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.
The very first step to work on
rustc is to clone the repository:
git clone https://github.com/rust-lang/rust.git cd rust
To start, copy
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] # Enables LLVM assertions, which will check that the LLVM bitcode generated # by the compiler is internally consistent. These are particularly helpful # if you edit `codegen`. assertions = true [rust] # This will make your build more parallel; it costs a bit of runtime # performance perhaps (less inlining) but it's worth it. codegen-units = 0 # This enables full debuginfo and debug assertions. The line debuginfo is also # enabled by `debuginfo-level = 1`. Full debuginfo is also enabled by # `debuginfo-level = 2`. Debug assertions can also be enabled with # `debug-assertions = true`. Note that `debug = true` will make your build # slower, so you may want to try individually enabling debuginfo and assertions # or enable only line debuginfo which is basically free. debug = 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
rm -rf build/ before rebuilding.
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
One thing to keep in mind is that
rustc is a bootstrapping
compiler. That is, since
rustc is written in Rust, we need to use an
older version of the compiler to compile the newer version. In
particular, the newer version of the compiler and some of the artifacts needed
to build it, such as
libstd and other tooling, may use some unstable features
internally, requiring a specific version which understands these unstable
The result is that compiling
rustc is done in stages:
- Stage 0: the stage0 compiler is usually (you can configure
x.pyto use something else) the current beta
rustccompiler and its associated dynamic libraries (which
x.pywill download for you). This stage0 compiler is then used only to compile
rustc. When compiling
rustc, this stage0 compiler uses the freshly compiled
std. There are two concepts at play here: a compiler (with its set of dependencies) and its 'target' or 'object' libraries (
rustc). Both are staged, but in a staggered manner.
- Stage 1: the code in your clone (for new version) is then
compiled with the stage0 compiler to produce the stage1 compiler.
However, it was built with an older compiler (stage0), so to
optimize the stage1 compiler we go to next the stage.
- In theory, the stage1 compiler is functionally identical to the
stage2 compiler, but in practice there are subtle differences. In
particular, the stage1 compiler itself was built by stage0 and
hence not by the source in your working directory: this means that
the symbol names used in the compiler source may not match the
symbol names that would have been made by the stage1 compiler. This is
important when using dynamic linking and the lack of ABI compatibility
between versions. This primarily manifests when tests try to link with any
rustc_*crates or use the (now deprecated) plugin infrastructure. These tests are marked with
- In theory, the stage1 compiler is functionally identical to the stage2 compiler, but in practice there are subtle differences. In particular, the stage1 compiler itself was built by stage0 and hence not by the source in your working directory: this means that the symbol names used in the compiler source may not match the symbol names that would have been made by the stage1 compiler. This is important when using dynamic linking and the lack of ABI compatibility between versions. This primarily manifests when tests try to link with any of the
- Stage 2: we rebuild our stage1 compiler with itself to produce the stage2 compiler (i.e. it builds itself) to have all the latest optimizations. (By default, we copy the stage1 libraries for use by the stage2 compiler, since they ought to be identical.)
- (Optional) Stage 3: to sanity check our new compiler, we can build the libraries with the stage2 compiler. The result ought to be identical to before, unless something has broken.
To read more about the bootstrap process, read this chapter.
To build a compiler, run
./x.py build. This will do the whole bootstrapping
process described above, producing a usable compiler toolchain from the source
code you have checked out. This takes a long time, so it is not usually what
you want to actually run (more on this later).
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 --stage 1 src/libstd
This may look like it only builds
libstd, but that is not the case.
What this command does is the following:
libstdusing the stage0 compiler (using incremental)
librustcusing the stage0 compiler (using incremental)
- This produces the stage1 compiler
libstdusing 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
libstd 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 if you just say
./x.py build) has quite a few more steps:
rustcwith the stage1 compiler.
- The resulting compiler here is called the "stage2" compiler.
libstdwith stage2 compiler.
librustdocand a bunch of other things with the stage2 compiler.
Build only the libcore library
./x.py build src/libcore
Build the libcore and libproc_macro library only
./x.py build src/libcore src/libproc_macro
Build only libcore up to Stage 1
./x.py build src/libcore --stage 1
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.
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 1.25.0-dev binary: rustc commit-hash: unknown commit-date: unknown host: x86_64-unknown-linux-gnu release: 1.25.0-dev LLVM version: 4.0
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 --stage 1– builds everything using the stage 1 compiler, not just up to
./x.py build– builds the stage2 compiler
- Running tests (see the section on running tests for
./x.py test --stage 1 src/libstd– runs the
./x.py test --stage 1 src/test/ui– runs the
./x.py test --stage 1 src/test/ui/const-generics- runs all the tests in the
const-generics/subdirectory of the
./x.py test --stage 1 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.