- Get the source code
- What is
- Create a
- Creating a rustup toolchain
- Building targets for cross-compilation
The compiler is built using a tool called
x.py. You will need to
have Python installed to run it.
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
Due to the size of the repository, cloning on a slower internet connection can take a long time.
To sidestep this, you can use the
--depth N option with the
git clone command.
git to perform a "shallow clone", cloning the repository but truncating it to
--depth 1 tells
git to clone the repository but truncate the history to the latest
commit that is on the
master branch, which is usually fine for browsing the source code or
building the compiler.
git clone --depth 1 https://github.com/rust-lang/rust.git
NOTE: A shallow clone limits which
gitcommands can be run. If you intend to work on and contribute to the compiler, it is generally recommended to fully clone the repository as shown above.
git blamerequire access to the commit history, so they don't work if the repository was cloned with
x.py is the build tool for the
rust repository. It can build docs, run tests, and compile the
compiler and standard library.
This chapter focuses on the basics to be productive, but
if you want to learn more about
x.py, read this chapter.
x rather than
x.py is recommended as:
./xis the most likely to work on every system (on Unix it runs the shell script that does python version detection, on Windows it will probably run the powershell script - certainly less likely to break than
./x.pywhich often just opens the file in an editor).1
(You can find the platform related scripts around the
Notice that this is not absolute, for instance, using Nushell in VSCode on Win10,
./x still open the
x.py in editor rather invoke the program :)
In the rest of this guide, we use
x rather than
x.py directly. The following
could be replaced by:
x.py command can be run directly on most Unix systems in the following format:
./x <subcommand> [flags]
This is how the documentation and examples assume you are running
Some alternative ways are:
# On a Unix shell if you don't have the necessary `python3` command
./x <subcommand> [flags]
# In Windows Powershell (if powershell is configured to run scripts)
./x <subcommand> [flags]
./x.ps1 <subcommand> [flags]
# On the Windows Command Prompt (if .py files are configured to run Python)
x.py <subcommand> [flags]
# You can also run Python yourself, e.g.:
python x.py <subcommand> [flags]
On Windows, the Powershell commands may give you an error that looks like this:
PS C:\Users\vboxuser\rust> ./x
./x : File C:\Users\vboxuser\rust\x.ps1 cannot be loaded because running scripts is disabled on this system. For more
information, see about_Execution_Policies at https:/go.microsoft.com/fwlink/?LinkID=135170.
At line:1 char:1
+ CategoryInfo : SecurityError: (:) , PSSecurityException
+ FullyQualifiedErrorId : UnauthorizedAccess
You can avoid this error by allowing powershell to run local scripts:
Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
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 clarify that this is another global installed binary util, which is
similar to the one declared in section What is
it works as an independent process to execute the
x.py rather than calling the
shell to run the platform related scripts.
To start, run
./x setup and select the
compiler defaults. This will do some initialization
and create a
config.toml for you with reasonable defaults. If you use a different default (which
you'll likely want to do if you want to contribute to an area of rust other than the compiler, such
as rustdoc), make sure to read information about that default (located in
as the build process may be different for other defaults.
Alternatively, you can write
config.toml by hand. See
config.example.toml for all the available
settings and explanations of them. See
src/bootstrap/defaults for common settings to change.
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 clean will not cause a rebuild of LLVM.
Here are the basic invocations of the
x commands most commonly used when
rustdoc, and other tools.
|When to use it
|Quick check to see if most things compile; rust-analyzer can run this automatically for you
|Runs all tests
|Formats all code
As written, these commands are reasonable starting points. However, there are
additional options and arguments for each of them that are worth learning for
serious development work. In particular,
./x build and
provide many ways to compile or test a subset of the code, which can save a lot
Also, note that
x supports all kinds of path suffixes for
src/tools directories. So, you can simply run
x test tidy instead of
x test src/tools/tidy. Or,
x build std instead of
x build library/std.
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.
Once you've created a
config.toml, you are now ready to run
x. There are a lot of options here, but let's start with what is
probably the best "go to" command for building a local compiler:
./x build library
This may look like it only builds the standard library, but that is not the case. What this command does is the following:
stdusing the stage0 compiler
rustcusing the stage0 compiler
- This produces the stage1 compiler
stdusing the stage1 compiler
This final product (stage1 compiler + libs built using that compiler)
is what you need to build other Rust programs (unless you use
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 avoiding rebuilds for std.
Sometimes you don't need a full build. When doing some kind of
"type-based refactoring", like renaming a method, or changing the
signature of some function, you can use
./x check instead for a much faster build.
Note that this whole command just gives you a subset of the full
build. The full
rustc build (what you get with
./x 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 build --stage 0 library
If you choose the
library profile when running
x setup, you can omit
--stage 0 (it's the
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 stage0 build/host/stage0-sysroot # beta compiler + stage0 std
rustup toolchain link stage1 build/host/stage1
rustup toolchain link stage2 build/host/stage2
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
LLVM version: 11.0
The rustup toolchain points to the specified toolchain compiled in your
so the rustup toolchain will be updated whenever
x build or
x 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.
Note: rust-analyzer and IntelliJ Rust plugin use a component called
rust-analyzer-proc-macro-srv to work with proc macros. If you intend to use a
custom toolchain for a project (e.g. via
rustup override set stage1) you may
want to build this component:
./x build proc-macro-srv-cli
To produce a compiler that can cross-compile for other targets,
pass any number of
target flags to
For example, if your host platform is
and your cross-compilation target is
wasm32-wasi, you can build with:
./x build --target x86_64-unknown-linux-gnu,wasm32-wasi
Note that if you want the resulting compiler to be able to build crates that
involve proc macros or build scripts, you must be sure to explicitly build target support for the
host platform (in this case,
If you want to always build for other targets without needing to pass flags to
you can configure this in the
[build] section of your
config.toml like so:
target = ["x86_64-unknown-linux-gnu", "wasm32-wasi"]
Note that building for some targets requires having external dependencies installed
(e.g. building musl targets requires a local copy of musl).
Any target-specific configuration (e.g. the path to a local copy of musl)
will need to be provided by your
config.example.toml for information on target-specific configuration keys.
For examples of the complete configuration necessary to build a target, please visit the rustc book, select any target under the "Platform Support" heading on the left, and see the section related to building a compiler for that target. For targets without a corresponding page in the rustc book, it may be useful to inspect the Dockerfiles that the Rust infrastructure itself uses to set up and configure cross-compilation.
If you have followed the directions from the prior section on creating a rustup toolchain, then once you have built your compiler you will be able to use it to cross-compile like so:
cargo +stage1 build --target wasm32-wasi
Here are a few other useful
x commands. We'll cover some of them in detail
in other sections:
- Building things:
./x build– builds everything using the stage 1 compiler, not just up to
./x build --stage 2– builds everything with the stage 2 compiler including
- Running tests (see the section on running tests for
./x test library/std– runs the unit tests and integration tests from
./x test tests/ui– runs the
./x test tests/ui/const-generics- runs all the tests in the
const-generics/subdirectory of the
./x test tests/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.