Module mutation 

Mutation testing for Rust (unit mutation --language rust, #201) — the rung above coverage. A test that runs a line still passes if you delete its assertions; a surviving mutant proves it. This module wraps cargo-mutants: it runs the engine, reads its outcomes.json, and reports the surviving mutants the suite failed to catch.

The gate is binary, not a percentage (equivalent mutants make a fixed score unreachable, and a score isn’t comparable across engines) and on by default: any un-exempted surviving mutant is a finding. This module stays a pure measurement — measure_rust returns the survivors and unexplained_survivors is the pure core over a parsed report; the CLI layer turns a non-empty result into the failure.

Diff-scoping (--base) is delegated to cargo-mutants’ own --in-diff: the <base>...HEAD diff is written out and passed through, so only mutants on changed lines are tested (“no unexplained surviving mutant on the lines you touched”).

Structs

LineCol

A line/column position; only the line is read.

MutantInfo

The mutant a scenario describes, pared to the location + description the report needs. cargo-mutants also carries function, genre, package, replacement; those are ignored.

MutantOutcome

One scenario’s outcome. summary is cargo-mutants’ result word — Success for the unmutated baseline, CaughtMutant / MissedMutant (and Timeout / Unviable) for each mutant.

MutantsReport

A cargo-mutants outcomes.json export, pared to what the rule reads. Unmodeled fields (total_mutants, caught, timings, …) are ignored.

Span

A source span; only the start line is read.

Survivor

A surviving mutant — a mutation the unit suite ran but failed to catch.

Enums

Scenario

The scenario a result came from: the unmutated baseline, or one mutant. Matches cargo-mutants’ externally-tagged JSON ("Baseline" vs {"Mutant": {…}}).

Functions

measure_rust

Run cargo-mutants over the crate at root and return its un-exempted survivors.

parse_mutants_report

Parse a cargo-mutants outcomes.json export.

unexplained_survivors

The surviving mutants not lifted by a mutation exemption — the rule’s findings.