Async-await hits beta!

Sept. 30, 2019 · Niko Matsakis

Big news! As of this writing, syntactic support for async-await is available in the Rust beta channel! It will be available in the 1.39 release, which is expected to be released on November 7th, 2019. Once async-await hits stable, that will mark the culmination of a multi-year effort to enable efficient and ergonomic asynchronous I/O in Rust. It will not, however, mark the end of the road: there is still more work to do, both in terms of polish (some of the error messages we get today are, um, not great) and in terms of feature set (async fn in traits, anyone?).

(If you're not familiar with what async-await is, don't despair! There's a primer and other details later on in this post!)

Async-await support in the ecosystem growing rapidly

But async-await has never been the entire story. To make good use of async-await, you also need strong libraries and a vibrant ecosystem. Fortunately, you've got a lot of good choices, and they keep getting better:

Restructuring Async I/O in the Rust org

Now that async-await is approaching stable, we are taking the opportunity to make some changes to our Rust team structure. The current structure includes two working groups: the Async Foundations WG, focused on building up core language support, and the Async Ecosystem WG, focused on supporting the ecosystem develop.

In light of all the activity going on in the ecosystem, however, there it not as much need for the Async Ecosystem WG, and as such we've decided to spin it down. We'll be deprecating the rustasync github org. The and websites will move to the main rust-lang org, but the fate of the other projects will be decided by the people who built them. A few will likely be deprecated, and the remainder will be moving out to be maintained independently.

The Async Foundations WG, meanwhile, will continue, but with a shift in focus. Now that async-await is en route to stabilization, the focus will be on polish, such as improving diagnostics, fixing smaller bugs, and improving documentation such as the async book. Once progress is made on that, we'll be considering what features to implement next.

(An aside: this is the first time that we've ever opted to spin down a working group, and we realized that we don't have a formal policy for that. We've created an issue with the governance working group to look into that for the future.)

Async await: a quick primer

So, what is async await? Async-await is a way to write functions that can "pause", return control to the runtime, and then pick up from where they left off. Typically those pauses are to wait for I/O, but there can be any number of uses.

You may be familiar with the async-await from other languages, such as JavaScript or C#. Rust's version of the feature is similar, but with a few key differences.

To use async-await, you start by writing async fn instead of fn:

async fn first_function() -> u32 { .. }

Unlike a regular function, calling an async fn doesn't do anything to start -- instead, it returns a Future. This is a suspended computation that is waiting to be executed. To actually execute the future, you have to use the .await operator:

async fn another_function() {
    // Create the future:
    let future = first_function();
    // Await the future, which will execute it (and suspend
    // this function if we encounter a need to wait for I/O): 
    let result: u32 = future.await;

This example shows the first difference between Rust and other languages: we write future.await instead of await future. This syntax integrates better with Rust's ? operator for propagating errors (which, after all, are very common in I/O). One can simply write future.await? to await the result of a future and propagate errors. It also has the advantage of making method chaining painless.

Zero-cost futures

The other difference between Rust futures and futures in other languages is that they are based on a "poll" model, which makes them zero cost. In other languages, invoking an async function immediately creates a future and schedules it for execution: awaiting the future isn't really necessary for it to execute. But this implies some overhead for each future that is created.

In contrast, in Rust, calling an async function does not do any scheduling in and of itself, which means that we can compose a complex nest of futures without incurring a per-future cost. As an end-user, though, the main thing you'll notice is that futures feel "lazy": they don't do anything until you await them.

If you'd like a closer look at how futures work under the hood, take a look at the executor section of the async book, or watch the excellent talk that withoutboats gave at Rust LATAM 2019 on the topic.


In summary, if you've an interest in using Async I/O in Rust, this is a very exciting time! With async-await syntax hitting stable in November, it's going to be easier than ever to write futures (in particular, if you tried using the combinator-based futures in the past, you'll find async-await integrates much better with Rust's borrowing system). Moreover, there are now a number of great runtimes and other libraries available in the ecosystem to work with. So get out there and build stuff!

(Oh, yeah, and please file bugs when you hit confusing or surprising problems, so we can improve the user experience!)