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 Stream for ReadDir

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type Item = Result<DirEntry, Error>

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

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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 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>
where T: Clone,

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type Item = T

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

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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<<Receiver<T> as Stream>::Item>>

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impl<T> Stream for Unblock<T>
where T: Iterator + Send + 'static, <T as Iterator>::Item: Send + 'static,

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type Item = <T as Iterator>::Item

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

Implementors§

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impl Stream for DirStream

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impl<'a, S, T> Stream for Cloned<S>
where T: 'a + Clone, S: Stream<Item = &'a T>,

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type Item = T

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impl<'a, S, T> Stream for Copied<S>
where T: 'a + Copy, S: Stream<Item = &'a T>,

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type Item = T

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impl<A, B> Stream for Zip<A, B>
where A: Stream, B: Stream,

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type Item = (<A as Stream>::Item, <B as Stream>::Item)

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impl<F> Stream for OnceFuture<F>
where F: Future,

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type Item = <F 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> Stream for Bytes<R>
where R: AsyncRead + Unpin,

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

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impl<R> Stream for Split<R>
where R: AsyncBufRead,

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

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type Item = <S as Stream>::Item

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

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

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

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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

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type Item = <S as Stream>::Item

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impl<S, F> Stream for Inspect<S, F>
where S: Stream, F: FnMut(&<S as Stream>::Item),

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type Item = <S as Stream>::Item

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

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type Item = <Fut as Future>::Output

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impl<S, F, T> Stream for FilterMap<S, F>
where S: Stream, F: FnMut(<S as Stream>::Item) -> Option<T>,

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type Item = T

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impl<S, F, T> Stream for Map<S, F>
where S: Stream, F: FnMut(<S as Stream>::Item) -> T,

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type Item = T

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impl<S, P> Stream for Filter<S, P>
where S: Stream, P: FnMut(&<S as Stream>::Item) -> bool,

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type Item = <S as Stream>::Item

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impl<S, P> Stream for SkipWhile<S, P>
where S: Stream, P: FnMut(&<S as Stream>::Item) -> bool,

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type Item = <S as Stream>::Item

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impl<S, P> Stream for TakeWhile<S, P>
where S: Stream, P: FnMut(&<S as Stream>::Item) -> bool,

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type Item = <S as Stream>::Item

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

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type Item = B

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impl<S, U> Stream for Chain<S, U>
where S: Stream, U: Stream<Item = <S as Stream>::Item>,

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type Item = <S as Stream>::Item

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impl<S, U> Stream for Flatten<S>
where S: Stream<Item = U>, U: Stream,

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type Item = <U as Stream>::Item

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impl<S, U, F> Stream for FlatMap<S, U, F>
where S: Stream, U: Stream, F: FnMut(<S as Stream>::Item) -> U,

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type Item = <U as Stream>::Item

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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 Once<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, E, F, Fut, Item> Stream for TryUnfold<T, F, Fut>
where F: FnMut(T) -> Fut, Fut: Future<Output = Result<Option<(Item, T)>, E>>,

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type Item = Result<Item, E>

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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> Stream for RepeatWith<F>
where F: FnMut() -> T,

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type Item = T

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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, S1, S2> Stream for Or<S1, S2>
where S1: Stream<Item = T>, S2: Stream<Item = T>,

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type Item = T

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impl<T, S1, S2> Stream for Race<S1, S2>
where S1: Stream<Item = T>, S2: Stream<Item = T>,

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type Item = T