- Asking Questions
- Cloning and Building
- Contributor Procedures
- Other Resources
This documentation is not intended to be comprehensive; it is meant to be a quick guide for the most useful things. For more information, see this chapter on how to build and run the compiler.
The compiler team (or
t-compiler) usually hangs out in Zulip in this
"stream"; it will be easiest to get questions answered there.
Please ask questions! A lot of people report feeling that they are "wasting
expert time", but nobody on
t-compiler feels this way. Contributors are
important to us.
Also, if you feel comfortable, prefer public topics, as this means others can see the questions and answers, and perhaps even integrate them back into this guide :)
t-compiler members are experts on all parts of
rustc; it's a pretty
large project. To find out who has expertise on different parts of the
compiler, consult this "experts map".
It's not perfectly complete, though, so please also feel free to ask questions even if you can't figure out who to ping.
We do ask that you be mindful to include as much useful information as you can in your question, but we recognize this can be hard if you are unfamiliar with contributing to Rust.
Just pinging someone without providing any context can be a bit annoying and
just create noise, so we ask that you be mindful of the fact that the
t-compiler folks get a lot of pings in a day.
The main repository is
rust-lang/rust. This contains the compiler,
the standard library (including
and a bunch of tools (e.g.
rustdoc, the bootstrapping infrastructure, etc).
There are also a bunch of submodules for things like LLVM,
etc. You don't need to clone these immediately, but the build tool will
automatically clone and sync them (more on this later).
See this chapter for detailed software requirements.
Most notably, you will need Python 2 or 3 to run
There are no hard hardware requirements, but building the compiler is computationally expensive, so a beefier machine will help, and I wouldn't recommend trying to build on a Raspberry Pi :P
- Recommended >=30GB of free disk space; otherwise, you will have to keep clearing incremental caches. More space is better, the compiler is a bit of a hog; it's a problem we are aware of.
- Recommended >=8GB RAM.
- Recommended >=2 cores; having more cores really helps.
- You will need an internet connection to build; the bootstrapping process involves updating git submodules and downloading a beta compiler. It doesn't need to be super fast, but that can help.
Building the compiler takes more than half an hour on my moderately powerful laptop. The first time you build the compiler, LLVM will also be built unless you use CI-built LLVM (see below).
cargo, the build system will use as many cores as possible. Sometimes
this can cause you to run low on memory. You can use
-j to adjust the number
concurrent jobs. If a full build takes more than ~45 minutes to an hour,
you are probably spending most of the time swapping memory in and out;
On a slow machine, the build times for rustc are very painful. Consider using
./x.py check instead of a full build and letting the automated tests run
when you push to GitHub.
If you don't have too much free disk space, you may want to turn off incremental compilation (see below). This will make compilation take longer (especially after a rebase), but will save a ton of space from the incremental caches.
You can just do a normal git clone:
git clone https://github.com/rust-lang/rust.git
You don't need to clone the submodules at this time. But if you want to, you can do the following:
# first time git submodule update --init --recursive # subsequent times (to pull new commits) git submodule update
The compiler has a configuration file which contains a ton of settings. We will provide some recommendations here that should work for most, but check out this chapter for more info.
In the top level of the repo:
$ x.py setup
This will walk you through an interactive setup for x.py that looks like this:
$ x.py setup Welcome to the Rust project! What do you want to do with x.py? a) Contribute to the standard library b) Contribute to the compiler c) Contribute to the compiler, and also modify LLVM or codegen d) Install Rust from source Please choose one (a/b/c/d): a `x.py` will now use the configuration at /home/joshua/rustc2/src/bootstrap/defaults/config.toml.library To get started, try one of the following commands: - `x.py check` - `x.py build` - `x.py test library/std` - `x.py doc` For more suggestions, see https://rustc-dev-guide.rust-lang.org/building/suggested.html
Note that by default,
x.py setup will use CI-built LLVM if available for your
platform so that you don't need to build LLVM in addition to building the
compiler. In some circumstances, such as when updating the version of LLVM used
rustc, you may want to temporarily disable this feature. See the "Updating
LLVM" section for more.
If you want to download LLVM from CI without running
x.py setup, you can set
download-ci-llvm option to
true in your
[llvm] download-ci-llvm = true
rustc is a bootstrapping compiler, which means that it is written in Rust
and thus needs to be compiled by itself. So where do you
get the original compiler from? We use the current beta compiler
to build a new compiler. Then, we use that compiler to build itself. Thus,
rustc has a 2-stage build. You can read more about bootstrapping
here, but you don't need to know much more to contribute.
We have a special tool
./x.py that drives this process. It is used for
building the compiler, the standard libraries, and
rustdoc. It is also used
for driving CI and building the final release artifacts.
Unfortunately, a proper 2-stage build takes a long time depending on your hardware, but it is the only correct way to build everything (e.g. it's what the CI and release processes use). However, in most cases, you can get by without a full 2-stage build. In the following section, we give instructions for how to do "the correct thing", but then we also give various tips to speed things up.
Here is a summary of the different commands for reference, but you probably should still read the rest of the section:
|Command||When to use it|
|Quick check to see if things compile; rust-analyzer can run this automatically for you|
|Build only the standard library, without building the compiler|
|Build just the 1st stage of the compiler, along with the standard library; this is faster than building stage 2 and usually good enough|
|Build the 1st stage of the compiler and skips rebuilding the standard library; this is useful after you've done an ordinary stage1 build to skip compilation time, but it can cause weird problems. (Just do a regular build to resolve.)|
|Run the test suite using the stage1 compiler|
|Run the test suite using the stage1 compiler and update expected test output.|
|Do a full 2-stage build. You almost never want to do this.|
|Do a full 2-stage build and run all tests. You almost never want to do this.|
To do a full 2-stage build of the whole compiler, you should run this (after
config.toml as mentioned above):
./x.py build --stage 2 compiler/rustc
In the process, this will also necessarily build the standard libraries, and it
rustdoc (which doesn't take too long).
To build and test everything:
For most contributions, you only need to build stage 1, which saves a lot of time:
# Build the compiler (stage 1) ./x.py build library/std # Subsequent builds ./x.py build --keep-stage 1 library/std
This will take a while, especially the first time. Be wary of accidentally
touching or formatting the compiler, as
./x.py will try to recompile it.
--keep-stage 1 will assume that the stage 0 standard library
does not need to be rebuilt, which is usually true, which will save some time.
However, if you are changing certain parts of the compiler, this may lead to
weird errors. Feel free to ask on zulip if you are running into issues.
This runs a ton of tests and takes a long time to complete. If you are
rustc, you can usually get by with only the UI tests. These
test are mostly for the frontend of the compiler, so if you are working on LLVM
or codegen, this shortcut will not test your changes. You can read more about the
different test suites in this chapter.
# First build ./x.py test src/test/ui # Subsequent builds ./x.py test src/test/ui --keep-stage 1
If your changes impact test output, you can use
--bless to automatically
.stderr files of the affected tests:
./x.py test src/test/ui --keep-stage 1 --bless
While working on the compiler, it can be helpful to see if the code just
compiles (similar to
cargo check) without actually building it. You can do
This command is really fast (relative to the other commands). It usually
completes in a couple of minutes on my laptop. A common workflow when working
on the compiler is to make changes and repeatedly check with
Then, run the tests as shown above when you think things should work.
Finally, the CI ensures that the codebase is using consistent style. To format the code:
# Actually format ./x.py fmt # Just check formatting, exit with error ./x.py fmt --check
Note: we don't use stable
rustfmt; we use a pinned version with a special
config, so this may result in different style from normal
rustfmt if you have
format-on-save turned on. It's a good habit to run
./x.py fmt before every
commit, as this reduces conflicts later. The pinned version is built under
build/<target>/stage0/bin/rustfmt, so if you want, you can use it for a
single file or for format-on-save in your editor, which can be faster than
As before, technically the proper way to build one of these libraries is to use
the stage-2 compiler, which of course requires a 2-stage build, described above
In practice, though, you don't need to build the compiler unless you are planning to use a recently added nightly feature. Instead, you can just build stage 0, which uses the current beta compiler.
./x.py build --stage 0
./x.py test --stage 0 library/std
(The same works for
rustc internals (and, of course, the standard library), so you
will have to build the compiler and
std once before you can build
As before, you can use
./x.py build to do this. The first time you build,
the stage-1 compiler will also be built.
# First build ./x.py build # Subsequent builds ./x.py build --keep-stage 1
As with the compiler, you can do a fast check build:
Rustdoc has two types of tests: content tests and UI tests.
# Content tests ./x.py test src/test/rustdoc # UI tests ./x.py test src/test/rustdoc-ui # Both at once ./x.py test src/test/rustdoc src/test/rustdoc-ui
There are a bunch of other projects that you can contribute to outside of the
rust-lang/rust repo, including
chalk, and many others.
These repos might have their own contributing guidelines and procedures. Many
of them are owned by working groups (e.g.
chalk is largely owned by
WG-traits). For more info, see the documentation in those repos' READMEs.
There are a bunch of other ways you can contribute, especially if you don't
feel comfortable jumping straight into the large
The following tasks are doable without much background knowledge but are incredibly helpful:
- Cleanup crew: find minimal reproductions of ICEs, bisect regressions, etc. This is a way of helping that saves a ton of time for others to fix an error later.
- Writing documentation: if you are feeling a bit more intrepid, you could try to read a part of the code and write doc comments for it. This will help you to learn some part of the compiler while also producing a useful artifact!
- Working groups: there are a bunch of working groups on a wide variety of rust-related things.
There are some official procedures to know about. This is a tour of the highlights, but there are a lot more details, which we will link to below.
When you open a PR on the
rust-lang/rust repo, a bot called
automatically assign a reviewer to the PR. The reviewer is the person that will
approve the PR to be tested and merged. If you want a specific reviewer (e.g. a
team member you've been working with), you can specifically request them by
r? @user (e.g.
r? @eddyb) in either the original post or a followup
comment (you can see this comment for example).
Please note that the reviewers are humans, who for the most part work on
in their free time. This means that they can take some time to respond and review
your PR. It also means that reviewers can miss some PRs that are assigned to them.
To try to move PRs forward, the Triage WG regularly goes through all PRs that are waiting for review and haven't been discussed for at least 2 weeks. If you don't get a review within 2 weeks, feel free to ask the Triage WG on Zulip (#t-release/triage). They have knowledge of when to ping, who might be on vacation, etc.
When the PR is ready to be merged, the reviewer will issue a command to
@bors, the CI bot. Usually, this is
@bors r+ or
@bors r=user to approve
a PR (there are few other commands, but they are less relevant here).
You can see this comment for example. This puts the PR in bors's queue
to be tested and merged. Be patient; this can take a while and the queue can
sometimes be long. PRs are never merged by hand.
For most PRs, no special procedures are needed. You can just open a PR, and it will be reviewed, approved, and merged. This includes most bug fixes, refactorings, and other user-invisible changes. The next few sections talk about exceptions to this rule.
Also, note that it is perfectly acceptable to open WIP PRs or GitHub Draft PRs. Some people prefer to do this so they can get feedback along the way or share their code with a collaborator. Others do this so they can utilize the CI to build and test their PR (e.g. if you are developing on a laptop).
Rust has strong backwards-compatibility guarantees. Thus, new features can't just be implemented directly in stable Rust. Instead, we have 3 release channels: stable, beta, and nightly.
- Stable: this is the latest stable release for general usage.
- Beta: this is the next release (will be stable within 6 weeks).
- Nightly: follows the
masterbranch of the repo. This is the only channel where unstable, incomplete, or experimental features are usable with feature gates.
In order to implement a new feature, usually you will need to go through the RFC process to propose a design, have discussions, etc. In some cases, small features can be added with only an FCP (see below). If in doubt, ask the compiler, language, or libs team (whichever is most relevant).
After a feature is approved to be added, a tracking issue is created on the
rust-lang/rust repo, which tracks the progress towards the implementation of
the feature, any bugs reported, and eventually stabilization.
The feature then needs to be implemented behind a feature gate, which prevents it from being accidentally used.
Finally, somebody may propose stabilizing the feature in an upcoming version of Rust. This requires a Final Comment Period (see below) to get the approval of the relevant teams.
After that, the feature gate can be removed and the feature turned on for all users.
For more details on this process, see this chapter on implementing new features.
As mentioned above, Rust has strong backwards-compatibility guarantees. To this end, we are reluctant to make breaking changes. However, sometimes they are needed to correct compiler bugs (e.g. code that compiled but should not) or make progress on some features.
Depending on the scale of the breakage, there are a few different actions that can be taken. If the reviewer believes the breakage is very minimal (i.e. very unlikely to be actually encountered by users), they may just merge the change. More often, they will request a Final Comment Period (FCP), which calls for rough consensus among the members of a relevant team. The team members can discuss the issue and either accept, reject, or request changes on the PR.
If the scale of breakage is large, a deprecation warning may be needed. This is a warning that the compiler will display to users whose code will break in the future. After some time, an FCP can be used to move forward with the actual breakage.
If the scale of breakage is unknown, a team member or contributor may request a crater run. This is a bot that will compile all crates.io crates and many public github repos with the compiler with your changes. A report will then be generated with crates that ceased to compile with or began to compile with your changes. Crater runs can take a few days to complete.
The compiler team has a special process for large changes, whether or not they cause breakage. This process is called a Major Change Proposal (MCP). MCP is a relatively lightweight mechanism for getting feedback on large changes to the compiler (as opposed to a full RFC or a design meeting with the team).
Example of things that might require MCPs include major refactorings, changes to important types, or important changes to how the compiler does something, or smaller user-facing changes.
Compiler performance is important. We have put a lot of effort over the last few years into gradually improving it.
If you suspect that your change may cause a performance regression (or improvement), you can request a "perf run" (your reviewer may also request one before approving). This is yet another bot that will compile a collection of benchmarks on a compiler with your changes. The numbers are reported here, and you can see a comparison of your changes against the latest master.