2019 is the year WebAssembly with Rust goes from "usable" to "stable, batteries-available, and production-ready."

To realize this goal, the Rust and WebAssembly domain working group will:

  • Cultivate a library ecosystem by collaborating on a modular toolkit

  • Bring multithreading to Rust-generated Wasm

  • Integrate best-in-class debugging support into our toolchain

  • Polish our toolchain and developer workflow, culminating in a 1.0 version of wasm-pack

  • Invest in monitoring, testing, and profiling infrastructure to keep our tools and libraries snappy, stable and production-ready.


This proposed roadmap draws upon

Detailed Explanation

Collaborating on a Modular Toolkit

The idea of building [high-level libraries] in a modular way that will allow others in the community to put the components together in a different way is very exciting to me. This hopefully will make the ecosystem as a whole much stronger.

In particular I’d love to see a modular effort towards implementing a virtual DOM library with JSX like syntax. There have been several efforts on this front but all have seemed relatively monolithic and “batteries included”. I hope this will change in 2019.

— Ryan Levick in Rust WebAssembly 2019

Don't create branded silos. Branding might perhaps be useful to achieve fame. But if we truly want Rust's Wasm story to succeed we should think of ways to collaborate instead of carving out territory.

— Yoshua Wuyts in Wasm 2019

In 2018, we created foundational libraries like js-sys and web-sys. In 2019, we should build modular, high-level libraries on top of them, and collect the libraries under an umbrella toolkit crate for a holistic experience. This toolkit and its libraries will make available all the batteries you want when targeting Wasm.

Building a greenfield Web application? Use the whole toolkit to hit the ground running. Carefully crafting a tiny Wasm module and integrating it back into an existing JavaScript project? Grab that one targeted library you need out from the toolkit and use it by itself.

  • Modular: Take or leave any individual component. Prefer interfaces over implementations.

  • Cultivate collaboration: We've already seen an ecosystem sprouting up in the Rust and WebAssembly domain, and lots of great experiments, but we haven't seen a lot of collaboration between projects. By deliberately creating a space for collaboration, we can reduce effort duplication, multiply impact, and help the ecosystem stay healthy.

Multithreading for Wasm

We must bring Rust’s fearless concurrency to the Web!

— Nick Fitzgerald in Rust and WebAssembly in 2019

Be the absolute cutting edge when it comes to WebAssembly, we should be thinking about being the first to satisfy [threads and atomics].

— richardanaya in My Rust 2019 Dream

Our toolchain already has experimental support for multithreading in Wasm. Browsers are currently shipping SharedArrayBuffer and atomics (the primitives of multithreading for Wasm) behind feature flags, and they expect to start shipping them enabled by default in 2019.

One of WebAssembly's selling points is the ability to effectively utilize available hardware. Multithreading extends that story from a single core to many. While multithreading will be literally possible for both JavaScript and any compiled-to-Wasm language, Rust's unique ownership system makes it economically realistic.

There are some technical snags (see the link above for details) that mean we can't get Rust's standard library's std::thread::* working on Wasm. But it is still crucial that we have shared implementations of core multithreading building blocks like thread pools and locks across the ecosystem. In 2019, we should transform our experimental multithreading support into a production-ready foundation for multithreading on Wasm, get popular crates like rayon working on the Web, and cash in on Rust's fearless concurrency.


Before [debugging] is working properly (including variable inspection, which doesn't work with wasm at all right now), everything else is just toying around.

— anlumo in a comment on r/rust

Having [source maps] would be excellent for debugging.

— Yoshua Wuyts in Wasm 2019

Debugging is tricky because much of the story is out of this working group's hands, and depends on both the WebAssembly standardization bodies and the folks implementing browser developer tools instead. However, there are some concrete steps we can take to improve debugging:

  1. Get println!, dbg!, and friends working out of the box with Wasm. To achieve this, we will build support for the WebAssembly reference sysroot and standard system calls for Wasm that are in the standardization pipeline.

  2. Create the ability to compile our Rust-generated Wasm to JavaScript with source maps when debugging. Source maps are a limited debug info format for JavaScript that enable stepping through source locations in a debugger, instead of stepping through compiler-generated JavaScript code.

  3. Add debugging-focused tracing and instrumentation features to our toolchain. For example, it is currently difficult to debug a JavaScript array buffer view of Wasm memory getting detached because Wasm memory was resized. We can make debugging easier by optionally instrumenting mem.grow instructions with logging.

In addition to that, we should work with the WebAssembly standardization bodies and browser developer tools makers, and actively participate in the WebAssembly debugging subcharter to create some movement in the debugging space. By keeping up the environmental and social pressure and lending a hand where we can, we will eventually have rich, source-level debugging for Wasm.

Toolchain and Workflow Polish

Setting up a Wasm project requires quite some boilerplate. It'd be nice if we could find ways to reduce this.

— Yoshua Wuyts in Wasm 2019

There are a few things that we intended to include in wasm-pack in 2018 that didn’t quite make the cut. [...] We should finish these tasks and polish wasm-pack into a 1.0 tool.

— Nick Fitzgerald in Rust and WebAssembly in 2019

In 2019, our toolchain and workflow should be feature complete and polished. wasm-pack, being the entry point to our toolchain, will bear the brunt of this work, but much of it will also be in tools that are invoked by wasm-pack rather than work in wasm-pack itself.

Given that this work is largely about plugging missing holes and improving user experience, it is a bit of a laundry list. But that is also good sign: it means that wasm-pack is actually fairly close to being feature complete.

After we've finished all these tasks, we should publish a 1.0 release of wasm-pack.

Monitoring, Profiling, and Testing Infrastructure

The main objection I've experienced when proposing rust/wasm is compile times, but the end-to-end latency actually looks pretty competitive so far [...] Having a couple of benchmarks in CI and a graph online somewhere would go a long way towards keeping it that way.

— @jamii in an RFC comment

If I want to run the tests of a library using both libtest and wasm-bindgen-test I need to write:

# #![allow(unused_variables)]
#fn main() {
#[cfg_attr(not(target_arch = "wasm32"), test)]
#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test)]
fn my_test() { ... }

instead of just

# #![allow(unused_variables)]
#fn main() {
fn my_test() { ... }

— @gnzlbg in an RFC comment

We should build infrastructure0 to keep an eye on

  • code size of popular and foundational wasm crates (such as those crates in our modular toolkit), and

  • our wasm-bindgen and wasm-pack build times.

By continually tracking this data over time, and just at once at a particular, we will hold ourselves accountable to delivering on our promises of a lightweight toolkit and "stable, production-ready" toolchain.

0 Or perhaps integrate our monitoring into if it makes sense and the maintainers are willing.

That is the infrastructure story at the macro-level, but we also need to support the needs of crates within the ecosystem at the micro-level. That means continuing to invest in unit testing and profiling Rust-generated Wasm binaries. Concretely, we should

  • add benchmarking support to wasm-bindgen-test, and

  • make wasm-bindgen-test future-compatible with the eRFC for custom test frameworks, paving the way forward for making regular #[test] and #[bench] Just Work™ with Wasm instead of requiring the use of #[wasm_bindgen_test] instead.

Rationale, Drawbacks, and Alternatives

We choose to focus our efforts in 2019 where:

  1. We — the Rust and WebAssembly working group — can build and ship features. Areas where we aren't potentially blocked by external factors, such as still-in-progress standards.

  2. We can leverage advantages that are unique to Rust in the WebAssembly domain.

Things We can Build and Ship

We don't want our fate in anyone's hands but our own.

The toolkit and toolchain polish work don't involve any external entities that could slow our progress to a halt. For debugging, where the larger story involves significant consensus with external groups and standards work, we explicitly choose to focus on what we can do ourselves to improve our own debugging story. We do not set ourselves up to block on anything produced by the WebAssembly community group's debugging subcharter, and we won't wait on browser vendors to implement new Wasm debugging support in developer tools.

Of the roadmap items, the multithreading story has the most risk: our success in this domain relies on browsers enabling Wasm's multithreading primitives by default. However, this seems like a relatively safe bet, since the multithreading primitives have moved past their most experimental phase, Chrome is already shipping them enabled by default, and all other major browsers have implementations that just aren't enabled by default yet.

Leveraging Unique Advantages

We want to focus our efforts where we get the biggest effort to impact efficiency, and establish ourselves as leaders in WebAssembly in ways that no one else even has a route towards catching up.

The multithreading story is perhaps the biggest example of unique advantage: multithreading is infamously bug prone (to say the least!) and Rust's ownership system eliminates data races at compile time.

By building a modular toolkit of libraries, we bolster our ability to target the full spectrum from tiny module surgically inserted into an existing JavaScript application, to building a complete Web application in Rust. Any language that relies on a garbage collector, fat runtime, or is overly opinionated about FFI and interaction with the outside world can't reach the tiny module end of that spectrum.

The toolchain polish and debugging work have less clearly unique advantages. But both are table stakes for a good development experience, and the par for the course for these things in the Wasm domain is currently so low that we can and should stand out from the crowd.

Considered Alternative Roadmap Items

Here are a few alternative items that were considered for the roadmap, perhaps because they were called out in #RustWasm2019 posts, but ultimately were not included.

Pushing anyref Integration into the Rust Language

We've already been well positioned to take advantage of host bindings and GC reference types once they ship in Wasm via wasm-bindgen. We could take it even further and imagine a future where the Rust language was able to pass around opaque references to objects in alternative memory spaces (some of which might be GC'd) in a first class way: structs that are split across memory spaces, fat pointers into multiple memory spaces, etc.

However, it isn't clear that pushing this all the way into the language will bring that much utility over the existing "anyref at the edges" implementation that wasm-bindgen already has. Additionally, cashing in on this work could easily be blocked in a couple ways: anyref isn't shipping in any mainstream wasm engine yet, and getting this language-level integration through the larger Rust RFC process with all of its stakeholders would happen at a glacial pace (if it even happened!)

A Focus Only on Pure-Rust Web Applications

We prefer to answer "yes and" to pure-Rust Web applications via the modular toolkit that can service the full spectrum of tiny module to whole Web app, than to focus only on the whole Web app end of the spectrum. Our hope with the toolkit is that a rising tide will lift all boats, regardless where your project lands on that spectrum.

Additionally, full Web apps are not a unique advantage for Rust. JavaScript has been doing it for a while, and as far as Wasm goes, there are better-funded "competitors" in the space that will be able to provide a more compelling monolithic Web app development experience more quickly (via integration with tooling, existing ecosystems, or throwing money and developers at the problem). Microsoft and Blazor, Google and Go, bringing existing native applications to the Web with Emscripten, etc. We should compete where we are best positioned to do so, and monolithic Web applications is not that.

All that said, if you want to build a whole Web application with Rust-generated Wasm and don't want to write any JavaScript at all, you should be able to do so. In fact, you already can with #[wasm_bindgen(start)] and the no-modules target. We will never remove this ability, and the new toolkit will only make developing a whole Web app easier.

Non-JavaScript and Non-Web Embeddings

While the whole of every non-Web and non-JavaScript WebAssembly embeddings looks very exciting, each embedding is a unique environment, and there is not yet a standard set of capabilities available. We don't want to block on waiting for a full set of standard capabilities to emerge, nor do we want to choose one particular embedding environment.

We do intend to support the reference sysroot work, and any follow up work that comes after it, but we will take advantage of these things on a opportunistic basis rather than making it a roadmap item.

We encourage anyone interested in non-JavaScript and non-Web embeddings to collaborate with the WebAssembly community group to push this story forward by defining standard Wasm capabilities!

Unresolved Questions

To be determined.