The compiler testing framework

The Rust project runs a wide variety of different tests, orchestrated by the build system ( test). The main test harness for testing the compiler itself is a tool called compiletest (located in the src/tools/compiletest directory). This section gives a brief overview of how the testing framework is setup, and then gets into some of the details on how to run tests as well as how to add new tests.

Compiletest test suites

The compiletest tests are located in the tree in the src/test directory. Immediately within you will see a series of subdirectories (e.g. ui, run-make, and so forth). Each of those directories is called a test suite – they house a group of tests that are run in a distinct mode.

Here is a brief summary of the test suites and what they mean. In some cases, the test suites are linked to parts of the manual that give more details.

  • ui – tests that check the exact stdout/stderr from compilation and/or running the test
  • run-pass-valgrind – tests that ought to run with valgrind
  • pretty – tests targeting the Rust "pretty printer", which generates valid Rust code from the AST
  • debuginfo – tests that run in gdb or lldb and query the debug info
  • codegen – tests that compile and then test the generated LLVM code to make sure that the optimizations we want are taking effect. See LLVM docs for how to write such tests.
  • codegen-units – tests for the monomorphization collector and CGU partitioning
  • assembly – similar to codegen tests, but verifies assembly output to make sure LLVM target backend can handle provided code.
  • mir-opt – tests that check parts of the generated MIR to make sure we are building things correctly or doing the optimizations we expect.
  • incremental – tests for incremental compilation, checking that when certain modifications are performed, we are able to reuse the results from previous compilations.
  • run-make – tests that basically just execute a Makefile; the ultimate in flexibility but quite annoying to write.
  • rustdoc – tests for rustdoc, making sure that the generated files contain the expected documentation.
  • rustfix – tests for applying diagnostic suggestions with the rustfix crate
  • *-fulldeps – same as above, but indicates that the test depends on things other than std (and hence those things must be built)

Other Tests

The Rust build system handles running tests for various other things, including:

  • Tidy – This is a custom tool used for validating source code style and formatting conventions, such as rejecting long lines. There is more information in the section on coding conventions.

    Example: ./ test tidy

  • Formatting – Rustfmt is integrated with the build system to enforce uniform style across the compiler. In the CI, we check that the formatting is correct. The formatting check is also automatically run by the Tidy tool mentioned above.

    Example: ./ fmt --check checks formatting and exits with an error if formatting is needed.

    Example: ./ fmt runs rustfmt on the codebase.

    Example: ./ test tidy --bless does formatting before doing other tidy checks.

  • Unit tests – The Rust standard library and many of the Rust packages include typical Rust #[test] unittests. Under the hood, will run cargo test on each package to run all the tests.

    Example: ./ test library/std

  • Doc tests – Example code embedded within Rust documentation is executed via rustdoc --test. Examples:

    ./ test src/doc – Runs rustdoc --test for all documentation in src/doc.

    ./ test --doc library/std – Runs rustdoc --test on the standard library.

  • Link checker – A small tool for verifying href links within documentation.

    Example: ./ test src/tools/linkchecker

  • Dist check – This verifies that the source distribution tarball created by the build system will unpack, build, and run all tests.

    Example: ./ test distcheck

  • Tool tests – Packages that are included with Rust have all of their tests run as well (typically by running cargo test within their directory). This includes things such as cargo, clippy, rustfmt, rls, miri, bootstrap (testing the Rust build system itself), etc.

  • Cargo test – This is a small tool which runs cargo test on a few significant projects (such as servo, ripgrep, tokei, etc.) just to ensure there aren't any significant regressions.

    Example: ./ test src/tools/cargotest

Testing infrastructure

When a Pull Request is opened on Github, GitHub Actions will automatically launch a build that will run all tests on some configurations (x86_64-gnu-llvm-8 linux. x86_64-gnu-tools linux, mingw-check linux). In essence, it runs ./ test after building for each of them.

The integration bot bors is used for coordinating merges to the master branch. When a PR is approved, it goes into a queue where merges are tested one at a time on a wide set of platforms using GitHub Actions (as of January 2021, over 50 different configurations). Due to the limit on the number of parallel jobs, we run CI under the rust-lang-ci organization except for PRs. Most platforms only run the build steps, some run a restricted set of tests, only a subset run the full suite of tests (see Rust's platform tiers).

Testing with Docker images

The Rust tree includes Docker image definitions for the platforms used on GitHub Actions in src/ci/docker. The script src/ci/docker/ is used to build the Docker image, run it, build Rust within the image, and run the tests.

You can run these images on your local development machine. This can be helpful to test environments different from your local system. First you will need to install Docker on a Linux, Windows, or macOS system (typically Linux will be much faster than Windows or macOS because the later use virtual machines to emulate a Linux environment). To enter interactive mode which will start a bash shell in the container, run src/ci/docker/ --dev <IMAGE> where <IMAGE> is one of the directory names in src/ci/docker (for example x86_64-gnu is a fairly standard Ubuntu environment).

The docker script will mount your local rust source tree in read-only mode, and an obj directory in read-write mode. All of the compiler artifacts will be stored in the obj directory. The shell will start out in the obj directory. From there, you can run ../src/ci/ which will run the build as defined by the image.

Alternatively, you can run individual commands to do specific tasks. For example, you can run python3 ../ test src/test/ui to just run UI tests. Note that there is some configuration in the src/ci/ script that you may need to recreate. Particularly, set submodules = false in your config.toml so that it doesn't attempt to modify the read-only directory.

Some additional notes about using the Docker images:

  • Some of the std tests require IPv6 support. Docker on Linux seems to have it disabled by default. Run the commands in to enable IPv6 before creating the container. This only needs to be done once.
  • The container will be deleted automatically when you exit the shell, however the build artifacts persist in the obj directory. If you are switching between different Docker images, the artifacts from previous environments stored in the obj directory may confuse the build system. Sometimes you will need to delete parts or all of the obj directory before building inside the container.
  • The container is bare-bones, with only a minimal set of packages. You may want to install some things like apt install less vim.
  • You can open multiple shells in the container. First you need the container name (a short hash), which is displayed in the shell prompt, or you can run docker container ls outside of the container to list the available containers. With the container name, run docker exec -it <CONTAINER> /bin/bash where <CONTAINER> is the container name like 4ba195e95cef.

Running tests on a remote machine

Tests may be run on a remote machine (e.g. to test builds for a different architecture). This is done using remote-test-client on the build machine to send test programs to remote-test-server running on the remote machine. remote-test-server executes the test programs and sends the results back to the build machine. remote-test-server provides unauthenticated remote code execution so be careful where it is used.

To do this, first build remote-test-server for the remote machine, e.g. for RISC-V

./ build src/tools/remote-test-server --target riscv64gc-unknown-linux-gnu

The binary will be created at ./build/$HOST_ARCH/stage2-tools/$TARGET_ARCH/release/remote-test-server. Copy this over to the remote machine.

On the remote machine, run the remote-test-server with the remote argument (and optionally -v for verbose output). Output should look like this:

$ ./remote-test-server -v remote
starting test server
listening on!

You can test if the remote-test-server is working by connecting to it and sending ping\n. It should reply pong:

$ nc $REMOTE_IP 12345

To run tests using the remote runner, set the TEST_DEVICE_ADDR environment variable then use as usual. For example, to run ui tests for a RISC-V machine with the IP address use

./ test src/test/ui --target riscv64gc-unknown-linux-gnu

If remote-test-server was run with the verbose flag, output on the test machine may look something like

run "/tmp/work/test1007/a"
run "/tmp/work/test1008/a"
run "/tmp/work/test1009/a"
run "/tmp/work/test1010/a"
run "/tmp/work/test1011/a"
run "/tmp/work/test1012/a"
run "/tmp/work/test1013/a"
run "/tmp/work/test1014/a"
run "/tmp/work/test1015/a"
run "/tmp/work/test1016/a"
run "/tmp/work/test1017/a"
run "/tmp/work/test1018/a"

Tests are built on the machine running not on the remote machine. Tests which fail to build unexpectedly (or ui tests producing incorrect build output) may fail without ever running on the remote machine.

Testing on emulators

Some platforms are tested via an emulator for architectures that aren't readily available. For architectures where the standard library is well supported and the host operating system supports TCP/IP networking, see the above instructions for testing on a remote machine (in this case the remote machine is emulated).

There is also a set of tools for orchestrating running the tests within the emulator. Platforms such as arm-android and arm-unknown-linux-gnueabihf are set up to automatically run the tests under emulation on GitHub Actions. The following will take a look at how a target's tests are run under emulation.

The Docker image for armhf-gnu includes QEMU to emulate the ARM CPU architecture. Included in the Rust tree are the tools remote-test-client and remote-test-server which are programs for sending test programs and libraries to the emulator, and running the tests within the emulator, and reading the results. The Docker image is set up to launch remote-test-server and the build tools use remote-test-client to communicate with the server to coordinate running tests (see src/bootstrap/

TODO: Is there any support for using an iOS emulator?

It's also unclear to me how the wasm or asm.js tests are run.


Crater is a tool for compiling and running tests for every crate on (and a few on GitHub). It is mainly used for checking for extent of breakage when implementing potentially breaking changes and ensuring lack of breakage by running beta vs stable compiler versions.

When to run Crater

You should request a crater run if your PR makes large changes to the compiler or could cause breakage. If you are unsure, feel free to ask your PR's reviewer.

Requesting Crater Runs

The rust team maintains a few machines that can be used for running crater runs on the changes introduced by a PR. If your PR needs a crater run, leave a comment for the triage team in the PR thread. Please inform the team whether you require a "check-only" crater run, a "build only" crater run, or a "build-and-test" crater run. The difference is primarily in time; the conservative (if you're not sure) option is to go for the build-and-test run. If making changes that will only have an effect at compile-time (e.g., implementing a new trait) then you only need a check run.

Your PR will be enqueued by the triage team and the results will be posted when they are ready. Check runs will take around ~3-4 days, with the other two taking 5-6 days on average.

While crater is really useful, it is also important to be aware of a few caveats:

  • Not all code is on! There is a lot of code in repos on GitHub and elsewhere. Also, companies may not wish to publish their code. Thus, a successful crater run is not a magically green light that there will be no breakage; you still need to be careful.

  • Crater only runs Linux builds on x86_64. Thus, other architectures and platforms are not tested. Critically, this includes Windows.

  • Many crates are not tested. This could be for a lot of reasons, including that the crate doesn't compile any more (e.g. used old nightly features), has broken or flaky tests, requires network access, or other reasons.

  • Before crater can be run, @bors try needs to succeed in building artifacts. This means that if your code doesn't compile, you cannot run crater.

Perf runs

A lot of work is put into improving the performance of the compiler and preventing performance regressions. A "perf run" is used to compare the performance of the compiler in different configurations for a large collection of popular crates. Different configurations include "fresh builds", builds with incremental compilation, etc.

The result of a perf run is a comparison between two versions of the compiler (by their commit hashes).

You should request a perf run if your PR may affect performance, especially if it can affect performance adversely.

Further reading

The following blog posts may also be of interest: