Auto-skip equivalent mutants in clamp patterns (if x < C { x = C })

[fchimpan]

Summary

Found while dogfooding mutest v0.6.0 (e6eca1b) against a real-world Go codebase (commit eb6320af). The clamp idiom produces a provably unkillable mutant:

// private/pkg/thread/thread.go:43 — reported SURVIVED
if parallelism < 1 {
    parallelism = 1
}

Mutating < to <= is equivalent for every input:

• x < 1: both clamp to 1
• x == 1: original does nothing, mutant assigns 1 — same result
• x > 1: neither fires

No test can ever kill this mutant, so it is guaranteed noise in every run, which conflicts with the project goal that every surviving mutant should represent a real test gap.

Proposal

Skip the boundary mutation when an if statement matches the clamp shape:

• Cond is x OP C with OP in <, <=, >, >= (either operand order)
• Body is exactly one assignment x = C where x and C are syntactically identical to the condition operands

Since the comparison mutator only swaps strict/non-strict operators, matching this shape can drop the mutation point entirely, same as the existing len(x) > 0 / cap(x) > 0 skips documented in "Skipped Mutations".

Notes

Variants worth covering: if C > x { x = C } (swapped operands) and max-clamps if x > C { x = C }. Patterns where the assigned value differs from the compared constant (e.g. if x < 0 { x = 1 }) are NOT equivalent and must stay mutated.