Mutation tester

A mutation tester changes the code in small ways (a < for a <=, a call removed) and runs the project's tests against each change. Each language has its own, so the one in use is found in the project rather than assumed.

Finding it

1. CI: a mutation job, usually a scheduled one, and the command it runs.
2. The tool's config: a file of its own (stryker.config.json), a plugin in the build file (pom.xml, build.gradle), or a section in pyproject.toml or setup.cfg.
3. The dependency manifest: the tool among the development dependencies.

When the project has none, choosing one for its language and test framework is a decision for a person.

Using it

• Start from a green suite: the tests must pass on the unchanged code first, or the score means nothing.
• Limit it while working to the changed files or the diff (--mutate in Stryker, --in-diff in cargo-mutants).
• Run the whole codebase on a schedule: one test run per mutant makes it the slowest check the project has.
• Keep a timeout per mutant, since a mutant can loop forever, and keep the report as JSON or HTML.

Examples: Stryker (JavaScript, C#), PIT (Java), mutmut (Python), cargo-mutants (Rust).