Mutation testing (cargo-mutants)

bomdrift’s risk-bearing modules are audited with cargo-mutants to verify the test suite actually catches the mutations the tests claim to catch. The audit is run manually before each release — it is not wired into CI (the runtime is prohibitive for per-PR feedback, and the signal is most useful at release-cut time).

How to reproduce

Run from a clean worktree (the tool rewrites source files in place during the run):

git worktree add ../bomdrift-mutants main
cd ../bomdrift-mutants
cargo mutants --in-place --no-shuffle --file '<path-glob>' --timeout 60

Use a separate worktree so concurrent editing in your main checkout doesn’t collide with the in-place rewrites.

Audit log

Each entry: module — caught / unviable / surviving / total — date. The target is <10% surviving.

src/diff/** — v0.9.9 baseline

• Total mutants: 19 (+ 1 baseline build)
• Caught: 16
• Unviable: 3 (compile-failures the tool excludes from the surviving-rate denominator)
• Surviving: 0
• Surviving rate: 0 / 19 = 0.0%
• Date: 2026-06-01

No test additions required. The diff engine’s existing unit and property tests fully cover the mutation surface.

The three unviable mutants were mutations that produced code that does not compile — they’re excluded from the surviving denominator because the compiler itself rejects them before the test suite runs.