> ## Documentation Index
> Fetch the complete documentation index at: https://developers.stellar.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Mutation Testing

> Mutation testing finds code not tested.

Mutation testing is making changes to a program, either manually or automatically, to identify changes that can be made that don't get caught by tests.

Mutation testing is similar to measuring [code coverage], sharing the same goal to identify code not covered by tests. But where code coverage focuses on checking if a line of code is executed during a test, mutation testing will actually check that a test fails when the line is changed. A line of code can look like it is covered by tests, but the outcomes and side-effects may not be asserted on in the tests.

## How to do Mutation Testing

The `cargo-mutants` tool can be used to automatically and iteratively modify the Rust code, and rerun the tests after each mutation, to identify code not tested.

1. Install `cargo-mutants`:

   ```shell theme={"theme":{"light":"github-light","dark":"github-dark"}}
   cargo install --locked cargo-mutants
   ```

2. Run the `cargo mutants` command inside your contract's crate directory.

   ```shell theme={"theme":{"light":"github-light","dark":"github-dark"}}
   $ cargo mutants
   Found 4 mutants to test
   ok       Unmutated baseline in 19.0s build + 0.6s test
    INFO Auto-set test timeout to 20s
   MISSED   src/lib.rs:14:9: replace IncrementContract::increment -> u32 with 1 in 0.4s build + 0.4s test
   4 mutants tested in 23s: 1 missed, 3 caught
   ```

   Code that is identified as not covered by a test will be outputted as a `MISSED` line in the output.

   Diffs of each change that was attempted can be found in the `mutants.out/diff` directory.

[code coverage]: https://developers.stellar.org/docs/build/guides/testing/code-coverage.md