diff --git a/text/0000-async-stream.md b/text/0000-async-stream.md new file mode 100644 index 00000000000..41c5d1a8690 --- /dev/null +++ b/text/0000-async-stream.md @@ -0,0 +1,792 @@ +- Feature Name: `async_stream` +- Start Date: 2020-09-29 +- 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 `Stream` trait definition in the +`futures-core` crate (which is "pub-used" by the `futures` crate) to the standard library. + +# Motivation +[motivation]: #motivation + +Streams are a core async abstraction. These behave similarly to `Iterator`, +but rather than blocking between each item yield, it allows other +tasks to run while it waits. + +People can do this currently using the `Stream` trait defined in the +[futures](https://crates.io/crates/futures) crate. However, we would like +to add `Stream` to the standard library. + +Including `Stream` in the standard library would clarify the stability guarantees of the trait. For example, if [Tokio](https://tokio.rs/) +wishes to declare a [5 year stability period](http://smallcultfollowing.com/babysteps/blog/2020/02/11/async-interview-6-eliza-weisman/#communicating-stability), +having the `Stream` trait in the standard library means there are no concerns +about the 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)). + +## Examples of current crates that are consuming streams + +### async-h1 + +* [async-h1](https://docs.rs/async-h1)'s server implementation takes `TcpStream` instances produced by a `TcpListener` in a loop. + +### async-sse + +* [async-sse](https://docs.rs/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 +[guide-level-explanation]: #guide-level-explanation + +A "stream" is the async version of an [iterator]. + +The `Iterator` trait includes a `next` method, which computes and returns the next item in the sequence. The `Stream` trait includes the `poll_next` method to assist with defining a stream. In the future, we should add a `next` method for use when consuming and interacting with a stream (see the [Future possiblilities](future-possibilities) section later in this RFC). + +## poll_next method + +When implementing a `Stream`, users will define a `poll_next` method. +The `poll_next` method asks if the next item is ready. If so, it returns +the item. 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 { + // Core items: + type Item; + fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll>; + + // Optional optimization hint, just like with iterators: + #[inline] + fn size_hint(&self) -> (usize, Option) { + (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 + +### Usage + +A user could create a stream as follows (Example taken from @yoshuawuyt's [implementation pull request](https://github.com/rust-lang/rust/pull/79023)). + +Creating a stream involves two steps: creating a `struct` to + hold the stream's state, and then implementing `Stream` for that + `struct`. + + Let's make a stream named `Counter` which counts from `1` to `5`: + +```rust +#![feature(async_stream)] +# use core::stream::Stream; +# use core::task::{Context, Poll}; +# use core::pin::Pin; + +// First, the struct: + +/// A stream which counts from one to five +struct Counter { + count: usize, +} + +// we want our count to start at one, so let's add a new() method to help. +// This isn't strictly necessary, but is convenient. Note that we start +// `count` at zero, we'll see why in `poll_next()`'s implementation below. +impl Counter { + fn new() -> Counter { + Counter { count: 0 } + } +} + +// Then, we implement `Stream` for our `Counter`: + +impl Stream for Counter { + // we will be counting with usize + type Item = usize; + + // poll_next() is the only required method + fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll> { + // Increment our count. This is why we started at zero. + self.count += 1; + + // Check to see if we've finished counting or not. + if self.count < 6 { + Poll::Ready(Some(self.count)) + } else { + Poll::Ready(None) + } + } +} +``` + +## Initial impls + +There are a number of simple "bridge" impls that are also provided: + +```rust +impl Stream for Box +where + S: Stream + Unpin + ?Sized, +{ + type Item = ::Item +} + +impl Stream for &mut S +where + S: Stream + Unpin + ?Sized, +{ + type Item = ::Item; +} + +impl Stream for Pin

+where + P: DerefMut + Unpin, + T: Stream, +{ + type Item = ::Item; +} + +impl Stream for AssertUnwindSafe +where + S: Stream, +{ + type Item = ::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`. + +It is possible that it could live in another area as well, though this follows +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: + +```rust +trait Stream { + type Item; + + async fn next(&mut self) -> Option; +} +``` + +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 important 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 possiblilities](future-possibilities) section, is dedicated [generator syntax]. + +# Rationale and alternatives +[rationale-and-alternatives]: #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? + +# Future possibilities +[future-possibilities]: #future-possibilities + +## Next method + +While users will be able to implement a `Stream` as defined in this RFC, they will not have a way to interact with it in the core library. As soon as we figure out a way to do it in an object safe manner, we should add a `next` method either in the `Stream` trait or elsewhere. + +The `Iterator` trait includes a `next` method, which computes and returns the next item in the sequence. We should also implement a `next` method for `Stream`, similar to [the implementation in the futures-util crate](https://docs.rs/futures-util/0.3.5/src/futures_util/stream/stream/next.rs.html#10-12). + +The core `poll_next` method is unergonomic; it does not let you 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, such as `next`. + +As @yoshuawuyts states in their [pull request which adds `core::stream::Stream` to the standard library](https://github.com/rust-lang/rust/pull/79023): + +Unlike `Iterator`, `Stream` makes a distinction between the `poll_next` +method which is used when implementing a `Stream`, and the `next` method +which is used when consuming a stream. Consumers of `Stream` only need to +consider `next`, which when called, returns a future which yields +`Option`. + +The future returned by `next` will yield `Some(Item)` as long as there are +elements, and once they've all been exhausted, will yield `None` to indicate +that iteration is finished. If we're waiting on something asynchronous to +resolve, the future will wait until the stream is ready to yield again. + +As defined in the [`Future` docs](https://doc.rust-lang.org/stable/std/future/trait.Future.html): + +Once a future has completed (returned Ready from poll), calling its poll method again may panic, block forever, or cause other kinds of problems; the Future trait places no requirements on the effects of such a call. However, as the poll 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 future's state. + +This is similar to the `Future` trait. The `Future::poll` method is rarely called +directly, it is almost always used to implement other Futures. Interacting +with futures is done through `async/await`. + +We need something like the `next()` method in order to iterate over the stream directly in an `async` block or function. It is essentially an adapter from `Stream` to `Future`. + +This would allow a user to await on a future: + +```rust +while let Some(v) = stream.next().await { + +} +``` + +We could also consider adding a `try_next` method, allowing +a user to write: + +```rust +while let Some(x) = s.try_next().await? +``` + +But this could also be written as: + +```rust +while let Some(x) = s.next().await.transpose()? +``` + +### More Usage Examples + +Using the example of `Stream` implemented on a struct called `Counter`, the user would interact with the stream like so: + +```rust +let mut counter = Counter::new(); + +let x = counter.next().await.unwrap(); +println!("{}", x); + +let x = counter.next().await.unwrap(); +println!("{}", x); + +let x = counter.next().await.unwrap(); +println!("{}", x); + +let x = counter.next().await.unwrap(); +println!("{}", x); + +let x = counter.next().await.unwrap(); +println!("{}", x); +# +} +``` + +This would print `1` through `5`, each on their own line. + +An earlier draft of the RFC prescribed an implementation of the `next` method on the `Stream` trait. Unfortunately, as detailed in [this comment](https://github.com/rust-lang/rust/pull/79023#discussion_r547425181), it made the stream non-object safe. More experimentation is required - and it may need to be an unstable language feature for more testing before it can be added to core. + +## More Convenience methods + +The `Iterator` trait also 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 + +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. + +```rust +pub trait IntoIterator where + ::Item == Self::Item, +{ + type Item; + + type IntoIter: Iterator; + + fn into_iter(self) -> Self::IntoIter; +} +``` + +Examples are taken from the Rust docs on [for loops and into_iter](https://doc.rust-lang.org/std/iter/index.html#for-loops-and-intoiterator) + +* `for x in iter` uses `impl IntoIterator for T` + +```rust +let values = vec![1, 2, 3, 4, 5]; + +for x in values { + println!("{}", x); +} +``` + +Desugars to: + +```rust +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: + +```rust +pub trait IntoStream +where + ::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](https://github.com/rust-lang/wg-async-foundations/pull/15/files#r449880986)) makes it easy to write streams in combination with [async stream](https://github.com/taiki-e/futures-async-stream). For example: + +```rust +type S(usize); + +impl IntoStream for S { + type Item = usize; + type IntoStream: impl Stream; + + 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. + +```rust +pub trait FromIterator { + + fn from_iter(iter: T) -> Self + where + T: IntoIterator; +} +``` + +It should be noted that this trait is rarely used directly, instead used through Iterator's collect method ([source](https://doc.rust-lang.org/std/iter/trait.FromIterator.html)). + +```rust +pub trait Iterator { + fn collect(self) -> B + where + B: FromIterator, + { ... } +} +``` + +Examples are taken from the Rust docs on [iter and collect](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.collect) + + +```rust +let a = [1, 2, 3]; + +let doubled: Vec = a.iter() + .map(|&x| x * 2) + .collect(); + +``` + +**Streams** + +We may want a trait similar to this for `Stream`. The `FromStream` trait would provide a way to convert a `Stream` into another type. + +This trait could look like this: + +```rust +pub trait FromStream { + async fn from_stream(stream: T) -> Self + where + T: IntoStream; +} +``` + +We could potentially include a collect method for Stream as well. + +```rust +pub trait Stream { + async fn collect(self) -> B + where + B: FromStream, + { ... } +} +``` + +When drafting this RFC, there was [discussion](https://github.com/rust-lang/wg-async-foundations/pull/15#discussion_r451182595) +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 to. 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. + +## Converting an Iterator to a Stream + +If a user wishes to convert an Iterator to a Stream, they may not be able to use IntoStream because a blanked impl for Iterator would conflict with more specific impls they may wish to write. Having a function that takes an `impl Iterator` and returns an `impl Stream` would be quite helpful. + +The [async-std](https://github.com/async-rs/async-std) crate has [stream::from_iter](https://docs.rs/async-std/1.6.5/async_std/stream/fn.from_iter.html). The [futures-rs](https://github.com/rust-lang/futures-rs) crate has [stream::iter](https://docs.rs/futures/0.3.5/futures/stream/fn.iter.html). Either of these approaches could work once we expose `Stream` in the standard library. + +Adding this functionality is out of the scope of this RFC, but is something we should revisit once `Stream` is in the standard library. + +## 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](https://docs.rs/async-std/1.6.0/async_std/stream/index.html) has created several async counterparts to the traits in [std::iter](https://doc.rust-lang.org/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. + +```rust +#[async] +for elem in stream { ... } +``` + +One of the complications of using `while let` syntax is the need to pin. +A `for` loop syntax that takes ownership of the stream would be able to +do the pinning for you. + +We may not want to make sequential processing "too easy" without also enabling +parallel/concurrent processing, which people frequently want. One challenge is +that parallel processing wouldn't naively permit early returns and other complex +control flow. We could add a `par_stream()` method, similar to +[Rayon's](https://github.com/rayon-rs/rayon) `par_iter()`. + +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](https://smallcultfollowing.com/babysteps/blog/2019/12/10/async-interview-2-cramertj-part-2/#the-need-for-streaming-streams-and-iterators) + + +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). Lending streams depend on [Generic Associated Types](https://rust-lang.github.io/rfcs/1598-generic_associated_types.html), which are not (at the time of this RFC) stable. + +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 `LendingStream`s. + +### Current Stream Trait + +```rust +pub trait Stream { + type Item; + + fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll>; + + #[inline] + fn size_hint(&self) -> (usize, Option) { + (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 + +```rust +trait LendingStream<'s> { + type Item<'a> where 's: 'a; + + fn poll_next<'a>( + self: Pin<&'a mut Self>, + cx: &mut Context<'_>, + ) -> Poll>>; +} + +impl LendingStream for S +where + S: Stream, +{ + type Item<'_> = S::Item; + + fn poll_next<'s>( + self: Pin<&'s mut Self>, + cx: &mut Context<'_>, + ) -> Poll>> { + Stream::poll_next(self, cx) + } +} +``` + +This is a "conversion" trait such that anything which implements `Stream` can also implement +`LendingStream`. + +This trait captures the case where 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: + +```rust +impl Stream for Box where T: Stream +``` + +and + +```rust +impl LendingStream for Box where T: LendingStream +``` + +There is a coherence conflict for `Box`, so presumably it will fail the coherence rules. + +[More examples are available here](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a667a7560f8dc97ab82a780e27dfc9eb). + +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. + +We may eventually want a super trait relationship available in the Rust language + +```rust +trait Stream: LendingStream +``` + +This would allow us to leverage `default impl`. + +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 +[generator syntax]: #generator-syntax + +In the future, we may wish to introduce a new form of function - +`gen fn` in iterators and `async gen fn` 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 +could yield references to local variables. Given a "detached" +or "owned" stream, the generator could yield owned values +or things that were borrowed from its caller. + +### In Iterators + +```rust +gen fn foo() -> Value { + yield value; +} +``` + +After desugaring, this would result in a function like: + +```rust +fn foo() -> impl Iterator +``` + +### In Async Code + +```rust +async gen fn foo() -> Value +``` + +After desugaring would result in a function like: + +```rust +fn foo() -> impl Stream +``` + +If we introduce `-> impl 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 are some crucial differences between the two. + +Consider Iterator's core `next` method: + +```rust +pub trait Iterator { + type Item; + + fn next(&mut self) -> Option; +} +``` + +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: + +```rust +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 + +```rust +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: + +```rust +trait PinIterator { + type Item; +} +impl + DerefMut> Iterator for Pin

{ + 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 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 currently understood as pinned receiver + context arg + return poll. + +Another key difference between `Iterators` and `Streams` is that futures are ultimately passed to some executor API like spawn which expects a `'static` future. To achieve that, the futures contain all the state they need and references are internal to that state. Iterators are almost never required to be `'static` by the APIs that consume them. + +It is, admittedly, somewhat confusing to have Async generators require Pinning and Iterator generators to not require pinning, users may feel they are creating code in an unnatural way when using the Async generators. This will need to be discussed more when generators are proposed in the future. + +### Disallowing self-borrowing generators in `gen fn` + +Another option is to make the generators returned by `gen fn` always be `Unpin` so that the user doesn't have to think about pinning unless they're already in an async context. + +In the spirit of experimentation, boats has written the [propane] +crate. This crate includes a `#[propane] fn` that changes the function signature +to return `impl Iterator` and lets you `yield`. The non-async version uses +(nightly-only) generators which are non-`static`, disallowing self-borrowing. +In other words, you can't hold a reference to something on the stack across a `yield`. + +This should still allow yielding from inside a for loop, as long as the for loop is +over a borrowed input and not something owned by the stack frame. + +[propane]: https://github.com/withoutboats/propane + +Further designing generator functions is out of the scope of this RFC.