Trait futures::stream::Stream

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

    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

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 is finished, i.e. Ready(None) has been returned, further calls to poll_next may result in a panic or other "bad behavior". 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

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

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

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

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

type Item = <S as Stream>::Item

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

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

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

type Item = <S as Stream>::Item

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

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

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

type Item = <S as Stream>::Item

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

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

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Implementors

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

type Item = <A as Stream>::Item

fn poll_next(
    self: Pin<&mut Either<A, B>>,
    cx: &mut Context
) -> Poll<Option<<A as Stream>::Item>>

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

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

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

impl<Fut> Stream for futures::future::IntoStream<Fut> where
    Fut: Future, 

type Item = <Fut as Future>::Output

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

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

impl<Fut> Stream for TryFlattenStream<Fut> where
    Fut: TryFuture,
    <Fut as TryFuture>::Ok: TryStream,
    <<Fut as TryFuture>::Ok as TryStream>::Error == <Fut as TryFuture>::Error, 

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

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

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

type Item = <Fut as Future>::Output

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

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

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

type Item = <Fut as Future>::Output

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

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

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

type Item = <Fut as Future>::Output

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

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

impl<Fut, S> Stream for FlattenSink<Fut, S> where
    Fut: TryFuture<Ok = S>,
    S: TryStream<Error = <Fut as TryFuture>::Error>, 

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

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

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

type Item = <I as Iterator>::Item

fn poll_next(
    self: Pin<&mut Iter<I>>,
    &mut Context
) -> Poll<Option<<I as Iterator>::Item>>

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

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

type Item = Result<String, Error>

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

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

type Item = <S as Stream>::Item

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

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

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

type Item = <S as Stream>::Item

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

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

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

type Item = <S as Stream>::Item

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

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

type Item = <S as Stream>::Item

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

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

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

type Item = <S as Stream>::Item

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

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

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

type Item = <S as Stream>::Item

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

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

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

type Item = <S as Stream>::Item

fn poll_next(
    self: Pin<&mut With<S, Item, U, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<S as Stream>::Item>>

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

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

type Item = <S as Stream>::Item

fn poll_next(
    self: Pin<&mut WithFlatMap<S, Item, U, St, F>>,
    cx: &mut Context
) -> Poll<Option<<S as Stream>::Item>>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

impl<St> Stream for futures::stream::IntoStream<St> where
    St: TryStream, 

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

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

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

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

type Item = <St as Stream>::Item

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

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

type Item = <St as Stream>::Item

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

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

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

type Item = <St as Stream>::Item

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

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

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

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

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

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

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

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

impl<St, E> Stream for ErrInto<St, E> where
    St: TryStream,
    <St as TryStream>::Error: Into<E>, 

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

fn poll_next(
    self: Pin<&mut ErrInto<St, E>>,
    cx: &mut Context
) -> Poll<Option<<ErrInto<St, E> as Stream>::Item>>

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

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

type Item = <St as Stream>::Item

fn poll_next(
    self: Pin<&mut Inspect<St, F>>,
    cx: &mut Context
) -> Poll<Option<<St as Stream>::Item>>

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

impl<St, F> Stream for InspectErr<St, F> where
    F: FnMut(&<St as TryStream>::Error),
    St: TryStream, 

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

fn poll_next(
    self: Pin<&mut InspectErr<St, F>>,
    cx: &mut Context
) -> Poll<Option<<InspectErr<St, F> as Stream>::Item>>

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

impl<St, F> Stream for InspectOk<St, F> where
    F: FnMut(&<St as TryStream>::Ok),
    St: TryStream, 

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

fn poll_next(
    self: Pin<&mut InspectOk<St, F>>,
    cx: &mut Context
) -> Poll<Option<<InspectOk<St, F> as Stream>::Item>>

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

impl<St, F, E> Stream for MapErr<St, F> where
    F: FnMut(<St as TryStream>::Error) -> E,
    St: TryStream, 

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

fn poll_next(
    self: Pin<&mut MapErr<St, F>>,
    cx: &mut Context
) -> Poll<Option<<MapErr<St, F> as Stream>::Item>>

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

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

type Item = T

fn poll_next(self: Pin<&mut Map<St, F>>, cx: &mut Context) -> Poll<Option<T>>

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

impl<St, F, T> Stream for MapOk<St, F> where
    F: FnMut(<St as TryStream>::Ok) -> T,
    St: TryStream, 

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

fn poll_next(
    self: Pin<&mut MapOk<St, F>>,
    cx: &mut Context
) -> Poll<Option<<MapOk<St, F> as Stream>::Item>>

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

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

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

fn poll_next(
    self: Pin<&mut AndThen<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<AndThen<St, Fut, F> as Stream>::Item>>

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

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

type Item = <St as Stream>::Item

fn poll_next(
    self: Pin<&mut Filter<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<St as Stream>::Item>>

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

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

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

fn poll_next(
    self: Pin<&mut OrElse<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<OrElse<St, Fut, F> as Stream>::Item>>

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

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

type Item = <St as Stream>::Item

fn poll_next(
    self: Pin<&mut SkipWhile<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<St as Stream>::Item>>

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

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

type Item = <St as Stream>::Item

fn poll_next(
    self: Pin<&mut TakeWhile<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<St as Stream>::Item>>

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

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

type Item = <Fut as Future>::Output

fn poll_next(
    self: Pin<&mut Then<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<Fut as Future>::Output>>

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

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

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

fn poll_next(
    self: Pin<&mut TryFilter<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<Result<<St as TryStream>::Ok, <St as TryStream>::Error>>>

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

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

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

fn poll_next(
    self: Pin<&mut TrySkipWhile<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<<TrySkipWhile<St, Fut, F> as Stream>::Item>>

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

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

type Item = T

fn poll_next(
    self: Pin<&mut FilterMap<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<T>>

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

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

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

fn poll_next(
    self: Pin<&mut TryFilterMap<St, Fut, F>>,
    cx: &mut Context
) -> Poll<Option<Result<T, <St as TryStream>::Error>>>

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

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

type Item = <St1 as Stream>::Item

fn poll_next(
    self: Pin<&mut Chain<St1, St2>>,
    cx: &mut Context
) -> Poll<Option<<Chain<St1, St2> as Stream>::Item>>

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

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

type Item = <St1 as Stream>::Item

fn poll_next(
    self: Pin<&mut Select<St1, St2>>,
    cx: &mut Context
) -> Poll<Option<<St1 as Stream>::Item>>

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

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

fn poll_next(
    self: Pin<&mut Zip<St1, St2>>,
    cx: &mut Context
) -> Poll<Option<<Zip<St1, St2> as Stream>::Item>>

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

impl<T> Stream for Receiver<T>

type Item = T

fn poll_next(self: Pin<&mut Receiver<T>>, cx: &mut Context) -> Poll<Option<T>>

impl<T> Stream for UnboundedReceiver<T>

type Item = T

fn poll_next(
    self: Pin<&mut UnboundedReceiver<T>>,
    cx: &mut Context
) -> Poll<Option<T>>

impl<T> Stream for Empty<T>

type Item = T

fn poll_next(
    self: Pin<&mut Empty<T>>,
    &mut Context
) -> Poll<Option<<Empty<T> as Stream>::Item>>

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

impl<T> Stream for Pending<T>

type Item = T

fn poll_next(
    self: Pin<&mut Pending<T>>,
    &mut Context
) -> Poll<Option<<Pending<T> as Stream>::Item>>

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

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

type Item = T

fn poll_next(
    self: Pin<&mut Repeat<T>>,
    &mut Context
) -> Poll<Option<<Repeat<T> as Stream>::Item>>

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

fn poll_next(self: Pin<&mut PollFn<F>>, cx: &mut Context) -> Poll<Option<T>>

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

fn poll_next(
    self: Pin<&mut Unfold<T, F, Fut>>,
    cx: &mut Context
) -> Poll<Option<<Unfold<T, F, Fut> as Stream>::Item>>

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