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.

For instructions on how to install Python and other prerequisites, see the next page.

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

There are also submodules for things like LLVM, clippy, 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

Create a config.toml

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 src/bootstrap/defaults.

Alternatively, you can write config.toml by hand. See config.toml.example 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.ccache):

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

What is x.py?

x.py is the script used to orchestrate the tooling in the rustc repository. It is the script that can build docs, run tests, and compile rustc. 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 here. To read more about the bootstrap process and why x.py is necessary, read this chapter.

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.

Building the Compiler

To build a compiler, run ./x.py build. This will build up to the stage1 compiler, including 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 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 "recommended workflows" below.

Note that this whole command just gives you a subset of the full rustc build. The full rustc build (what you get with ./x.py 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.py build --stage 0 library

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 stage1 build/<host-triple>/stage1
rustup toolchain link stage2 build/<host-triple>/stage2

The <host-triple> would typically be one of the following:

  • Linux: x86_64-unknown-linux-gnu
  • Mac: x86_64-apple-darwin or aarch64-apple-darwin
  • Windows: x86_64-pc-windows-msvc

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.py build or x.py 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.py 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.py 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.py build --target x86_64-unknown-linux-gnu --target 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.py 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.toml.example 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.py commands

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 std
    • ./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 more details):
    • ./x.py test library/std – runs the unit tests and integration tests from std
    • ./x.py test src/test/ui – runs the ui test suite
    • ./x.py test src/test/ui/const-generics - runs all the tests in the const-generics/ subdirectory of the ui test suite
    • ./x.py test src/test/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.py clean

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