# Trait futures_util::stream::Stream [−] [src]

    #[must_use = "streams do nothing unless polled"]pub trait Stream {
        type Item;
        pub fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>
        ) -> Poll<Option<Self::Item>>;

        pub fn size_hint(&self) -> (usize, Option<usize>) { ... }
    }

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.

## Associated Types

### `type Item` [src]

Values yielded by the stream.

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## Required methods

### `pub fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

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.

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## Provided methods

### `pub fn size_hint(&self) -> (usize, Option<usize>)` [src]

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.

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## Implementations on Foreign Types

### `impl<P> Stream for Pin<P> where
P: DerefMut + Unpin,
<P as Deref>::Target: Stream,` [src]

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

#### `pub fn poll_next(
self: Pin<&mut Pin<P>>,
cx: &mut Context<'_>
) -> Poll<Option<<Pin<P> as Stream>::Item>>` [src]

#### `pub fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<S> Stream for Box<S, Global> where
S: Stream + Unpin + ?Sized,` [src]

#### `type Item = <S as Stream>::Item`

#### `pub fn poll_next(
self: Pin<&mut Box<S, Global>>,
cx: &mut Context<'_>
) -> Poll<Option<<Box<S, Global> as Stream>::Item>>` [src]

#### `pub fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<S> Stream for AssertUnwindSafe<S> where
S: Stream,` [src]

#### `type Item = <S as Stream>::Item`

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

#### `pub fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<'_, S> Stream for &'_ mut S where
S: Stream + Unpin + ?Sized,` [src]

#### `type Item = <S as Stream>::Item`

#### `pub fn poll_next(
self: Pin<&mut &'_ mut S>,
cx: &mut Context<'_>
) -> Poll<Option<<&'_ mut S as Stream>::Item>>` [src]

#### `pub fn size_hint(&self) -> (usize, Option<usize>)` [src]

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## Implementors

### `impl<A, B> Stream for Either<A, B> where
A: Stream,
B: Stream<Item = A::Item>,` [src]

#### `type Item = A::Item`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<A, F: FnMut() -> A> Stream for RepeatWith<F>` [src]

#### `type Item = A`

#### `fn poll_next(
self: Pin<&mut Self>,
_: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `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>>,` [src]

#### `type Item = B`

#### `fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<F> Stream for FlattenStream<F> where
Flatten<F, <F as Future>::Output>: Stream,
F: Future,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<F> Stream for futures_util::future::IntoStream<F> where
Once<F>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<Fut> Stream for TryFlattenStream<Fut> where
TryFlatten<Fut, Fut::Ok>: Stream,
Fut: TryFuture,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<Fut: Future> Stream for Once<Fut>` [src]

#### `type Item = Fut::Output`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<I> Stream for Iter<I> where
I: Iterator,` [src]

#### `type Item = I::Item`

#### `fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<I::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<S: Stream> Stream for Fuse<S>` [src]

#### `type Item = S::Item`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<S: Stream> Stream for Peekable<S>` [src]

#### `type Item = S::Item`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St1, St2> Stream for Chain<St1, St2> where
St1: Stream,
St2: Stream<Item = St1::Item>,` [src]

#### `type Item = St1::Item`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St1, St2> Stream for Select<St1, St2> where
St1: Stream,
St2: Stream<Item = St1::Item>,` [src]

#### `type Item = St1::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<St1::Item>>` [src]

### `impl<St1, St2> Stream for Zip<St1, St2> where
St1: Stream,
St2: Stream,` [src]

#### `type Item = (St1::Item, St2::Item)`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St> Stream for Cycle<St> where
St: Clone + Stream,` [src]

#### `type Item = St::Item`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St> Stream for Flatten<St> where
Flatten<St, St::Item>: Stream,
St: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St> Stream for Take<St> where
St: Stream,` [src]

#### `type Item = St::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<St::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St> Stream for TryFlatten<St> where
St: TryStream,
St::Ok: TryStream,
<St::Ok as TryStream>::Error: From<St::Error>,` [src]

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

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

### `impl<St, E> Stream for ErrInto<St, E> where
MapErr<St, IntoFn<E>>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, F> Stream for Inspect<St, F> where
Map<St, InspectFn<F>>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, F> Stream for InspectErr<St, F> where
Inspect<IntoStream<St>, InspectErrFn<F>>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, F> Stream for InspectOk<St, F> where
Inspect<IntoStream<St>, InspectOkFn<F>>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, F> Stream for Map<St, F> where
St: Stream,
F: FnMut1<St::Item>,` [src]

#### `type Item = F::Output`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, F> Stream for MapErr<St, F> where
Map<IntoStream<St>, MapErrFn<F>>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, F> Stream for MapOk<St, F> where
Map<IntoStream<St>, MapOkFn<F>>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, Fut> Stream for TakeUntil<St, Fut> where
St: Stream,
Fut: Future,` [src]

#### `type Item = St::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<St::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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

#### `type Item = Result<Fut::Ok, St::Error>`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `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>,` [src]

#### `type Item = St::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<St::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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

#### `type Item = Result<St::Ok, Fut::Error>`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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

#### `type Item = St::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<St::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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

#### `type Item = St::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<St::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, Fut, F> Stream for Then<St, Fut, F> where
St: Stream,
F: FnMut(St::Item) -> Fut,
Fut: Future,` [src]

#### `type Item = Fut::Output`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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

#### `type Item = Result<St::Ok, St::Error>`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `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>,` [src]

#### `type Item = Result<St::Ok, St::Error>`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `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>,` [src]

#### `type Item = Result<St::Ok, St::Error>`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `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>>,` [src]

#### `type Item = T`

#### `fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `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,` [src]

#### `type Item = Result<T, St::Error>`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St, U, F> Stream for FlatMap<St, U, F> where
Flatten<Map<St, F>, U>: Stream,` [src]

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

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St: Stream> Stream for Enumerate<St>` [src]

#### `type Item = (usize, St::Item)`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St: Stream> Stream for Skip<St>` [src]

#### `type Item = St::Item`

#### `fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<St::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<St: TryStream> Stream for futures_util::stream::IntoStream<St>` [src]

#### `type Item = Result<St::Ok, St::Error>`

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

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<T> Stream for Empty<T>` [src]

#### `type Item = T`

#### `fn poll_next(
self: Pin<&mut Self>,
_: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<T> Stream for Pending<T>` [src]

#### `type Item = T`

#### `fn poll_next(
self: Pin<&mut Self>,
_: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

### `impl<T> Stream for Repeat<T> where
T: Clone,` [src]

#### `type Item = T`

#### `fn poll_next(
self: Pin<&mut Self>,
_: &mut Context<'_>
) -> Poll<Option<Self::Item>>` [src]

#### `fn size_hint(&self) -> (usize, Option<usize>)` [src]

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

#### `type Item = T`

#### `fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>>` [src]

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

#### `type Item = Result<Item, Fut::Error>`

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

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

#### `type Item = Item`

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

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