Draft RFC to add Stream to the standard library

rfc-drafts/stream.md

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+- Feature Name: `async_stream`
+- Start Date: 2020-05-13
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Introduce the `Stream` trait into the standard library, using the
+design from `futures`. Redirect the futures-stream definition to the
+standard library.
+
+# Motivation
+[motivation]: #motivation
+
+* Why include stream trait in the std library at all?
+    * Streams are a core async abstraction
+        * we want to enable portable libraries that produce/consume streams without being tied to particular executors
+        * examples of crates that are consuming streams?
+            * [async-h1](https://docs.rs/async-h1)'s server implementation takes `TcpStream` instances produced by a `TcpListener` in a loop.
+        * examples of crates that are producing streams?
+            * [async-sse](https://docs.rs/async-sse/) parses incoming buffers into a stream of messages.
+        * people can do this today using futures crate, but the stability guarantees are less clear
+            * e.g., if tokio wishes to declare a [5 year stability period](http://smallcultfollowing.com/babysteps/blog/2020/02/11/async-interview-6-eliza-weisman/#communicating-stability), having something in std means there are no concerns about trait changing during that time ([citation](http://smallcultfollowing.com/babysteps/blog/2019/12/23/async-interview-3-carl-lerche/#what-should-we-do-next-stabilize-stream))
+    * We eventually want dedicated syntax for working with streams, which will require a shared trait
+        * Producing streams
+        * Consuming streams
+* Why is the stream trait defined how it is?
+    * It is the "pollable iterator"
+    * dyn compatibility
+
+# Guide-level explanation
+[guide-level-explanation]: #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.
+
+[iterator]: https://doc.rust-lang.org/std/iter/trait.Iterator.html
+[`Future`]: https://doc.rust-lang.org/std/future/trait.Future.html
+[`Poll::pending`]: https://doc.rust-lang.org/std/task/enum.Poll.html#variant.Pending
+
+```rust
+// Defined in std::stream module
+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)
+    }
+}
+```
+
+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.
+
+[`Future::poll`]: https://doc.rust-lang.org/std/future/trait.Future.html#tymethod.poll
+[pinned]: https://doc.rust-lang.org/std/pin/struct.Pin.html
+[context]: https://doc.rust-lang.org/std/task/struct.Context.html
+[Waker]: https://doc.rust-lang.org/std/task/struct.Waker.html
+
+## Initial impls
+
+There are a number of simple "bridge" impls that are also provided:
+
+```rust
+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;
+}
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #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`.
+
+## Why use a `poll` method?
+
+An alternative design for the stream trait would be to have a trait
+that defines an async `next` method:
+
+```rust
+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](https://rust-lang.github.io/wg-async-foundations/design_notes/async_fn_in_traits.html)
+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][future-possibilities] section, is dedicated [generator syntax].
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Why should we *not* do this?
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+## Where should stream live?
+
+* 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
+[prior-art]: #prior-art
+
+Discuss prior art, both the good and the bad, in relation to this proposal.
+A few examples of what this can include are:
+
+- For language, library, cargo, tools, and compiler proposals: Does this feature exist in other programming languages and what experience have their community had?
+- For community proposals: Is this done by some other community and what were their experiences with it?
+- For other teams: What lessons can we learn from what other communities have done here?
+- Papers: Are there any published papers or great posts that discuss this? If you have some relevant papers to refer to, this can serve as a more detailed theoretical background.
+
+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
+[unresolved-questions]: #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
+[future-possibilities]: #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.
+
+[`StreamExt`]: https://docs.rs/futures/0.3.5/futures/stream/trait.StreamExt.html
+[`futures`]: https://crates.io/crates/futures
+
+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.
+
+[outstanding design issues]: https://rust-lang.github.io/wg-async-foundations/design_notes/async_closures.html
+
+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, mirroring iterators
+
+* currently blocked on async fn in traits
+* The exact bounds are unclear.
+* the same as combinators
+* These would be needed to provide similar iteration semantics as Iterator:
+    * `for x in iter` uses `impl IntoIterator for T`
+    * `for x in &iter` uses `impl IntoIterator for &T`
+    * `for x in &mut iter` uses `impl IntoIterator for &mut T`
+
+## Async iteration syntax
+
+We may wish to introduce some dedicated syntax, analogous to `for` 
+
+## Generator syntax
+[generator syntax]: #generator-syntax
+
+```rust
+gen async fn foo() -> X {
+    yield value;
+}
+```
+
+## "Attached" streams
+
+Just as with iterators, there is a 
+
+