item = <Flatten<F, <F as Future>::Output> as Stream>::Item

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

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type Item = <Once<F> as Stream>::Item

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

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type Item = <TryFlatten<Fut, <Fut as TryFuture>::Ok> as Stream>::Item

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

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type Item = <TryFlatten<Fut, Si> as Stream>::Item

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

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type Item = <Fut as Future>::Output

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

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type Item = <Fut as Future>::Output

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

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type Item = <Fut as Future>::Output

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

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type Item = <I as Iterator>::Item

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

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

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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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,

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type Item = <S as Stream>::Item

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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>>,

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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

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type Item = <St1 as Stream>::Item

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

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type Item = <St1 as Stream>::Item

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

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type Item = (<St1 as Stream>::Item, <St2 as Stream>::Item)

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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,

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type Item = <St1 as Stream>::Item

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

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type Item = <St as Stream>::Item

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

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type Item = <<St as Stream>::Item as Future>::Output

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

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type Item = <<St as Stream>::Item as Future>::Output

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

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type Item = <St as Stream>::Item

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

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type Item = <Flatten<St, <St as Stream>::Item> as Stream>::Item

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

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type Item = <St as Stream>::Item

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

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type Item = Result<<<St as TryStream>::Ok as TryFuture>::Ok, <St as TryStream>::Error>

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

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type Item = Result<<<St as TryStream>::Ok as TryFuture>::Ok, <St as TryStream>::Error>

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

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type Item = Result<<<St as TryStream>::Ok as TryStream>::Ok, <<St as TryStream>::Ok as TryStream>::Error>

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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>,

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type Item = <FlattenUnorderedWithFlowController<NestedTryStreamIntoEitherTryStream<St>, PropagateBaseStreamError<St>> as Stream>::Item

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

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type Item = <MapErr<St, IntoFn<E>> as Stream>::Item

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

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type Item = <Map<St, InspectFn<F>> as Stream>::Item

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

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type Item = <Inspect<IntoStream<St>, InspectErrFn<F>> as Stream>::Item

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

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type Item = <Inspect<IntoStream<St>, InspectOkFn<F>> as Stream>::Item

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

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type Item = <F as FnOnce1<<St as Stream>::Item>>::Output

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

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type Item = <Map<IntoStream<St>, MapErrFn<F>> as Stream>::Item

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

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type Item = <Map<IntoStream<St>, MapOkFn<F>> as Stream>::Item

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

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type Item = <St as Stream>::Item

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

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type Item = Result<<Fut as TryFuture>::Ok, <St as TryStream>::Error>

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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>,

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type Item = <St as Stream>::Item

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

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type Item = Result<<St as TryStream>::Ok, <Fut as TryFuture>::Error>

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

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type Item = <St as Stream>::Item

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

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type Item = <St as Stream>::Item

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

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type Item = <Fut as Future>::Output

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

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type Item = Result<<St as TryStream>::Ok, <St as TryStream>::Error>

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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>,

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type Item = Result<<St as TryStream>::Ok, <St as TryStream>::Error>

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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>,

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type Item = Result<<St as TryStream>::Ok, <St as TryStream>::Error>

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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>>,

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type Item = T

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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,

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type Item = Result<T, <St as TryStream>::Error>

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

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type Item = <Flatten<Map<St, F>, U> as Stream>::Item

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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,

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type Item = <FlattenUnorderedWithFlowController<Map<St, F>, ()> as Stream>::Item

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

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type Item = <St as Stream>::Item

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

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type Item = Result<<St as Stream>::Item, Box<dyn Any + Send>>

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

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type Item = Vec<<St as Stream>::Item>

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

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type Item = (usize, <St as Stream>::Item)

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

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type Item = Vec<<St as Stream>::Item>

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impl<St: Stream> Stream for Skip<St>

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type Item = <St as Stream>::Item

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

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type Item = Result<<St as TryStream>::Ok, <St as TryStream>::Error>

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

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type Item = Result<Vec<<St as TryStream>::Ok>, TryChunksError<<St as TryStream>::Ok, <St as TryStream>::Error>>

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impl<T> Stream for Empty<T>

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type Item = T

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impl<T> Stream for Pending<T>

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type Item = T

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

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type Item = T

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

A Stream implementation that can be polled repeatedly until the future is done. The stream will never return Poll::Pending so polling it in a tight loop is worse than using a blocking synchronous function.

use futures::task::Poll;
use futures::{StreamExt, future, pin_mut};
use future::FusedFuture;

let f = async { 1_u32 };
pin_mut!(f);
let mut r = future::poll_immediate(f);
assert_eq!(r.next().await, Some(Poll::Ready(1)));

let f = async {futures::pending!(); 42_u8};
pin_mut!(f);
let mut p = future::poll_immediate(f);
assert_eq!(p.next().await, Some(Poll::Pending));
assert!(!p.is_terminated());
assert_eq!(p.next().await, Some(Poll::Ready(42)));
assert!(p.is_terminated());
assert_eq!(p.next().await, None);
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type Item = Poll<T>

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

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type Item = T

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

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type Item = Result<Item, <Fut as TryFuture>::Error>

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

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type Item = Item

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

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type Item = Poll<T>