Is This Magic!? Ferris Explores Rustc!

This is the companion guide to the “Is This Magic!? Ferris Explores rustc” talk presented at RustConf 2019 by QuietMisdreavus and J Haigh. We’ve published this on both of our sites! J’s version is over here.

This post is directed at Rust developers who would like to contribute to the Rust compiler, but don’t know where to start. This post is going to focus on the details of finding a good first issue, getting your development environment set up, working through an issue, and learning about the pull request workflow.

Intro to compiler and crates

What is the Rust compiler?

To some Rustaceans, the Rust compiler is little more than “the thing that yells at my code when I run cargo build”. But there’s a complex world inside the compiler, waiting for an intrepid explorer like yourself to peer through its depths.

If you’re here, that means you’re probably interested in the Rust compiler itself - how it works, how it’s structured, and how to get involved. The Rust compiler is broken down into many crates, which are roughly arranged into the various phases of compilation, where the compiler takes your code and applies successive steps on it to turn it into your desired output. Some of the crates in the compiler are:

(*The actual entry point is a small file in src/rustc, but it effectively immediately calls a function in rustc_driver.)

Side note: This is why it can sometimes seem like you fix all your errors only to find that there were more lurking underneath! It means you made it to the next phase.

We’re not going into the details of the complete process of compilation here - there’s a lovely set of documentation about the compiler internals over at, which gets into way more detail than we can fit into a half-hour talk!

Intro to teams

Instead, let’s talk about the people who work on the Rust project. The Rust project is broken down into various teams, each with their own focus area that they work on. There’s a team for the internals of the compiler, a team for language design, a team for tooling that works with the compiler, a team for the standard library, a team for documentation and learning material, a team for the infrastructure that keeps the compiler shipping, a team for community outreach, and loads more.

In addition to teams there are also working groups that are either created by another team with a tighter focus, or were created by the Project as a whole to focus on improving Rust in a particular niche. They usually have a specific goal in mind - “land and stabilize async/await”, “specify a complete grammar for Rust syntax”, “set up a system to translate the Rust website and materials”, and so on.

Finding a Good First Issue

So say you want to jump into it. Probably the first thing you want to do is find some small thing you can work on, to get your feet wet with the codebase, the build and test process, and the contribution workflow. Sure, you can read through the code and build and run the tests without having something to fix or enhance, but modifying the code makes the process that much more real.

So how do you get a “good first issue”? There are generally two ways. I like to refer to them as the “quiet method” and the “social method”, each with variations. The “quiet method” usually involves finding ways to fix things that people already want, with instructions already written. The “social method” involves making yourself known to existing contributors, so that they can walk you through a contribution.

quiet method #1

The compiler repo puts labels on their issues. Two specific labels that are good to look out for are E-easy and E-mentor. These respectively mean that the necessary change doesn’t require much domain-specific knowledge, or that a contributor has provided instructions (or an offer for communication) in the issue. Finding both of these on an issue at the same time is a lucky occurrence - these are precious commodities that get claimed quickly!

quiet method #2

Without a doubt, the easiest way to get your name in the contributor history of the compiler is to fix a typo in the documentation. Typo fix PRs are easy to make, easy to review, and easy to merge. The docs team loves these!

social method #1

I want to be where the people are…

All the teams generally have spaces where they communicate in semi-real time. Most teams have a dedicated Discord channel, and the compiler and language teams even have Zulip instances where discussion can break out into much more fine-grain threads. Lurking in these spaces can be invaluable in hitting the ground running when you start writing code. You’ll have the opportunity to get comfortable with the lingo, see contribution opportunities before they get issues filed, and meet and greet the teams in their own space.

social method #2

In the right spaces, simply asking for contribution opportunities can be enough! Sometimes people have pet issues that they wouldn’t fix themselves, but would happily pass off to new contributors.

This isn’t specifically Rust compiler advice. This can work for a lot of open source projects. After you get involved people may send issues to you that they think you might enjoy writing code for. Maintainers may even have issues they’d like fixed that they haven’t posted yet and talking with maintainers can allow you to help discover issues that hadn’t been considered yet!

Failing that, sending a tweet to @rustlang on Twitter can suffice, as they’re usually happy to signal-boost people looking to help out in the community! (Please don’t do it all at once during this talk, though, we don’t want to overburden them. >_>;;)

social method #3

Similarly, following the right people on Twitter can make an opportunity appear. Contributors, like Niko Matsakis, post about new developments in their domains and new opportunities for contributions.

Setting up your development environment

Once you’ve decided on an issue to work on (or even ahead of time, just for fun), it’s time to clone the git repo for the compiler. Before we can build it, there are a few things to set up first. The build process has a handful of dependencies, to coordinate the process, build LLVM, and sync up some required items. Check out the README in the repo for the complete list, but here’s a general list of “build and test dependencies” for the compiler:

Typically, you can launch directly into the build process. However, there are many knobs you can tweak to make the experience nicer. The build process has its own configuration file, called config.toml. This file is missing by default, with defaults documented in the provided file config.toml.example. If you copy that file to make config.toml, you can start to tweak the settings in the build. Here are a handful of good ones to keep in mind:

Building and testing code

Fun fact: You don’t use cargo to build the compiler! Instead you use the provided script, which coordinates the build process of the compiler. This operates somewhat similarly to cargo. In the script also includes special flags to specific building and testing the compiler. The script is actually a shim over the bootstrap binary, also located in the repo. The script is also responsible for sourcing which version of Rust to build the compiler with and building the bootstrap binary itself, among other things. Here are some example commands for compiling the repo:

$ build
$ build --stage 1
$ build --stage 1 src/libcore
$ build --stage 1 src/tools/rustdoc
$ check

The first command builds the compiler, the standard library, and (if configured in config.toml) a handful of default tools. The second one does something similar, but stops short of providing a complete build. The full details are beyond the scope of this talk, but here’s a brief rundown.

The compiler is built in stages. This means the compiler builds itself multiple times to ensure it works properly. For example, this is necessary for dynamic linking and proc-macros. If you plan to mess with codegen or name mangling it’s safest to build stage 2 (the default built for build). When in doubt, ask around.

The third and fourth commands show that you can filter specific crates rather than building “everything”. These can be helpful if you’re only working in one part and you want to make sure it builds, or if you’re working on something like rustdoc that isn’t part of the default build process.

Finally, there is an analogue to cargo check available to the compiler. This provides a quick verification that your code would build without going through the entire build process. This is helpful in the same way that cargo check is - if you’re in the middle of writing a fix but want to double-check that some code you wrote parses correctly or passes borrow-check. It’s not necessary to spend the extra time putting a binary together.

The Rust compiler has an extensive test suite built in, built up over time as features and bug fixes are added. Here are some sample commands that run tests in the compiler.

$ test
$ test src/librustc
$ test --stage 1 src/test/run-pass

The first one runs everything in the test suite - unit tests, doctests, integration tests, the works. The second runs the unit tests in the librustc crate. The last one builds stage 1 and runs the integration tests in the run-pass directory. There are many kinds of integration tests in src/test, each with their own requirements. The Rustc Guide has more information!

Another important command to keep in mind is

$ test src/tools/tidy

This is a basic tidy tool we have to ensure lines aren’t too long, there’s no trailing whitespace, etc. Run this command before opening your PR.

You can even set up your freshly-built compiler as a toolchain in Rustup, if you want/need to build some code that doesn’t live in a test:

$ rustup toolchain link local /path/to/rust/build/$HOST/stage1
... in some rust project ...
$ cargo +local build

Lots and lots of details are in the rustc guide!

Working through an issue

You’ve found an issue, your dev environment is all set up, and you’re ready to start writing code! Awesome!

At this point you’ve probably read through the issue. Make sure you know the problem you’re trying to solve and ask questions if there’s anything unclear. Asking questions either on the issue or in the team channel are the most likely spots to get answers. If you’re working with someone or have been talking with the team you may ask questions somewhere else or work directly with a mentor. Just ask the team what their preference is.

Let’s walk through an example issue and see about exploring the code and adding our contribution to it!

panicbit filed an issue that they would to see re-exported crates and crate items inlined in rustdoc.

This would look like:

pub extern crate foo;

This code would display foo as if it was a module in the docs.

extern crate foo;
pub use foo::*;

This code would inline the reexported items as if foo were a module.

Rustdoc’s code lives in the compiler repo, in src/librustdoc. We can find the existing code for how crates and exported items are currently handled and then change it to how we want it to be!

From a high level, we know rustdoc walks through the generated abstract syntax tree and then cleans up the AST to remove duplicates. This is where the implementation for how the docs should be generated lives. Let’s look at the section of code for crates and crate items.


impl Clean<Item> for doctree::ExternCrate {
    fn clean(&self, cx: &DocContext) -> Item {
        Item {
            name: None,
            attrs: self.attrs.clean(cx),
            source: self.whence.clean(cx),
            def_id: DefId { krate: self.cnum, index: CRATE_DEF_INDEX },
            visibility: self.vis.clean(cx),
            stability: None,
            deprecation: None,
            inner: ExternCrateItem(, self.path.clone())

Our implementation for showing the exported crate and items needs to go inside the clean trait function for an Item. We know that we may return multiple Items since there are many Items that can be exported from a crate. We want to add Item names to a please_inline list if we have the doc inline attribute, #[doc(inline)], above the Item. If the Item is in please_inline we then want to iterate through our items and if the Item is in please_inline, we want to try to inline that Item. Luckily, we also saw there is a try_inline function!


pub fn try_inline(
    cx: &DocContext<'_>,
    res: Res,
    name: ast::Name,
    attrs: Option<Attrs<'_>>,
    visited: &mut FxHashSet<DefId>
) -> Option<Vec<clean::Item>> {
    // ...

There are certain situations where an item can’t be inlined (it’s actually local, it’s not something we generate a page for, etc), so we need to take that into account when solving this problem.

This logic ends up looking like this:

// If there's a `pub` on it, start looking through the attributes
let please_inline = self.vis.node.is_pub() && self.attrs.iter().any(|a| {
    // If there's a `#[doc(...)]` attribute, look through the `(...)` == "doc" && match a.meta_item_list() {
        // If there's an `inline` in the `(...)`, that's what we're
        // looking for
        Some(l) => attr::list_contains_name(&l, "inline"),
        None => false,

if please_inline {
    // We need an empty `HashSet` for `try_inline`
    let mut visited = FxHashSet::default();

    // We also need a `Def`, which the compiler uses to identify
    // items the compiler uses. This is a `DefId` for the root of the
    // crate we're importing
    let def = Def::Mod(DefId {
        krate: self.cnum,
        index: CRATE_DEF_INDEX,

    // `try_inline` returns an `Option`. If it returns the crate's
    // items, use that instead of the `ExternCrateItem`.
    if let Some(items) = inline::try_inline(cx, def,, &mut visited) {
        return items;

The function signature also changes a little bit here. Instead of returning a single Item, the crate, we might return the crate and the items exported from the crate. We did a few changes to allow us to use and return a Vec<Item>.

Back where this function is called, we replaced our previous use of map with flat_map.


impl Clean<Item> for doctree::Module {
    let attrs = self.attrs.clean(cx);

    let mut items: Vec<Item> = vec![];
    items.extend(self.extern_crates.iter().flat_map(|x| x.clean(cx)));
    // ...

We changed the impl Clean to be for a Vec<Item> instead of a single Item.


impl Clean<Vec<Item>> for doctree::ExternCrate {
    fn clean(&self, cx: &DocContext) -> Vec<Item> {
        // ...

We also used the vec! macro to create a vector from our Item array at the end of our implementation of Clean on ExternCrate.

But we aren’t done yet! We also need to add some new crates to test the code we just wrote and write some docs to make sure that users can verify this works and know how to use it in their Rust code! We added a sample case of this to the test suite.


#![crate_name = "inner"]
pub struct SomeStruct;


// @has pub_extern_crate/index.html
// @!has - '//code' 'pub extern crate inner'
// @has - '//a/@href' 'inner/index.html'
// @has pub_extern_crate/inner/index.html
// @has pub_extern_crate/inner/struct.SomeStruct.html
pub extern crate inner;

The rustdoc tests are a little special compared to the rest of the compiler so we’re not going to get into the details of how these tests work. If you’re interested you can read more about rustdoc tests here.

After you’ve looked over your code and tested it and updated relevant docs it’s time to submit a PR! You can also do a WIP PR if you’d like feedback as you go. Just want sure to talk with the team involved with that part of the project.

PR workflow

All changes to the compiler and tools are coordinated through GitHub Pull Requests. With a fork on GitHub, you can push up a branch and create a PR, which will be assigned to and reviewed by a team member. When you create your PR, it’s a good idea to tag the issue it fixes, if any, and to add a description of the changes you’ve made. If there is any additional information that could be helpful to someone reviewing your PR you should add it here too.

When you post a PR, the @rust-highfive bot will assign someone at random, or you can add r? @username to your PR description if you want to assign someone specific. This can be useful if you’ve been working with someone in particular. They may reassign the PR if necessary, by using the same message.

Finally, when the reviewer looks over your code, they might request changes or accept/reject your PR. Make any changes, hash discussion out in chat on the PR or with the team, and bam! You got your contribution!

We also use a bot to coordinate running all the tests on PRs before they merge. Because of this, when a reviewer is ready to merge your PR, they don’t merge it directly. Instead they’ll summon our CI bot, @bors, to add the PR to the testing queue, with @bors r+. This makes sure that your PR passes tests on all our supported platforms before landing in master.

Not every PR will land, but it’s still a contribution of your time and knowledge and walking away from every issue teach you something new. It’s awesome to get code in, but you’re always going to walk away with a contribution.

More information can be found in!