futures_async_stream/

lib.rs

// SPDX-License-Identifier: Apache-2.0 OR MIT

/*!
<!-- Note: Document from sync-markdown-to-rustdoc:start through sync-markdown-to-rustdoc:end
     is synchronized from README.md. Any changes to that range are not preserved. -->
<!-- tidy:sync-markdown-to-rustdoc:start -->

Async stream for Rust and the futures crate.

This crate provides useful features for streams, using `async_await` and
unstable [`coroutines`](https://github.com/rust-lang/rust/issues/43122).

## Usage

Add this to your `Cargo.toml`:

```toml
[dependencies]
futures-async-stream = "0.2"
futures = "0.3"
```

*Compiler support: requires rustc nightly-2024-04-25+*

## `#[for_await]`

Processes streams using a for loop.

This is a reimplement of [futures-await]'s `#[async]` for loops for
futures 0.3 and is an experimental implementation of [the idea listed as the
next step of async/await](https://rust-lang.github.io/rfcs/2394-async_await.html#for-await-and-processing-streams).

```
#![feature(proc_macro_hygiene, stmt_expr_attributes)]

use futures::stream::Stream;
use futures_async_stream::for_await;

async fn collect(stream: impl Stream<Item = i32>) -> Vec<i32> {
    let mut vec = vec![];
    #[for_await]
    for value in stream {
        vec.push(value);
    }
    vec
}
```

`value` has the `Item` type of the stream passed in. Note that async for
loops can only be used inside of `async` functions, closures, blocks,
`#[stream]` functions and `stream_block!` macros.

## `#[stream]`

Creates streams via coroutines.

This is a reimplement of [futures-await]'s `#[stream]` for futures 0.3 and
is an experimental implementation of [the idea listed as the next step of
async/await](https://rust-lang.github.io/rfcs/2394-async_await.html#generators-and-streams).

```
#![feature(coroutines)]

use futures::stream::Stream;
use futures_async_stream::stream;

// Returns a stream of i32
#[stream(item = i32)]
async fn foo(stream: impl Stream<Item = String>) {
    // `for_await` is built into `stream`. If you use `for_await` only in `stream`, there is no need to import `for_await`.
    #[for_await]
    for x in stream {
        yield x.parse().unwrap();
    }
}
```

To early exit from a `#[stream]` function or block, use `return`.

`#[stream]` on async fn must have an item type specified via
`item = some::Path` and the values output from the stream must be yielded
via the `yield` expression.

`#[stream]` can also be used on async blocks:

```
#![feature(coroutines, proc_macro_hygiene, stmt_expr_attributes)]

use futures::stream::Stream;
use futures_async_stream::stream;

fn foo() -> impl Stream<Item = i32> {
    #[stream]
    async move {
        for i in 0..10 {
            yield i;
        }
    }
}
```

Note that `#[stream]` on async block does not require the `item` argument,
but it may require additional type annotations.

## Using async stream functions in traits

You can use async stream functions in traits by passing `boxed` or
`boxed_local` as an argument.

```
#![feature(coroutines)]

use futures_async_stream::stream;

trait Foo {
    #[stream(boxed, item = u32)]
    async fn method(&mut self);
}

struct Bar(u32);

impl Foo for Bar {
    #[stream(boxed, item = u32)]
    async fn method(&mut self) {
        while self.0 < u32::MAX {
            self.0 += 1;
            yield self.0;
        }
    }
}
```

A async stream function that received a `boxed` argument is converted to a
function that returns `Pin<Box<dyn Stream<Item = item> + Send + 'lifetime>>`.
If you passed `boxed_local` instead of `boxed`, async stream function
returns a non-thread-safe stream (`Pin<Box<dyn Stream<Item = item> + 'lifetime>>`).

```
#![feature(coroutines)]

use std::pin::Pin;

use futures::stream::Stream;
use futures_async_stream::stream;

// The trait itself can be defined without unstable features.
trait Foo {
    fn method(&mut self) -> Pin<Box<dyn Stream<Item = u32> + Send + '_>>;
}

struct Bar(u32);

impl Foo for Bar {
    #[stream(boxed, item = u32)]
    async fn method(&mut self) {
        while self.0 < u32::MAX {
            self.0 += 1;
            yield self.0;
        }
    }
}
```

## `#[try_stream]`

`?` operator can be used with the `#[try_stream]`. The `Item` of the
returned stream is `Result` with `Ok` being the value yielded and `Err` the
error type returned by `?` operator or `return Err(...)`.

```
#![feature(coroutines)]

use futures::stream::Stream;
use futures_async_stream::try_stream;

#[try_stream(ok = i32, error = Box<dyn std::error::Error>)]
async fn foo(stream: impl Stream<Item = String>) {
    #[for_await]
    for x in stream {
        yield x.parse()?;
    }
}
```

`#[try_stream]` can be used wherever `#[stream]` can be used.

To early exit from a `#[try_stream]` function or block, use `return Ok(())`.

<!--
## List of features that may be added in the future as an extension of this feature:

- `async_sink` (https://github.com/rust-lang-nursery/futures-rs/pull/1548#issuecomment-486205382)
- Support `.await` in macro (https://github.com/rust-lang-nursery/futures-rs/pull/1548#discussion_r285341883)
- Parallel version of `for_await` (https://github.com/rustasync/runtime/pull/25)
-->

## How to write the equivalent code without this API?

### `#[for_await]`

You can write this by combining `while let` loop, `.await`, `pin!` macro,
and `StreamExt::next()` method:

```
use std::pin::pin;

use futures::stream::{Stream, StreamExt};

async fn collect(stream: impl Stream<Item = i32>) -> Vec<i32> {
    let mut vec = vec![];
    let mut stream = pin!(stream);
    while let Some(value) = stream.next().await {
        vec.push(value);
    }
    vec
}
```

### `#[stream]`

You can write this by manually implementing the combinator:

```
use std::{
    pin::Pin,
    task::{ready, Context, Poll},
};

use futures::stream::Stream;
use pin_project::pin_project;

fn foo<S>(stream: S) -> impl Stream<Item = i32>
where
    S: Stream<Item = String>,
{
    Foo { stream }
}

#[pin_project]
struct Foo<S> {
    #[pin]
    stream: S,
}

impl<S> Stream for Foo<S>
where
    S: Stream<Item = String>,
{
    type Item = i32;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        if let Some(x) = ready!(self.project().stream.poll_next(cx)) {
            Poll::Ready(Some(x.parse().unwrap()))
        } else {
            Poll::Ready(None)
        }
    }
}
```

[futures-await]: https://github.com/alexcrichton/futures-await

<!-- tidy:sync-markdown-to-rustdoc:end -->
*/

#![no_std]
#![doc(test(
    no_crate_inject,
    attr(
        deny(warnings, rust_2018_idioms, single_use_lifetimes),
        allow(dead_code, unused_variables)
    )
))]
#![warn(
    // Lints that may help when writing public library.
    missing_debug_implementations,
    missing_docs,
    clippy::alloc_instead_of_core,
    clippy::exhaustive_enums,
    clippy::exhaustive_structs,
    clippy::impl_trait_in_params,
    clippy::missing_inline_in_public_items,
    clippy::std_instead_of_alloc,
    clippy::std_instead_of_core,
)]
#![feature(coroutine_trait)]

#[cfg(test)]
extern crate std;

#[cfg(test)]
mod tests;

#[cfg(test)]
#[path = "gen/tests/assert_impl.rs"]
mod assert_impl;
#[cfg(test)]
#[path = "gen/tests/track_size.rs"]
mod track_size;

#[doc(inline)]
pub use futures_async_stream_macro::for_await;
#[doc(inline)]
pub use futures_async_stream_macro::stream;
#[doc(inline)]
pub use futures_async_stream_macro::stream_block;
#[doc(inline)]
pub use futures_async_stream_macro::try_stream;
#[doc(inline)]
pub use futures_async_stream_macro::try_stream_block;

mod future {
    use core::{
        future::Future,
        ops::{Coroutine, CoroutineState},
        pin::Pin,
        ptr::NonNull,
        task::{Context, Poll},
    };

    use pin_project::pin_project;

    // Based on https://github.com/rust-lang/rust/blob/1.66.0/library/core/src/future/mod.rs.
    // TODO: can we avoid GenFuture? https://github.com/rust-lang/rust/commit/9f36f988ad873f5d34cd9c08e4903c597ffc9532

    /// This type is needed because:
    ///
    /// a) Coroutines cannot implement `for<'a, 'b> Coroutine<&'a mut Context<'b>>`, so we need to pass
    ///    a raw pointer (see <https://github.com/rust-lang/rust/issues/68923>).
    /// b) Raw pointers and `NonNull` aren't `Send` or `Sync`, so that would make every single future
    ///    non-Send/Sync as well, and we don't want that.
    ///
    /// It also simplifies the lowering of `.await`.
    #[doc(hidden)]
    #[derive(Debug, Clone, Copy)]
    pub struct ResumeTy(pub(crate) NonNull<Context<'static>>);

    // SAFETY: the caller of the `get_context` function that dereferences a
    // pointer must guarantee that no data races will occur.
    // Note: Since https://github.com/rust-lang/rust/pull/95985, `Context` is
    // `!Send` and `!Sync`.
    unsafe impl Send for ResumeTy {}
    // SAFETY: see `Send` impl
    unsafe impl Sync for ResumeTy {}

    // Needed to work around old nightly bug (fixed in nightly-2024-05-24 by https://github.com/rust-lang/rust/pull/125392)
    impl core::panic::UnwindSafe for ResumeTy {}
    impl core::panic::RefUnwindSafe for ResumeTy {}

    /// Wrap a coroutine in a future.
    ///
    /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
    /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
    #[doc(hidden)]
    #[inline]
    pub fn from_coroutine<G>(g: G) -> impl Future<Output = G::Return>
    where
        G: Coroutine<ResumeTy, Yield = ()>,
    {
        GenFuture(g)
    }

    #[pin_project]
    pub(crate) struct GenFuture<G>(#[pin] G);

    impl<G> Future for GenFuture<G>
    where
        G: Coroutine<ResumeTy, Yield = ()>,
    {
        type Output = G::Return;

        #[inline]
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            let this = self.project();
            // Resume the coroutine, turning the `&mut Context` into a `NonNull` raw pointer. The
            // `.await` lowering will safely cast that back to a `&mut Context`.
            match this.0.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
                CoroutineState::Yielded(()) => Poll::Pending,
                CoroutineState::Complete(x) => Poll::Ready(x),
            }
        }
    }

    #[doc(hidden)]
    #[inline]
    #[must_use]
    pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
        // SAFETY: the caller must guarantee that `cx.0` is a valid pointer
        // that fulfills all the requirements for a mutable reference.
        unsafe { &mut *cx.0.as_ptr().cast::<Context<'b>>() }
    }
}

mod stream {
    use core::{
        future::Future,
        ops::{Coroutine, CoroutineState},
        pin::Pin,
        ptr::NonNull,
        task::{Context, Poll},
    };

    use futures_core::stream::Stream;
    use pin_project::pin_project;

    use crate::future::ResumeTy;

    /// Wrap a coroutine in a stream.
    ///
    /// This function returns a `GenStream` underneath, but hides it in `impl Trait` to give
    /// better error messages (`impl Stream` rather than `GenStream<[closure.....]>`).
    #[doc(hidden)]
    #[inline]
    pub fn from_coroutine<G, T>(g: G) -> impl Stream<Item = T>
    where
        G: Coroutine<ResumeTy, Yield = Poll<T>, Return = ()>,
    {
        GenStream(g)
    }

    #[pin_project]
    pub(crate) struct GenStream<G>(#[pin] G);

    impl<G, T> Stream for GenStream<G>
    where
        G: Coroutine<ResumeTy, Yield = Poll<T>, Return = ()>,
    {
        type Item = T;

        #[inline]
        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            let this = self.project();
            match this.0.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
                CoroutineState::Yielded(x) => x.map(Some),
                CoroutineState::Complete(()) => Poll::Ready(None),
            }
        }
    }

    // This is equivalent to the `futures::stream::StreamExt::next` method.
    // But we want to make this crate dependency as small as possible, so we define our `next` function.
    #[doc(hidden)]
    #[inline]
    pub fn next<S>(stream: &mut S) -> impl Future<Output = Option<S::Item>> + '_
    where
        S: Stream + Unpin,
    {
        Next(stream)
    }

    pub(crate) struct Next<'a, S>(&'a mut S);

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

        #[inline]
        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            Pin::new(&mut self.0).poll_next(cx)
        }
    }
}

mod try_stream {
    use core::{
        ops::{Coroutine, CoroutineState},
        pin::Pin,
        ptr::NonNull,
        task::{Context, Poll},
    };

    use futures_core::stream::{FusedStream, Stream};
    use pin_project::pin_project;

    use crate::future::ResumeTy;

    /// Wrap a coroutine in a stream.
    ///
    /// This function returns a `GenStream` underneath, but hides it in `impl Trait` to give
    /// better error messages (`impl Stream` rather than `GenStream<[closure.....]>`).
    #[doc(hidden)]
    #[inline]
    pub fn from_coroutine<G, T, E>(g: G) -> impl FusedStream<Item = Result<T, E>>
    where
        G: Coroutine<ResumeTy, Yield = Poll<T>, Return = Result<(), E>>,
    {
        GenTryStream(Some(g))
    }

    #[pin_project]
    pub(crate) struct GenTryStream<G>(#[pin] Option<G>);

    impl<G, T, E> Stream for GenTryStream<G>
    where
        G: Coroutine<ResumeTy, Yield = Poll<T>, Return = Result<(), E>>,
    {
        type Item = Result<T, E>;

        #[inline]
        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            let mut this = self.project();
            if let Some(g) = this.0.as_mut().as_pin_mut() {
                let res = match g.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
                    CoroutineState::Yielded(x) => x.map(|x| Some(Ok(x))),
                    CoroutineState::Complete(Err(e)) => Poll::Ready(Some(Err(e))),
                    CoroutineState::Complete(Ok(())) => Poll::Ready(None),
                };
                if let Poll::Ready(Some(Err(_)) | None) = &res {
                    this.0.set(None);
                }
                res
            } else {
                Poll::Ready(None)
            }
        }
    }

    impl<G, T, E> FusedStream for GenTryStream<G>
    where
        G: Coroutine<ResumeTy, Yield = Poll<T>, Return = Result<(), E>>,
    {
        #[inline]
        fn is_terminated(&self) -> bool {
            self.0.is_none()
        }
    }
}

// Not public API.
#[doc(hidden)]
pub mod __private {
    #[doc(hidden)]
    pub use core::{
        marker::Send,
        option::Option::{None, Some},
        pin::Pin,
        result::Result::{self, Ok},
        task::Poll,
    };

    #[doc(hidden)]
    pub mod future {
        #[doc(hidden)]
        pub use core::future::Future;

        #[doc(hidden)]
        pub use crate::future::{ResumeTy, from_coroutine, get_context};
    }

    #[doc(hidden)]
    pub mod stream {
        #[doc(hidden)]
        pub use futures_core::stream::Stream;

        #[doc(hidden)]
        pub use crate::stream::{from_coroutine, next};
    }

    #[doc(hidden)]
    pub mod try_stream {
        #[doc(hidden)]
        pub use crate::try_stream::from_coroutine;
    }
}