Async-await status report #2

8 July 2019

I wanted to give an update on the status of the “async-await foundations” working group. This post aims to cover three things:

  • the “async await MVP” that we are currently targeting;
  • how that fits into the bigger picture;
  • and how you can help, if you’re so inclined;

Current target: async-await MVP

We are currently working on stabilizing what we call the async-await MVP – as in, “minimal viable product”. As the name suggests, the work we’re doing now is basically the minimum that is needed to “unlock” async-await. After this work is done, it will be easier to build async I/O based applications in Rust, though a number of rough edges remain.

The MVP consists of the following pieces:

The future trait

The first of these bullets, the future trait, was stabilized in the 1.36.0 release. This is important because the Future trait is the core building block for the whole Async I/O ecosystem. Having a stable future trait means that we can begin the process of consolidating the ecosystem around it.

Basic async-await syntax

Now that the future trait is stable, the next step is to stabilize the basic “async-await” syntax. We are presently shooting to stabilize this in 1.38. We’ve finished the largest work items, but there are still a number of things left to get done before that date – if you’re interested in helping out, see the “how you can help” section at the end of this post!

The current support we are aiming to stabilize permits async fn, but only outside of traits and trait implementations. This means that you can write free functions like this one:1

// When invoked, returns a future that (once awaited) will yield back a result:
async fn process(data: TcpStream) -> Result<(), Box<dyn Error>> {
    let mut buf = vec![0u8; 1024];
    // Await data from the stream:
    let len = buf).await?;

or inherent methods:

impl MyType {
    // Same as above, but defined as a method on `MyType`:
    async fn process(data: TcpStream) -> Result<(), Box<dyn Error>> { .. }

You can also write async blocks, which generate a future “in place” without defining a separate function. These are particularly useful to pass as arguments to helpers like runtime::spawn:

let data: TcpStream;
runtime::spawn(async move {
    let mut buf = vec![0u8; 1024];
    let len = buf).await?;

Eventually, we plan to permit async fn in other places, but there are some complications to be resolved first, as will be discussed shortly.

The async book

One of the goals of this stabilization is that, once async-await syntax becomes available, there should be really strong documentation to help people get started. To that end, we’re rejuvenating the “async Rust” book. This book covers the nuts and bolts of Async I/O in Rust, ranging from simple examples with async fn all the way down to the details of how the future trait works, writing your own executors, and so forth. Take a look!

(Eventually, I expect some of this material may make its way into more standard books like The Rust Programming Language, but in the meantime we’re evolving it separately.)

Future work: the bigger picture

The current stabilization push, as I mentioned above, is aimed at getting an MVP stabilized – just enough to enable people to run off and start to build things. So you’re probably wondering, what are some of the things that come next? Here is a (incomplete) list of possible future work:

  • A core set of async traits and combinators. Basically a 1.0 version of the futures-rs repository, offering key interfaces like AsyncRead.
  • Better stream support. The futures-rs repository contains a Stream trait, but there remains some “support work” to make it better supported. This may include some form of for-await syntax (although that is not a given).
  • Generators and async generators. The same core compiler transform that enables async await should enable us to support Python- or JS-like generators as a way to write iterators. Those same generators can then be made asynchronous to produce streams of data.
  • Async fn in traits and trait impls. Writing generic crates and interfaces that work with async fn is possible in the MVP, but not as clean or elegant as it could be. Supporting async fn in traits is an obvious extension to make that nicer, though we have to figure out all of the interactions with the rest of the trait system.
  • Async closures. We would like to support the obvious async || syntax that would generate a closure. This may require tinkering with the Fn trait hierarchy.

How you can get involved

There’s been a lot of great work on the async fn implementation since my first post – we’ve closed over 40 blocker issues! I want to give a special shout out to the folks who worked on those issues:2

  • davidtwco reworked the desugaring so that the drop order for parameters in an async fn and fn is analagous, and then heroically fixed a number of minor bugs that were filed as fallout from this change.
  • tmandry dramatically reduced the size of futures at runtime.
  • gilescope improved a number of error messages and helped to reduce errors.
  • matthewjasper reworked some details of the compiler transform to solve a large number of ICEs.
  • doctorn fixed an ICE when await was used in inappropriate places.
  • centril has been helping to enumerate tests and generally work on triage work.
  • cramertj implemented the await syntax, wrote a bunch of tests, and, of course, did all of the initial implementation work.
  • and hey, I extended the region inferencer to support multiple lifetime parameters. I guess I get some credit too. =)

If you’d like to help push async fn over the finish line, take a look at our list of blocking issues. Anything that is not assigned is fair game! Just find an issue you like that is not assigned and use @rustbot claim to claim it. You can find out more about how our working group works on the async-await working group page. In particular, that page includes a link to the calendar event for our weekly meeting, which takes place in the the #wg-async-foundations channel on the rust-lang Zulip – the next meeting is tomorrow (Tuesday)!. But feel free to drop in any time with questions.


  1. Sadly, it seems like [rouge] hasn’t been updated yet to highlight the async or await keywords. Or maybe I just don’t understand how to upgrade it. =) [rouge]: ↩︎

  2. I culled this list by browsing the closed issues and who they were assigned to. I’m sorry if I forgot someone or minimized your role! Let me know and I’ll edit the post. <3 ↩︎