Trait futures_util::stream::Stream

#[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

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>>

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>)

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, 

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>>

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

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

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>>

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

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Implementors

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

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>>

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

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

type Item = A::Item

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

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>>, 

type Item = B

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

type Item = Fut::Output

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

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

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

type Item = Fut::Output

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

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

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

type Item = Fut::Output

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

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

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

type Item = I::Item

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

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

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

type Item = S::Item

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

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

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

type Item = S::Item

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

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

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, 

type Item = S::Item

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

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

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>>, 

type Item = S::Item

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

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

impl<S: Stream> Stream for Fuse<S>

type Item = S::Item

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

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

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

type Item = S::Item

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

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

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

type Item = S::Item

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

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

type Item = S::Item

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

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

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

type Item = St1::Item

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

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

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

type Item = St1::Item

pub fn poll_next(
    self: Pin<&mut Self>,
    cx: &mut Context<'_>
) -> Poll<Option<St1::Item>>

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

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

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

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

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

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

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

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

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

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

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

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

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

type Item = St::Item

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

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

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

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

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

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

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

type Item = St::Item

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

type Item = F::Output

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

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

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

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

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

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

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

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

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

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

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

type Item = St::Item

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

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

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

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

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

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

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>, 

type Item = St::Item

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

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

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

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

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

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

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

type Item = St::Item

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

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

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

type Item = St::Item

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

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

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

type Item = Fut::Output

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

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

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

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

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

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

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>, 

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

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

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

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>, 

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

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

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

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>>, 

type Item = T

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

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

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, 

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

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

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

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

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

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

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

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

type Item = St::Item

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

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

type Item = Vec<St::Item>

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

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

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

type Item = (usize, St::Item)

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

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

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

type Item = Vec<St::Item>

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

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

impl<St: Stream> Stream for Skip<St>

type Item = St::Item

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

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

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

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

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

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

impl<T> Stream for Empty<T>

type Item = T

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

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

impl<T> Stream for Pending<T>

type Item = T

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

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

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

type Item = T

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

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

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

type Item = T

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

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

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

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

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

type Item = Item

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

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