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.

Create a config.toml

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

> cd $RUST_CHECKOUT
> 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]
# 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

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.

Running x.py and building a stage1 compiler

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 features.

The result is that compiling rustc is done in stages:

• Stage 0: the stage0 compiler is usually (you can configure x.py to use something else) the current beta rustc compiler and its associated dynamic libraries (which x.py will download for you). This stage0 compiler is then used only to compile rustbuild, std, test, and rustc. When compiling test and 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 (std, test, and 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 can be important when using dynamic linking (e.g., with derives. Sometimes this means that some tests don't work when run with stage1.
• 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.

A note on stage meanings

When running x.py you will see output such as:

Building stage0 std artifacts
Copying stage0 std from stage0
Building stage0 test artifacts
Copying stage0 test from stage0
Building stage0 compiler artifacts
Copying stage0 rustc from stage0
Building LLVM for x86_64-apple-darwin
Building stage0 codegen artifacts
Assembling stage1 compiler
Building stage1 std artifacts
Copying stage1 std from stage1
Building stage1 test artifacts
Copying stage1 test from stage1
Building stage1 compiler artifacts
Copying stage1 rustc from stage1
Building stage1 codegen artifacts
Assembling stage2 compiler
Uplifting stage1 std
Copying stage2 std from stage1
Generating unstable book md files
Building stage0 tool unstable-book-gen
Building stage0 tool rustbook
Documenting standalone
Building rustdoc for stage2
Documenting book redirect pages
Documenting stage2 std
Building rustdoc for stage1
Documenting stage2 test
Documenting stage2 whitelisted compiler
Documenting stage2 compiler
Documenting stage2 rustdoc
Documenting error index
Uplifting stage1 test
Copying stage2 test from stage1
Uplifting stage1 rustc
Copying stage2 rustc from stage1
Building stage2 tool error_index_generator

A deeper look into x.py's phases can be seen here:

Keep in mind this diagram is a simplification, i.e. rustdoc can be built at different stages, the process is a bit different when passing flags such as --keep-stage, or if there are non-host targets.

The following tables indicate the outputs of various stage actions:

--stage=0 stops here.

--stage=1 stops here.

--stage=2 stops here.

Note that the convention x.py uses is that:

• A "stage N artifact" is an artifact that is produced by the stage N compiler.
• The "stage (N+1) compiler" is assembled from "stage N artifacts".
• A --stage N flag means build with stage N.

In short, stage 0 uses the stage0 compiler to create stage0 artifacts which will later be uplifted to stage1.

Every time any of the main artifacts (std, test, rustc) are compiled, two steps are performed. When std is compiled by a stage N compiler, that std will be linked to programs built by the stage N compiler (including test and rustc built later on). It will also be used by the stage (N+1) compiler to link against itself. This is somewhat intuitive if one thinks of the stage (N+1) compiler as "just" another program we are building with the stage N compiler. In some ways, rustc (the binary, not the rustbuild step) could be thought of as one of the few no_core binaries out there.

So "stage0 std artifacts" are in fact the output of the downloaded stage0 compiler, and are going to be used for anything built by the stage0 compiler: e.g. rustc, test artifacts. When it announces that it is "building stage1 std artifacts" it has moved on to the next bootstrapping phase. This pattern continues in latter stages.

Also note that building host std and target std are different based on the stage (e.g. see in the table how stage2 only builds non-host std targets. This is because during stage2, the host std is uplifted from the "stage 1" std -- specifically, when "Building stage 1 artifacts" is announced, it is later copied into stage2 as well (both the compiler's libdir and the sysroot).

This std is pretty much necessary for any useful work with the compiler. Specifically, it's used as the std for programs compiled by the newly compiled compiler (so when you compile fn main() { } it is linked to the last std compiled with x.py build --stage 1 src/libstd).

The rustc generated by the stage0 compiler is linked to the freshly-built libstd, which means that for the most part only std needs to be cfg-gated, so that rustc can use featured added to std immediately after their addition, without need for them to get into the downloaded beta. The libstd built by the stage1/bin/rustc compiler, also known as "stage1 std artifacts", is not necessarily ABI-compatible with that compiler. That is, the rustc binary most likely could not use this std itself. It is however ABI-compatible with any programs that the stage1/bin/rustc binary builds (including itself), so in that sense they're paired.

This is also where --keep-stage 1 src/libstd comes into play. Since most changes to the compiler don't actually change the ABI, once you've produced a libstd in stage 1, you can probably just reuse it with a different compiler. If the ABI hasn't changed, you're good to go, no need to spend the time recompiling that std. --keep-stage simply assumes the previous compile is fine and copies those artifacts into the appropriate place, skipping the cargo invocation.

The reason we first build std, then test, then rustc, is largely just because we want to minimize cfg(stage0) in the code for rustc. Currently rustc is always linked against a "new" std/test so it doesn't ever need to be concerned with differences in std; it can assume that the std is as fresh as possible.

The reason we need to build it twice is because of ABI compatibility. The beta compiler has it's own ABI, and then the stage1/bin/rustc compiler will produce programs/libraries with the new ABI. We used to build three times, but because we assume that the ABI is constant within a codebase, we presume that the libraries produced by the "stage2" compiler (produced by the stage1/bin/rustc compiler) is ABI-compatible with the stage1/bin/rustc compiler's produced libraries. What this means is that we can skip that final compilation -- and simply use the same libraries as the stage2/bin/rustc compiler uses itself for programs it links against.

This stage2/bin/rustc compiler is shipped to end-users, along with the stage 1 {std,test,rustc} artifacts.

If you want to learn more about x.py, read its README.md here.

Build Flags

There are other 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:

• Build libstd using the stage0 compiler (using incremental)
• Build librustc using the stage0 compiler (using incremental)
  • This produces the stage1 compiler
• Build libstd 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 libstd 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 just say ./x.py build) has quite a few more steps:

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

Build specific components

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.

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 1.25.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.25.0-dev
LLVM version: 4.0

Suggested workflows for faster builds of the compiler

There are two workflows that are useful for faster builds of the compiler.

Check, check, and check again. The first workflow, which is useful when doing simple refactorings, is to run ./x.py check continuously. Here you are just checking that the compiler can build, but often that is all you need (e.g., when renaming a method). You can then run ./x.py build when you actually need to run tests.

In fact, it is sometimes useful to put off tests even when you are not 100% sure the code will work. You can then keep building up refactoring commits and only run the tests at some later time. You can then use git bisect to track down precisely which commit caused the problem. A nice side-effect of this style is that you are left with a fairly fine-grained set of commits at the end, all of which build and pass tests. This often helps reviewing.

Incremental builds with --keep-stage. Sometimes just checking whether the compiler builds is not enough. A common example is that you need to add a debug! statement to inspect the value of some state or better understand the problem. In that case, you really need a full build. By leveraging incremental, though, you can often get these builds to complete very fast (e.g., around 30 seconds): the only catch is this requires a bit of fudging and may produce compilers that don't work (but that is easily detected and fixed).

The sequence of commands you want is as follows:

• Initial build: ./x.py build -i --stage 1 src/libstd
  • As documented above, this will build a functional stage1 compiler as part of running all stage0 commands (which include building a libstd compatible with the stage1 compiler) as well as the first few steps of the "stage 1 actions" up to "stage1 (sysroot stage1) builds libstd".
• Subsequent builds: ./x.py build -i --stage 1 src/libstd --keep-stage 1
  • Note that we added the --keep-stage 1 flag here

As mentioned, the effect of --keep-stage 1 is that we just assume that the old standard library can be re-used. If you are editing the compiler, this is almost always true: you haven't changed the standard library, after all. But sometimes, it's not true: for example, if you are editing the "metadata" part of the compiler, which controls how the compiler encodes types and other states into the rlib files, or if you are editing things that wind up in the metadata (such as the definition of the MIR).

The TL;DR is that you might get weird behavior from a compile when using --keep-stage 1 -- for example, strange ICEs or other panics. In that case, you should simply remove the --keep-stage 1 from the command and rebuild. That ought to fix the problem.

You can also use --keep-stage 1 when running tests. Something like this:

• Initial test run: ./x.py test -i --stage 1 src/test/ui
• Subsequent test run: ./x.py test -i --stage 1 src/test/ui --keep-stage 1

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 clean – clean up the build directory (rm -rf build works too, but then you have to rebuild LLVM)
  • ./x.py build --stage 1 – builds everything using the stage 1 compiler, not just up to libstd
  • ./x.py build – builds the stage2 compiler
• Running tests (see the section on running tests for more details):
  • ./x.py test --stage 1 src/libstd – runs the #[test] tests from libstd
  • ./x.py test --stage 1 src/test/run-pass – runs the run-pass test suite
  • ./x.py test --stage 1 src/test/ui/const-generics - runs all the tests in the const-generics/ subdirectory of the ui test suite
  • ./x.py test --stage 1 src/test/ui/const-generics/const-types.rs - runs the single test const-types.rs from the ui test suite

ctags

One of the challenges with rustc is that the RLS can't handle it, since it's a bootstrapping compiler. This makes code navigation difficult. One solution is to use ctags.

ctags has a long history and several variants. Exhuberant CTags seems to be quite commonly distributed but it does not have out-of-box Rust support. Some distributions seem to use Universal Ctags, which is a maintained fork and does have built-in Rust support.

The following script can be used to set up Exhuberant Ctags: https://github.com/nikomatsakis/rust-etags.

ctags integrates into emacs and vim quite easily. The following can then be used to build and generate tags:

$ rust-ctags src/lib* && ./x.py build <something>

This allows you to do "jump-to-def" with whatever functions were around when you last built, which is ridiculously useful.

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

Compiler Documentation

The documentation for the rust components are found at rustc doc.