The compiler is built using a tool called
x.py. You will need to
have Python installed to run it. But before we get to that, if you're going to
be hacking on
rustc, you'll want to tweak the configuration of the compiler.
The default configuration is oriented towards running the compiler as a user,
not a developer.
For instructions on how to install Python and other prerequisites, see the next page.
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).
The very first step to work on
rustc is to clone the repository:
git clone https://github.com/rust-lang/rust.git cd rust
There are also submodules for things like LLVM,
miri, etc. The
build tool will automatically clone and sync these for you. 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
To start, run
./x.py setup. This will do some initialization and create a
config.toml for you with reasonable defaults. These defaults are specified
indirectly via the
profile setting, which points to one of the TOML files in
Alternatively, you can write
config.toml by hand. See
for all the available settings and explanations of them. The following settings
are of particular interest, and
config.toml.example has full explanations.
You may want to change some of the following settings (and possibly others, such as
[llvm] # Whether to use Rust CI built LLVM instead of locally building it. download-ci-llvm = true # Download a pre-built LLVM? assertions = true # LLVM assertions on? ccache = "/path/to/ccache" # Use ccache when building LLVM? [rust] debug-logging = true # Leave debug! and trace! calls in rustc? incremental = true # Build rustc with incremental compilation?
If you set
download-ci-llvm = true, in some circumstances, such as when
updating the version of LLVM used by
rustc, you may want to temporarily
disable this feature. See the "Updating LLVM" section for more.
If you have already built
rustc and you change settings related to LLVM, then you may have to
rm -rf build for subsequent configuration changes to take effect. Note that
./x.py clean will not cause a rebuild of LLVM.
x.py is the script used to orchestrate the tooling in the
It is the script that can build docs, run tests, and compile
It is the now preferred way to build
rustc and it replaces the old makefiles
from before. Below are the different ways to utilize
x.py in order to
effectively deal with the repo for various common tasks.
This chapter focuses on the basics to be productive, but
if you want to learn more about
x.py, read its README.md
To read more about the bootstrap process and why
x.py is necessary,
read this chapter.
There is a binary that wraps
src/tools/x. All it does is
x.py, but it can be installed system-wide and run from any subdirectory
of a checkout. It also looks up the appropriate version of
python to use.
You can install it with
cargo install --path src/tools/x.
To build a compiler, run
./x.py build. This will build up to the stage1 compiler,
rustdoc, producing a usable compiler toolchain from the source
code you have checked out.
Note that building will require a relatively large amount of storage space. You may want to have upwards of 10 or 15 gigabytes available to build the compiler.
There are many flags you can pass to the build command of
x.py that can be
beneficial to cutting down compile times or fitting other things you might
need to change. They are:
Options: -v, --verbose use verbose output (-vv for very verbose) -i, --incremental use incremental compilation --config FILE TOML configuration file for build --build BUILD build target of the stage0 compiler --host HOST host targets to build --target TARGET target targets to build --on-fail CMD command to run on failure --stage N stage to build --keep-stage N stage to keep without recompiling --src DIR path to the root of the Rust checkout -j, --jobs JOBS number of jobs to run in parallel -h, --help print this help message
For hacking, often building the stage 1 compiler is enough, which saves a lot of time. But for final testing and release, the stage 2 compiler is used.
./x.py check is really fast to build the Rust compiler.
It is, in particular, very useful when you're doing some kind of
"type-based refactoring", like renaming a method, or changing the
signature of some function.
Once you've created a
config.toml, you are now ready to run
x.py. There are a lot of options here, but let's start with what is
probably the best "go to" command for building a local rust:
./x.py build -i library/std
This may look like it only builds
std, but that is not the case.
What this command does is the following:
stdusing the stage0 compiler (using incremental)
rustcusing the stage0 compiler (using incremental)
- This produces the stage1 compiler
stdusing the stage1 compiler (cannot use incremental)
This final product (stage1 compiler + libs built using that compiler)
is what you need to build other Rust programs (unless you use
The command includes the
-i switch which enables incremental compilation.
This will be used to speed up the first two steps of the process:
in particular, if you make a small change, we ought to be able to use your old
results to make producing the stage1 compiler faster.
Unfortunately, incremental cannot be used to speed up making the
stage1 libraries. This is because incremental only works when you run
the same compiler twice in a row. In this case, we are building a
new stage1 compiler every time. Therefore, the old incremental
results may not apply. As a result, you will probably find that
building the stage1
std is a bottleneck for you -- but fear not,
there is a (hacky) workaround. See the section on "recommended
Note that this whole command just gives you a subset of the full
build. The full
rustc build (what you get with
./x.py build --stage 2 compiler/rustc) has quite a few more steps:
rustcwith the stage1 compiler.
- The resulting compiler here is called the "stage2" compiler.
stdwith stage2 compiler.
librustdocand a bunch of other things with the stage2 compiler.
You almost never need to do this.
If you are working on the standard library, you probably don't need to build the compiler unless you are planning to use a recently added nightly feature. Instead, you can just build using the bootstrap compiler.
./x.py build --stage 0 library/std
Sometimes you might just want to test if the part you’re working on can
compile. Using these commands you can test that it compiles before doing
a bigger build to make sure it works with the compiler. As shown before
you can also pass flags at the end such as
Once you have successfully built
rustc, you will have created a bunch
of files in your
build directory. In order to actually run the
rustc, we recommend creating rustup toolchains. The first
one will run the stage1 compiler (which we built above). The second
will execute the stage2 compiler (which we did not build, but which
you will likely need to build at some point; for example, if you want
to run the entire test suite).
rustup toolchain link stage1 build/<host-triple>/stage1 rustup toolchain link stage2 build/<host-triple>/stage2
<host-triple> would typically be one of the following:
Now you can run the
rustc you built with. If you run with
should see a version number ending in
-dev, indicating a build from
your local environment:
$ rustc +stage1 -vV rustc 1.48.0-dev binary: rustc commit-hash: unknown commit-date: unknown host: x86_64-unknown-linux-gnu release: 1.48.0-dev LLVM version: 11.0
The rustup toolchain points to the specified toolchain compiled in your
so the rustup toolchain will be updated whenever
x.py build or
x.py test are run for
Note: the toolchain we've built does not include
cargo. In this case,
fall back to using
cargo from the installed
(in that order). If you need to use unstable
cargo flags, be sure to run
rustup install nightly if you haven't already. See the
rustup documentation on custom toolchains.
Here are a few other useful
x.py commands. We'll cover some of them in detail
in other sections:
- Building things:
./x.py build– builds everything using the stage 1 compiler, not just up to
./x.py build --stage 2– builds everything with the stage 2 compiler including
rustdoc(which doesn't take too long)
- Running tests (see the section on running tests for
./x.py test library/std– runs the
./x.py test src/test/ui– runs the
./x.py test src/test/ui/const-generics- runs all the tests in the
const-generics/subdirectory of the
./x.py test src/test/ui/const-generics/const-types.rs- runs the single test
Sometimes you need to start fresh, but this is normally not the case.
If you need to run this then
rustbuild is most likely not acting right and
you should file a bug as to what is going wrong. If you do need to clean
everything up then you only need to run one command!
rm -rf build works too, but then you have to rebuild LLVM, which can take
a long time even on fast computers.