[−][src]Trait futures_util::stream::Stream
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.
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 subsequentpoll_next
calls. -
Poll::Ready(None)
means that the stream has terminated, andpoll_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.
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.
Implementations on Foreign Types
impl<P> Stream for Pin<P> where
P: DerefMut + Unpin,
<P as Deref>::Target: Stream,
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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>>
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self: Pin<&mut Pin<P>>,
cx: &mut Context
) -> Poll<Option<<Pin<P> as Stream>::Item>>
fn size_hint(&self) -> (usize, Option<usize>)
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Implementors
impl<'_, S> Stream for &'_ mut S where
S: Stream + Unpin + ?Sized,
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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>>
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self: Pin<&mut &'_ mut S>,
cx: &mut Context
) -> Poll<Option<<&'_ mut S as Stream>::Item>>
fn size_hint(&self) -> (usize, Option<usize>)
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impl<A, B> Stream for Either<A, B> where
A: Stream,
B: Stream<Item = A::Item>,
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A: Stream,
B: Stream<Item = A::Item>,
type Item = A::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<A::Item>>
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impl<Fut> Stream for FlattenStream<Fut> where
Fut: Future,
Fut::Output: Stream,
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Fut: Future,
Fut::Output: Stream,
type Item = <Fut::Output as Stream>::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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impl<Fut> Stream for TryFlattenStream<Fut> where
Fut: TryFuture,
Fut::Ok: TryStream<Error = Fut::Error>,
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Fut: TryFuture,
Fut::Ok: TryStream<Error = Fut::Error>,
type Item = Result<<Fut::Ok as TryStream>::Ok, Fut::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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impl<Fut, S> Stream for FlattenSink<Fut, S> where
Fut: TryFuture<Ok = S>,
S: TryStream<Error = Fut::Error>,
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Fut: TryFuture<Ok = S>,
S: TryStream<Error = Fut::Error>,
type Item = Result<S::Ok, Fut::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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impl<Fut: Future> Stream for futures_util::future::IntoStream<Fut>
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type Item = Fut::Output
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<Fut: Future> Stream for Once<Fut>
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type Item = Fut::Output
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<I> Stream for Iter<I> where
I: Iterator,
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I: Iterator,
type Item = I::Item
fn poll_next(self: Pin<&mut Self>, _: &mut Context) -> Poll<Option<I::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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S: Sink<Item> + Stream,
S::Error: Into<E>,
type Item = S::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<S::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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S: Stream + Sink<Item>,
F: FnMut(U) -> Fut,
Fut: Future,
type Item = S::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<S::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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S: Stream + Sink<Item>,
F: FnMut(U) -> St,
St: Stream<Item = Result<Item, S::Error>>,
type Item = S::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<S::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<S: Stream> Stream for Fuse<S>
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type Item = S::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<S::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<S: Stream> Stream for Peekable<S>
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type Item = S::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<S: Stream, F> Stream for SinkMapErr<S, F>
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type Item = S::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<S::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St> Stream for Flatten<St> where
St: Stream,
St::Item: Stream,
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St: Stream,
St::Item: Stream,
type Item = <St::Item as Stream>::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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impl<St> Stream for Take<St> where
St: Stream,
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St: Stream,
type Item = St::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<St::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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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>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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impl<St, E> Stream for ErrInto<St, E> where
St: TryStream,
St::Error: Into<E>,
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St: TryStream,
St::Error: Into<E>,
type Item = Result<St::Ok, E>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, F> Stream for Inspect<St, F> where
St: Stream,
F: FnMut(&St::Item),
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St: Stream,
F: FnMut(&St::Item),
type Item = St::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<St::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, F> Stream for InspectErr<St, F> where
St: TryStream,
F: FnMut(&St::Error),
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St: TryStream,
F: FnMut(&St::Error),
type Item = Result<St::Ok, St::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, F> Stream for InspectOk<St, F> where
St: TryStream,
F: FnMut(&St::Ok),
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St: TryStream,
F: FnMut(&St::Ok),
type Item = Result<St::Ok, St::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, F, E> Stream for MapErr<St, F> where
St: TryStream,
F: FnMut(St::Error) -> E,
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St: TryStream,
F: FnMut(St::Error) -> E,
type Item = Result<St::Ok, E>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, F, T> Stream for Map<St, F> where
St: Stream,
F: FnMut(St::Item) -> T,
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St: Stream,
F: FnMut(St::Item) -> T,
type Item = T
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<T>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, F, T> Stream for MapOk<St, F> where
St: TryStream,
F: FnMut(St::Ok) -> T,
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St: TryStream,
F: FnMut(St::Ok) -> T,
type Item = Result<T, St::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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St: TryStream,
F: FnMut(St::Ok) -> Fut,
Fut: TryFuture<Error = St::Error>,
type Item = Result<Fut::Ok, St::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, Fut, F> Stream for Filter<St, Fut, F> where
St: Stream,
F: FnMut(&St::Item) -> Fut,
Fut: Future<Output = bool>,
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St: Stream,
F: FnMut(&St::Item) -> Fut,
Fut: Future<Output = bool>,
type Item = St::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<St::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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St: TryStream,
F: FnMut(St::Error) -> Fut,
Fut: TryFuture<Ok = St::Ok>,
type Item = Result<St::Ok, Fut::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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St: Stream,
F: FnMut(&St::Item) -> Fut,
Fut: Future<Output = bool>,
type Item = St::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<St::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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St: Stream,
F: FnMut(&St::Item) -> Fut,
Fut: Future<Output = bool>,
type Item = St::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<St::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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St: Stream,
F: FnMut(St::Item) -> Fut,
Fut: Future,
type Item = Fut::Output
fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context
) -> Poll<Option<Fut::Output>>
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self: Pin<&mut Self>,
cx: &mut Context
) -> Poll<Option<Fut::Output>>
fn size_hint(&self) -> (usize, Option<usize>)
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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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St: TryStream,
Fut: Future<Output = bool>,
F: FnMut(&St::Ok) -> Fut,
type Item = Result<St::Ok, St::Error>
fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context
) -> Poll<Option<Result<St::Ok, St::Error>>>
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self: Pin<&mut Self>,
cx: &mut Context
) -> Poll<Option<Result<St::Ok, St::Error>>>
fn size_hint(&self) -> (usize, Option<usize>)
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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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St: TryStream,
F: FnMut(&St::Ok) -> Fut,
Fut: TryFuture<Ok = bool, Error = St::Error>,
type Item = Result<St::Ok, St::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St, Fut, F, T> Stream for FilterMap<St, Fut, F> where
St: Stream,
F: FnMut(St::Item) -> Fut,
Fut: Future<Output = Option<T>>,
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St: Stream,
F: FnMut(St::Item) -> Fut,
Fut: Future<Output = Option<T>>,
type Item = T
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<T>>
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fn size_hint(&self) -> (usize, Option<usize>)
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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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St: TryStream,
Fut: TryFuture<Ok = Option<T>, Error = St::Error>,
F: FnMut(St::Ok) -> Fut,
type Item = Result<T, St::Error>
fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context
) -> Poll<Option<Result<T, St::Error>>>
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self: Pin<&mut Self>,
cx: &mut Context
) -> Poll<Option<Result<T, St::Error>>>
fn size_hint(&self) -> (usize, Option<usize>)
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impl<St1, St2> Stream for Chain<St1, St2> where
St1: Stream,
St2: Stream<Item = St1::Item>,
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St1: Stream,
St2: Stream<Item = St1::Item>,
type Item = St1::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St1, St2> Stream for Select<St1, St2> where
St1: Stream,
St2: Stream<Item = St1::Item>,
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St1: Stream,
St2: Stream<Item = St1::Item>,
type Item = St1::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<St1::Item>>
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impl<St1, St2> Stream for Zip<St1, St2> where
St1: Stream,
St2: Stream,
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St1: Stream,
St2: Stream,
type Item = (St1::Item, St2::Item)
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St: Stream> Stream for Enumerate<St>
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type Item = (usize, St::Item)
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St: Stream> Stream for Skip<St>
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type Item = St::Item
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<St::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<St: TryStream> Stream for futures_util::stream::IntoStream<St>
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type Item = Result<St::Ok, St::Error>
fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<T> Stream for Empty<T>
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type Item = T
fn poll_next(self: Pin<&mut Self>, _: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<T> Stream for Pending<T>
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type Item = T
fn poll_next(self: Pin<&mut Self>, _: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<T> Stream for Repeat<T> where
T: Clone,
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T: Clone,
type Item = T
fn poll_next(self: Pin<&mut Self>, _: &mut Context) -> Poll<Option<Self::Item>>
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fn size_hint(&self) -> (usize, Option<usize>)
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impl<T, F> Stream for PollFn<F> where
F: FnMut(&mut Context) -> Poll<Option<T>>,
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F: FnMut(&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)>>,
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F: FnMut(T) -> Fut,
Fut: Future<Output = Option<(Item, T)>>,