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Summary

Introduce the Stream trait into the standard library, using the design from futures. Redirect the Stream trait definition in the futures-core crate (which is "pub-used" by the futures crate) to the standard library.

Motivation

Streams are a core async abstraction. We want to enable portable libraries that produce/consume streams without being tied to a particular executor.

People can do this currently using the Stream trait defined in the futures crate. However, the stability guarantee of that trait would be clearer if it were added to the standard library. For example, if Tokio wishes to declare a 5 year stability period, having the stream trait in the standard library means there are no concerns about the trait changing during that time (citation).

Examples of crates that are consuming streams

async-h1

  • async-h1's server implementation takes TcpStream instances produced by a TcpListener in a loop.

async-sse

  • async-sse parses incoming buffers into a stream of messages.

Why a shared trait?

We eventually want dedicated syntax for working with streams, which will require a shared trait. This includes a trait for producing streams and a trait for consuming streams.

Guide-level explanation

A "stream" is the async version of an iterator. The Stream trait matches the definition of an iterator, except that the next method is defined to "poll" for the next item. In other words, where the next method on an iterator simply computes (and returns) the next item in the sequence, the poll_next method on stream asks if the next item is ready. If so, it will be returned, but otherwise poll_next will return Poll::pending. Just as with a Future, returning Poll::pending implies that the stream has arranged for the current task to be re-awoken when the data is ready.

// Defined in std::stream module
pub trait Stream {
    // Core items:
    type Item;
    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>;
    
    // Optional optimization hint, just like with iterators:
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, None)
    }

    // Convenience methods (covered later on in the RFC):
    fn next(&mut self) -> Next<'_, Self>
    where
        Self: Unpin;
}

The arguments to poll_next match that of the Future::poll method:

  • The self must be a pinned reference, ensuring both unique access to the stream and that the stream value itself will not move. Pinning allows the stream to save pointers into itself when it suspends, which will be required to support generator syntax at some point.
  • The context cx defines details of the current task. In particular, it gives access to the [Waker] for the task, which will allow the task to be re-awoken once data is ready.

Why does next require Self:Unpin?

When drafting this RFC, there was a good deal of discussion around why the next method requires Self:Unpin.

To understand this, it helps to take a closer look at the definition of Next (this struct is further discussed later in this RFC) in the futures-util crate.

pub struct Next<'a, St: ?Sized> {
    stream: &'a mut St,
}

Since Stream::poll_next takes a pinned reference, the next future needs S to be Unpin in order to safely construct a Pin<&mut S> from a &mut S.

An alternative approach we could take would be to have the next method take Pin<&mut S>, rather than &mut S. However, this would require pinning even when the type is Unpin. The current approach requires pinning only when the type is not Unpin.

At the moment, we do not see many Unpin! streams in practice (though there is one in the futures-intrusive crate). Where they will become important is when we introduce async generators, as discussed in future-possibilities.

Initial impls

There are a number of simple "bridge" impls that are also provided:

impl<S> Stream for Box<S>
where
    S: Stream + Unpin + ?Sized,
{
    type Item = <S as Stream>::Item
}

impl<S> Stream for &mut S
where
    S: Stream + Unpin + ?Sized,
{
    type Item = <S as Stream>::Item;
}

impl<S, T> Stream for Pin<P>
where
    P: DerefMut<Target=T> + Unpin,
    T::Target: Stream,
{
    type Item = <T as Stream>::Item;
}

impl<S> Stream for AssertUnwindSafe<S>
where
    S: Stream, 
{
    type Item = <S as Stream>::Item;
}

Next method/struct

We should also implement a next method, similar to the implementation in the futures-util crate.

In general, we have purposefully kept the core trait definition minimal. There are a number of useful extension methods that are available, for example, in the futures-stream crate, but we have not included them because they involve closure arguments, and we have not yet finalized the design of async closures.

However, the core methods alone are extremely unergonomic. You can't even iterate over the items coming out of the stream. Therefore, we include a few minimal convenience methods that are not dependent on any unstable features. Most notably, next

/// A future that advances the stream and returns the next value.
///
/// This `struct` is created by the [`next`] method on [`Stream`]. See its
/// documentation for more.
///
/// [`next`]: trait.Stream.html#method.next
/// [`Stream`]: trait.Stream.html
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct Next<'a, S: ?Sized> {
    stream: &'a mut S,
}

impl<St: ?Sized + Unpin> Unpin for Next<'_, St> {}

impl<'a, St: ?Sized + Stream + Unpin> Next<'a, St> {
    pub(super) fn new(stream: &'a mut St) -> Self {
        Next { stream }
    }
}

impl<St: ?Sized + Stream + Unpin> Future for Next<'_, St> {
    type Output = Option<St::Item>;

    fn poll(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Self::Output> {
        Pin::new(&mut *self.stream).poll_next(cx)
    }
}

This would allow a user to await on a future:

while let Some(v) = stream.next().await {

}

Reference-level explanation

This section goes into details about various aspects of the design and why they ended up the way they did.

Where does Stream live in the std lib?

Stream will live in the core::stream module and be re-exported as std::stream.

It is possible that it could live in another area as well, though this followes the pattern of core::future.

Why use a poll method?

An alternative design for the stream trait would be to have a trait that defines an async next method:

trait Stream {
    type Item;
    
    async fn next(&mut self) -> Option<Self::Item>;
}

Unfortunately, async methods in traits are not currently supported, and there are a number of challenges to be resolved before they can be added.

Moreover, it is not clear yet how to make traits that contain async functions be dyn safe, and it is imporant to be able to pass around dyn Stream values without the need to monomorphize the functions that work with them.

Unfortunately, the use of poll does mean that it is harder to write stream implementations. The long-term fix for this, discussed in the [Future possibilities] section, is dedicated generator syntax.

Drawbacks

Why should we not do this?

Rationale and alternatives

Where should stream live?

As mentioned above, core::stream is analogous to core::future. But, do we want to find some other naming scheme that can scale up to other future additions, such as io traits or channels?

Prior art

Discuss prior art, both the good and the bad, in relation to this proposal.

This section is intended to encourage you as an author to think about the lessons from other languages, provide readers of your RFC with a fuller picture. If there is no prior art, that is fine - your ideas are interesting to us whether they are brand new or if it is an adaptation from other languages.

Note that while precedent set by other languages is some motivation, it does not on its own motivate an RFC. Please also take into consideration that rust sometimes intentionally diverges from common language features.

Unresolved questions

  • What parts of the design do you expect to resolve through the RFC process before this gets merged?
  • What parts of the design do you expect to resolve through the implementation of this feature before stabilization?
  • What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?

Future possibilities

Convenience methods

The Iterator trait defines a number of useful combinators, like map. The Stream trait being proposed here does not include any such conveniences. Instead, they are available via extension traits, such as the StreamExt trait offered by the futures crate.

The reason that we have chosen to exclude combinators is that a number of them would require access to async closures. As of this writing, async closures are unstable and there are a number of outstanding design issues to be resolved before they are added. Therefore, we've decided to enable progress on the stream trait by stabilizing a core, and to come back to the problem of extending it with combinators.

Another reason to defer adding combinators is because of the possibility that some combinators may work best

This path does carry some risk. Adding combinator methods can cause existing code to stop compiling due to the ambiguities in method resolution. We have had problems in the past with attempting to migrate iterator helper methods from itertools for this same reason.

While such breakage is technically permitted by our semver guidelines, it would obviously be best to avoid it, or at least to go to great lengths to mitigate its effects. One option would be to extend the language to allow method resolution to "favor" the extension trait in existing code, perhaps as part of an edition migration.

Designing such a migration feature is out of scope for this RFC.

IntoStream / FromStream traits

IntoStream

Iterators

Iterators have an IntoIterator that is used with for loops to convert items of other types to an iterator.

pub trait IntoIterator where
    <Self::IntoIter as Iterator>::Item == Self::Item, 
{
    type Item;

    type IntoIter: Iterator;

    fn into_iter(self) -> Self::IntoIter;
}

Examples taken from the Rust docs on for loops and into_iter

  • for x in iter uses impl IntoIterator for T
let values = vec![1, 2, 3, 4, 5];

for x in values {
    println!("{}", x);
}

Desugars to:

let values = vec![1, 2, 3, 4, 5];
{
    let result = match IntoIterator::into_iter(values) {
        mut iter => loop {
            let next;
            match iter.next() {
                Some(val) => next = val,
                None => break,
            };
            let x = next;
            let () = { println!("{}", x); };
        },
    };
    result
}
  • for x in &iter uses impl IntoIterator for &T
  • for x in &mut iter uses impl IntoIterator for &mut T

Streams

We may want a trait similar to this for Stream. The IntoStream trait would provide a way to convert something into a Stream.

This trait could look like this:

pub trait IntoStream
where 
    <Self::IntoStream as Stream>::Item == Self::Item,
{
    type Item;

    type IntoStream: Stream;

    fn into_stream(self) -> Self::IntoStream;
}

This trait (as expressed by @taiki-e in a comment on a draft of this RFC) makes it easy to write streams in combination with async stream. For example:

type S(usize);

impl IntoStream for S {
    type Item = usize;
    type IntoStream: impl Stream<Item = Self::Item>;

    fn into_stream(self) -> Self::IntoStream {
        #[stream]
        async move {
            for i in 0..self.0 {
                yield i;
            }
        }
    }
}

FromStream

Iterators

Iterators have an FromIterator that is used to convert iterators into another type.

pub trait FromIterator<A> {

    fn from_iter<T>(iter: T) -> Self
    where
        T: IntoIterator<Item = A>;
}

It should be noted that this trait is rarely used directly, instead used through Iterator's collect method (source).

pub trait Interator {
    fn collect<B>(self) -> B
    where
        B: FromIterator<Self::Item>,
    { ... }
}

Examples taken from the Rust docs on iter and collect

let a = [1, 2, 3];

let doubled: Vec<i32> = a.iter()
                         .map(|&x| x * 2)
                         .collect();

Streams

We may want a trait similar to this for Stream. The FromStream trait would provide way to convert a Stream into another type.

This trait could look like this:

pub trait FromStream<A> {
    async fn from_stream<T>(stream: T) -> Self
    where
        T: IntoStream<Item = A>;
}

We could potentially include a collect method for Stream as well.

pub trait Stream {
    async fn collect<B>(self) -> B
    where
        B: FromStream<Self::Item>,
    { ... }
}

When drafting this RFC, there was discussion about whether to implement from_stream for all T where T: FromIterator as well. FromStream is perhaps more general than FromIterator because the await point is allowed to suspend execution of the current function, but doesn't have too. Therefore, many (if not all) existing impls of FromIterator would work for FromStream as well. While this would be a good point for a future discussion, it is not in the scope of this RFC.

Other Traits

Eventually, we may also want to add some (if not all) of the roster of traits we found useful for Iterator.

async_std::stream has created several async counterparts to the traits in std::iter. These include:

  • DoubleEndedStream: A stream able to yield elements from both ends.
  • ExactSizeStream: A stream that knows its exact length.
  • Extend: Extends a collection with the contents of a stream.
  • FromStream: Conversion from a Stream.
  • FusedStream: A stream that always continues to yield None when exhausted.
  • IntoStream: Conversion into a Stream.
  • Product: Trait to represent types that can be created by multiplying the elements of a stream.
  • Stream: An asynchronous stream of values.
  • Sum: Trait to represent types that can be created by summing up a stream.

As detailed in previous sections, the migrations to add these traits are out of scope for this RFC.

Async iteration syntax

Currently, if someone wishes to iterate over a Stream as defined in the futures crate, they are not able to use for loops, they must use while let and next/try_next instead.

We may wish to extend the for loop so that it works over streams as well.

#[async]
for elem in stream { ... }

Designing this extension is out of scope for this RFC. However, it could be prototyped using procedural macros today.

"Lending" streams

There has been much discussion around lending streams (also referred to as attached streams).

Definitions

Source

In a lending stream (also known as an "attached" stream), the Item that gets returned by Stream may be borrowed from self. It can only be used as long as the self reference remains live.

In a non-lending stream (also known as a "detached" stream), the Item that gets returned by Stream is "detached" from self. This means it can be stored and moved about independently from self.

This RFC does not cover the addition of lending streams (streams as implemented through this RFC are all non-lending streams).

We can add the Stream trait to the standard library now and delay adding in this distinction between the two types of streams - lending and non-lending. The advantage of this is it would allow us to copy the Stream trait from futures largely 'as is'.

The disadvantage of this is functions that consume streams would first be written to work with Stream, and then potentially have to be rewritten later to work with LendingStreams.

Current Stream Trait

pub trait Stream {
    type Item;

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

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, None)
    }
}

This trait, like Iterator, always gives ownership of each item back to its caller. This offers flexibility - such as the ability to spawn off futures processing each item in parallel.

Potential Lending Stream Trait

impl<S> LendingStream for S
where
    S: Stream,
{
    type Item<'_> = S::Item;
    
    fn poll_next<'s>(
        self: Pin<&'s mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Self::Item<'s>>> {
        Stream::poll_next(self, cx)
    }
}

This is a "conversion" trait such that anything which implements Stream can also implement Lending Stream.

This trait captures the case we re-use internal buffers. This would be less flexible for consumers, but potentially more efficient. Types could implement the LendingStream where they need to re-use an internal buffer and Stream if they do not. There is room for both.

We would also need to pursue the same design for iterators - whether through adding two traits or one new trait with a "conversion" from the old trait.

This also brings up the question of whether we should allow conversion in the opposite way - if every non-lending stream can become a lending one, should some lending streams be able to become non-lending ones?

Coherence

The impl above has a problem. As the Rust language stands today, we cannot cleanly convert impl Stream to impl LendingStream due to a coherence conflict.

If you have other impls like:

impl<T> Stream for Box<T> where T: Stream

and

impl<T> LendingStream for Box<T> where T: LendingStream

There is a coherence conflict for Box<impl Stream>, so presumably it will fail the coherence rules.

More examples are available here.

Resolving this would require either an explicit “wrapper” step or else some form of language extension.

It should be noted that the same applies to Iterator, it is not unique to Stream.

These use cases for lending/non-lending streams need more thought, which is part of the reason it is out of the scope of this particular RFC.

Generator syntax

In the future, we may wish to introduce a new form of function - gen fn in iterators and async gen in async code that can contain yield statements. Calling such a function would yield a impl Iterator or impl Stream, for sync and async respectively. Given an "attached" or "borrowed" stream, the generator yield could return references to local variables. Given a "detached" or "owned" stream, the generator yield could return things that you own or things that were borrowed from your caller.

In Iterators

gen fn foo() -> Value {
    yield value;
}

After desugaring, this would result in a function like:

fn foo() -> impl Iterator<Item = Value>

In Async Code

async gen fn foo() -> Value

After desugaring would result in a function like:

fn foo() -> impl Stream<Item = Value>

If we introduce -> Stream first, we will have to permit LendingStream in the future. Additionally, if we introduce LendingStream later, we'll have to figure out how to convert a LendingStream into a Stream seamlessly.

Differences between Iterator generators and Async generators

We want Stream and Iterator to work as analogously as possible, including when used with generators. However, in the current design, there is a crucial difference between the two.

Consider Iterator's core next method:

pub trait Iterator {
    type Item;

    fn next(&mut self) -> Option<Self::Item>;
}

And then compare it to the proposed Stream next method:

pub trait Stream {
    type Item;

    fn next(&mut self) -> Next<'_, Self>
    where
        Self: Unpin;
}

Iterator does not require pinning its core next method. In order for a gen fn to operate with the Iterator ecosystem, there must be some kind of initial pinning step that converts its result into an iterator. This will be tricky, since you can't return a pinned value except by boxing.

The general shape will be:

gen_fn().pin_somehow().adapter1().adapter2()

With streams, the core interface is pinned, so pinning occurs at the last moment.

The general shape would be

async_gen_fn().adapter1().adapter2().pin_somehow()

Pinning at the end, like with a stream, lets you build and return those adapters and then apply pinning at the end. This may be the more efficient setup and implies that, in order to have a gen fn that produces iterators, we will need to potentially disallow borrowing yields or implement some kind of PinnedIterator trait that can be "adapted" into an iterator by pinning.

For example:

trait PinIterator {
    type Item;
    fn next(self: Pin<&mut Self>) -> Self::Item;
    // combinators can go here (duplicating Iterator for the most part)
}
impl<I: PinIterator, P: Deref<Target = I> + DerefMut> Iterator for Pin<P> {
    type Item = <I as PinIterator>::Item;
    fn next(&mut self) -> Self::Item { self.as_mut().next() }
}

// this would be nice.. but would lead to name resolution ambiguity for our combinators 😬 
default impl<T: Iterator> PinIterator for T { .. }

Pinning also applies to the design of AsyncRead/AsyncWrite, which currently uses Pin even through there is no clear plan to make them implemented with generator type syntax. The asyncification of a signature is current understood as pinned receiver + context arg + return poll.

Further design of generator functions is out of the scope of this RFC.