Merge pull request #409 from nikomatsakis/async-await-hits-beta

Add "async-await hits beta" blog post

Author: nikomatsakis

posts/2019-09-30-Async-await-hits-beta.md

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+---
+layout: post
+title: "Async-await hits beta!"
+author: 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?).
+
+[not great]: https://github.com/rust-lang/rust/issues/64130
+[async fn in traits]: https://github.com/dtolnay/async-trait
+
+(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:** 
+
+- the async runtime [tokio], for example, recently announced an [alpha
+  release][] based on async-await;
+- the [recently announced][] [async-std][] library was built from the
+  start on the new async-await syntax;
+- using [wasm-bindgen-futures], you can even bridge Rust Futures with
+  [JavaScript promises];
+- the [hyper library][hyper] has [migrated][hyper#1805] to adopt standard Rust futures;
+- the 0.3.0 version of the [futures-rs library][futures] will support
+  async-await and will be released by the time async-await hits stable
+  (you can use the [0.3.0-alpha][] releases now);
+- finally, async-await support is starting to become available in higher-level
+  [web frameworks][wf] as well.
+  
+[futures]: https://crates.io/crates/futures-preview
+[0.3.0-alpha]: https://rust-lang-nursery.github.io/futures-rs/blog/2018/07/19/futures-0.3.0-alpha.1.html
+[wasm-bindgen-futures]: https://docs.rs/crate/wasm-bindgen-futures/0.2.16
+[tokio]: https://tokio.rs/
+[actix]: https://actix.rs/
+[alpha release]: https://tokio.rs/blog/2019-08-alphas/
+[adding support]: https://github.com/actix/actix-web/issues/955#issuecomment-523266936
+[async-std]: https://async.rs/
+[recently announced]: https://async.rs/blog/announcing-async-std/
+[wf]: https://www.arewewebyet.org/topics/frameworks/
+[JavaScript promises]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises
+[hyper]: https://hyper.rs
+[hyper#1805]: https://github.com/hyperium/hyper/issues/1805
+
+### 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 [areweasyncyet.rs] and [arewewebyet.org] 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.
+
+[areweasyncyet.rs]: https://areweasyncyet.rs/
+[arewewebyet.org]: https://www.arewewebyet.org/
+[rustasync github org]: https://github.com/rustasync/
+[rust-lang org]: https://github.com/rust-lang/
+[Async Foundations WG]: https://rust-lang.github.io/compiler-team/working-groups/async-await/
+[Async Ecosystem WG]: https://github.com/rustasync/team
+[async book]: https://github.com/rust-lang/async-book
+
+**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][gov25] with the [governance working
+group][gov-wg] to look into that for the future.)
+
+[gov25]: https://github.com/rust-lang/wg-governance/issues/25
+[gov-wg]: https://github.com/rust-lang/wg-governance/
+
+### 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`:
+
+```rust
+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:
+
+```rust
+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][video] that [withoutboats] gave at [Rust LATAM 2019]
+on the topic.
+
+[the executor section]: https://rust-lang.github.io/async-book/02_execution/04_executor.html
+[video]: https://www.youtube.com/watch?v=skos4B5x7qE
+[Rust LATAM 2019]: https://rustlatam.org/
+[withoutboats]: https://github.com/withoutboats
+
+### Summary
+
+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][bc]). Moreover, there are a 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!)
+
+[bc]: http://aturon.github.io/tech/2018/04/24/async-borrowing/