Testing the compiler

The Rust project runs a wide variety of different tests, orchestrated by the build system (./x test). This section gives a brief overview of the different testing tools. Subsequent chapters dive into running tests and adding new tests.

Kinds of tests

There are several kinds of tests to exercise things in the Rust distribution. Almost all of them are driven by ./x test, with some exceptions noted below.


The main test harness for testing the compiler itself is a tool called compiletest. It supports running different styles of tests, called test suites. The tests are all located in the tests directory. The Compiletest chapter goes into detail on how to use this tool.

Example: ./x test tests/ui

Package tests

The standard library and many of the compiler packages include typical Rust #[test] unit tests, integration tests, and documentation tests. You can pass a path to x.py to almost any package in the library or compiler directory, and x will essentially run cargo test on that package.


./x test library/stdRuns tests on std only
./x test library/coreRuns tests on core only
./x test compiler/rustc_data_structuresRuns tests on rustc_data_structures

The standard library relies very heavily on documentation tests to cover its functionality. However, unit tests and integration tests can also be used as needed. Almost all of the compiler packages have doctests disabled.

All standard library and compiler unit tests are placed in separate tests file (which is enforced in tidy). This ensures that when the test file is changed, the crate does not need to be recompiled. For example:

mod tests;

If it wasn't done this way, and you were working on something like core, that would require recompiling the entire standard library, and the entirety of rustc.

./x test includes some CLI options for controlling the behavior with these tests:

  • --doc — Only runs documentation tests in the package.
  • --no-doc — Run all tests except documentation tests.


Tidy 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: ./x test tidy


Rustfmt is integrated with the build system to enforce uniform style across the compiler. The formatting check is automatically run by the Tidy tool mentioned above.


./x fmt --checkChecks formatting and exits with an error if formatting is needed.
./x fmtRuns rustfmt across the entire codebase.
./x test tidy --blessFirst runs rustfmt to format the codebase, then runs tidy checks.

Book documentation tests

All of the books that are published have their own tests, primarily for validating that the Rust code examples pass. Under the hood, these are essentially using rustdoc --test on the markdown files. The tests can be run by passing a path to a book to ./x test.

Example: ./x test src/doc/book

Links across all documentation is validated with a link checker tool.

Example: ./x test src/tools/linkchecker

Example: ./x test linkchecker

This requires building all of the documentation, which might take a while.

Dist check

distcheck verifies that the source distribution tarball created by the build system will unpack, build, and run all tests.

Example: ./x test distcheck

Tool tests

Packages that are included with Rust have all of their tests run as well. This includes things such as cargo, clippy, rustfmt, miri, bootstrap (testing the Rust build system itself), etc.

Most of the tools are located in the src/tools directory. To run the tool's tests, just pass its path to ./x test.

Example: ./x test src/tools/cargo

Usually these tools involve running cargo test within the tool's directory.

In CI, some tools are allowed to fail. Failures send notifications to the corresponding teams, and is tracked on the toolstate website. More information can be found in the toolstate documentation.

Integration testing

Rust tests integration with real-world code to catch regressions and make informed decisions about the evolution of the language. There are several kinds of integration tests, including Crater. See the Integration testing chapter for more details.

Performance testing

A separate infrastructure is used for testing and tracking performance of the compiler. See the Performance testing chapter for more details.

Further reading

The following blog posts may also be of interest: