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. 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.

Get the source code

The very first step to work on rustc is to clone the repository:

git clone https://github.com/rust-lang/rust.git
cd rust

Create a config.toml

To start, copy config.toml.example to config.toml:

cp config.toml.example config.toml

Then you will want to open up the file and change the following settings (and possibly others, such as llvm.ccache):

[llvm]
# Indicates whether the LLVM assertions are enabled or not
assertions = true

[rust]
# Whether or not to leave debug! and trace! calls in the rust binary.
# Overrides the `debug-assertions` option, if defined.
#
# Defaults to rust.debug-assertions value
#
# If you see a message from `tracing` saying
# `max_level_info` is enabled and means logging won't be shown,
# set this value to `true`.
debug-logging = true

# Whether to always use incremental compilation when building rustc
incremental = true

If you have already built rustc, 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, so if your configuration change affects LLVM, you will need to manually rm -rf build/ before rebuilding.

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, 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:

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

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 workflows" below.

Note that this whole command just gives you a subset of the full rustc build. The full rustc build (what you get if you say ./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.

Build specific components

  • Build only the core library
./x.py build --stage 0 library/core
  • Build only the core and proc_macro libraries
./x.py build --stage 0 library/core library/proc_macro

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 --stage.

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
  • 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

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 the stage2 compiler
  • Running tests (see the section on running tests for more details):
    • ./x.py test library/std – runs the #[test] 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.