Rust in 2016

Aug. 14, 2015 · Nicholas Matsakis and Aaron Turon

This week marks three months since Rust 1.0 was released. As we're starting to hit our post-1.0 stride, we'd like to talk about what 1.0 meant in hindsight, and where we see Rust going in the next year.

What 1.0 was about

Rust 1.0 focused on stability, community, and clarity.

Altogether, Rust is exciting because it is empowering: you can hack without fear. And you can do so in contexts you might not have before, dropping down from languages like Ruby or Python, making your first foray into systems programming.

That's Rust 1.0; but what comes next?

Where we go from here

After much discussion within the core team, early production users, and the broader community, we've identified a number of improvements we'd like to make over the course of the next year or so, falling into three categories:

  • Doubling down on infrastructure;
  • Zeroing in on gaps in key features;
  • Branching out into new places to use Rust.

Let's look at some of the biggest plans in each of these categories.

Doubling down: infrastructure investments


Our basic stability promise for Rust is that upgrades between versions are "hassle-free". To deliver on this promise, we need to detect compiler bugs that cause code to stop working. Naturally, the compiler has its own large test suite, but that is only a small fraction of the code that's out there "in the wild". Crater is a tool that aims to close that gap by testing the compiler against all the packages found in, giving us a much better idea whether any code has stopped compiling on the latest nightly.

Crater has quickly become an indispensable tool. We regularly compare the nightly release against the latest stable build, and we use crater to check in-progress branches and estimate the impact of a change.

Interestingly, we have often found that when code stops compiling, it's not because of a bug in the compiler. Rather, it's because we fixed a bug, and that code happened to be relying on the older behavior. Even in those cases, using crater helps us improve the experience, by suggestion that we should phase fixes in slowly with warnings.

Over the next year or so, we plan to improve crater in numerous ways:

  • Extend the coverage to other platforms beyond Linux, and run test suites on covered libraries as well.
  • Make it easier to use: leave an @crater: test comment to try out a PR.
  • Produce a version of the tool that library authors can use to see effects of their changes on downstream code.
  • Include code from other sources beyond

Incremental compilation

Rust has always had a "crate-wide" compilation model. This means that the Rust compiler reads in all of the source files in your crate at once. These are type-checked and then given to LLVM for optimization. This approach is great for doing deep optimization, because it gives LLVM full access to the entire set of code, allowing for more better inlining, more precise analysis, and so forth. However, it can mean that turnaround is slow: even if you only edit one function, we will recompile everything. When projects get large, this can be a burden.

The incremental compilation project aims to change this by having the Rust compiler save intermediate by-products and re-use them. This way, when you're debugging a problem, or tweaking a code path, you only have to recompile those things that you have changed, which should make the "edit-compile-test" cycle much faster.

Part of this project is restructuring the compiler to introduce a new intermediate representation, which we call MIR. MIR is a simpler, lower-level form of Rust code that boils down the more complex features, making the rest of the compiler simpler. This is a crucial enabler for language changes like non-lexical lifetimes (discussed in the next section).

IDE integration

Top-notch IDE support can help to make Rust even more productive. Up until now, pioneering projects like Racer, Visual Rust, and Rust DT have been working largely without compiler support. We plan to extend the compiler to permit deeper integration with IDEs and other tools; the plan is to focus initially on two IDEs, and then grow from there.

Zeroing in: closing gaps in our key features


The idea of zero-cost abstractions breaks down into two separate goals, as identified by Stroustrup:

  • What you don't use, you don't pay for.
  • What you do use, you couldn't hand code any better.

Rust 1.0 has essentially achieved the first goal, both in terms of language features and the standard library. But it doesn't quite manage to achieve the second goal. Take the following trait, for example:

pub trait Extend<A> {
    fn extend<T>(&mut self, iterable: T) where T: IntoIterator<Item=A>;

The Extend trait provides a nice abstraction for inserting data from any kind of iterator into a collection. But with traits today, that also means that each collection can provide only one implementation that works for all iterator types, which requires actually calling .next() repeatedly. In some cases, you could hand code it better, e.g. by just calling memcpy.

To close this gap, we've proposed specialization, allowing you to provide multiple, overlapping trait implementations as long as one is clearly more specific than the other. Aside from giving Rust a more complete toolkit for zero-cost abstraction, specialization also improves its story for code reuse. See the RFC for more details.

Borrow checker improvements

The borrow checker is, in a way, the beating heart of Rust; it's the part of the compiler that lets us achieve memory safety without garbage collection, by catching use-after-free bugs and the like. But occasionally, the borrower checker also "catches" non-bugs, like the following pattern:

match map.find(&key) {
    Some(...) => { ... }
    None => {
        map.insert(key, new_value);

Code like the above snippet is perfectly fine, but the borrow checker struggles with it today because the map variable is borrowed for the entire body of the match, preventing it from being mutated by insert. We plan to address this shortcoming soon by refactoring the borrow checker to view code in terms of finer-grained ("non-lexical") regions -- a step made possible by the move to the MIR mentioned above.


There are some really neat things you can do in Rust today -- if you're willing to use the Nightly channel. For example, the regex crate comes with macros that, at compile time, turn regular expressions directly into machine code to match them. Or take the rust-postgres-macros crate, which checks strings for SQL syntax validity at compile time. Crates like these make use of a highly-unstable compiler plugin system that currently exposes far too many compiler internals. We plan to propose a new plugin design that is more robust and provides built-in support for hygienic macro expansion as well.

Branching out: taking Rust to new places


While cross-compiling with Rust is possible today, it involves a lot of manual configuration. We're shooting for push-button cross-compiles. The idea is that compiling Rust code for another target should be easy:

  1. Download a precompiled version of libstd for the target in question, if you don't already have it.
  2. Execute cargo build --target=foo.
  3. There is no step 3.

Cargo install

Cargo and is a really great tool for distributing libraries, but it lacks any means to install executables. RFC 1200 describes a simple addition to cargo, the cargo install command. Much like the conventional make install, cargo install will place an executable in your path so that you can run it. This can serve as a simple distribution channel, and is particularly useful for people writing tools that target Rust developers (who are likely to be familiar with running cargo).

Tracing hooks

One of the most promising ways of using Rust is by "embedding" Rust code into systems written in higher-level languages like Ruby or Python. This embedding is usually done by giving the Rust code a C API, and works reasonably well when the target sports a "C friendly" memory management scheme like reference counting or conservative GC.

Integrating with an environment that uses a more advanced GC can be quite challenging. Perhaps the most prominent examples are JavaScript engines like V8 (used by node.js) and SpiderMonkey (used by Firefox and Servo). Integrating with those engines requires very careful coding to ensure that all objects are properly rooted; small mistakes can easily lead to crashes. These are precisely the kind of memory management problems that Rust is intended to eliminate.

To bring Rust to environments with advanced GCs, we plan to extend the compiler with the ability to generate "trace hooks". These hooks can be used by a GC to sweep the stack and identify roots, making it possible to write code that integrates with advanced VMs smoothly and easily. Naturally, the design will respect Rust's "pay for what you use" policy, so that code which does not integrate with a GC is unaffected.

Epilogue: RustCamp 2015, and Rust's community in 2016

We recently held the first-ever Rust conference, RustCamp 2015, which sold out with 160 attendees. It was amazing to see so much of the Rust community in person, and to see the vibe of our online spaces translate into a friendly and approachable in-person event. The day opened with a keynote from Nicholas Matsakis and Aaron Turon laying out the core team's view of where we are and where we're headed. The slides are available online (along with several other talks), and the above serves as the missing soundtrack. Update: now you can see the talks as well!

There was a definite theme of the day: Rust's greatest potential is to unlock a new generation of systems programmers. And that's not just because of the language; it's just as much because of a community culture that says "Don't know the difference between the stack and the heap? Don't worry, Rust is a great way to learn about it, and I'd love to show you how."

The technical work we outlined above is important for our vision in 2016, but so is the work of those on our moderation and community teams, and all of those who tirelessly -- enthusiastically -- welcome people coming from all kinds of backgrounds into the Rust community. So our greatest wish for the next year of Rust is that, as its community grows, it continues to retain the welcoming spirit that it has today.