How to build and run the compiler

The compiler is built using a tool called x.py. You will need to have Python installed to run it.

Get the source code

The main repository is rust-lang/rust. This contains the compiler, the standard library (including core, alloc, test, proc_macro, etc), 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

Shallow clone the repository

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. This instructs git to perform a "shallow clone", cloning the repository but truncating it to the last N commits.

Passing --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
cd rust

NOTE: A shallow clone limits which git commands 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.

For example, git bisect and git blame require access to the commit history, so they don't work if the repository was cloned with --depth 1.

What is x.py?

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.

Also, using x rather than x.py is recommended as:

./x is 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.py which often just opens the file in an editor).1

(You can find the platform related scripts around the x.py, like x.ps1)

Notice that this is not absolute, for instance, using Nushell in VSCode on Win10, typing x or ./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 command:

./x check

could be replaced by:

./x.py check

Running x.py

The 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 x.py. 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
+ ./x
+ ~~~
    + CategoryInfo          : SecurityError: (:) [], PSSecurityException
    + FullyQualifiedErrorId : UnauthorizedAccess

You can avoid this error by allowing powershell to run local scripts:

Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser

Running x.py slightly more conveniently

There is a binary that wraps x.py called x in src/tools/x. All it does is run 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 x.py, but it works as an independent process to execute the x.py rather than calling the shell to run the platform related scripts.

Create a config.toml

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 src/bootstrap/defaults) 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 execute rm -rf build for subsequent configuration changes to take effect. Note that ./x clean will not cause a rebuild of LLVM.

Common x commands

Here are the basic invocations of the x commands most commonly used when working on rustc, std, rustdoc, and other tools.

CommandWhen to use it
./x checkQuick check to see if most things compile; rust-analyzer can run this automatically for you
./x buildBuilds rustc, std, and rustdoc
./x testRuns all tests
./x fmtFormats 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 ./x test provide many ways to compile or test a subset of the code, which can save a lot of time.

Also, note that x supports all kinds of path suffixes for compiler, library, and 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.

See the chapters on testing and rustdoc for more details.

Building the compiler

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:

  • Build std using the stage0 compiler
  • Build rustc using the stage0 compiler
    • This produces the stage1 compiler
  • Build std using 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 #![no_std] or #![no_core]).

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 rustc build. The full rustc build (what you get with ./x build --stage 2 compiler/rustc) has quite a few more steps:

  • Build rustc with the stage1 compiler.
    • The resulting compiler here is called the "stage2" compiler.
  • Build std with stage2 compiler.
  • Build librustdoc and a bunch of other things with the stage2 compiler.

You almost never need to do this.

Build specific components

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 default).

Creating a rustup toolchain

Once you have successfully built rustc, you will have created a bunch of files in your build directory. In order to actually run the resulting 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 -vV, you 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 build directory, so the rustup toolchain will be updated whenever x build or x test are run for that toolchain/stage.

Note: the toolchain we've built does not include cargo. In this case, rustup will fall back to using cargo from the installed nightly, beta, or stable toolchain (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

Building targets for cross-compilation

To produce a compiler that can cross-compile for other targets, pass any number of target flags to x build. For example, if your host platform is x86_64-unknown-linux-gnu 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, x86_64-unknown-linux-gnu).

If you want to always build for other targets without needing to pass flags to x build, you can configure this in the [build] section of your config.toml like so:

[build]
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.toml. Please see 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

Other x commands

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 std
    • ./x build --stage 2 – builds everything with the stage 2 compiler including rustdoc
  • Running tests (see the section on running tests for more details):
    • ./x test library/std – runs the unit tests and integration tests from std
    • ./x test tests/ui – runs the ui test suite
    • ./x test tests/ui/const-generics - runs all the tests in the const-generics/ subdirectory of the ui test suite
    • ./x test tests/ui/const-generics/const-types.rs - runs the single test const-types.rs from the ui test suite

Cleaning out build directories

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!

./x clean

rm -rf build works too, but then you have to rebuild LLVM, which can take a long time even on fast computers.

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