fix(infection): skip mutations to #[TestInline] attribute arguments (#159)

[rossaddison]

Problem

When Infection mutates values inside a #[TestInline(arguments: [...], result: ...)] attribute it is mutating test data, not production logic.

Every such mutation is "killed" — Assert::same catches the changed expected value — but the kills are semantically meaningless: they verify that the assertion mechanism works, not that the source code handles a production code mutation correctly. This inflates the mutation score with noise and wastes CI runner time.

Concrete example in plugin/inline/tests/Self/StaticMethods.php:

#[TestInline(arguments: [1, 1], result: 2)]   // ← Infection: result: 2 → 3 (IncrementInteger)
public static function publicSum(int $a, int $b): int
{
    return $a + $b;   // ← this is the actual production code worth mutating
}

When result: 2 is changed to result: 3, Assert::same(2, 3) fails — the mutant is "killed", but no useful information was gained about the production code.

Fix

Add global-ignoreSourceCodeByRegex to infection.json:

"mutators": {
    "global-ignoreSourceCodeByRegex": [
        "#\[TestInline"
    ]
}

Infection skips any mutation whose source line contains #[TestInline. This covers:

• All #[TestInline(...)] attribute lines in plugin/inline/tests/Self/ fixtures
• Any production code that uses the attribute inline on its methods

Method bodies on adjacent lines are unaffected and continue to be mutated normally — so regressions in publicSum, privateMul, etc. are still caught.

Verification

All 23 inline plugin tests pass unchanged. The fix is a pure config addition with no code changes.

Fixes #159

[rossaddison]

Summary

What the fix does: adds global-ignoreSourceCodeByRegex to infection.json with the pattern #\[TestInline. Infection then skips any mutation whose source line contains #[TestInline — so attribute argument values (result: 2, arguments: [1, 1]) are never mutated. Method bodies on the lines immediately below are untouched and continue to be mutated normally.

One line change to one config file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[rossaddison]

Fix: 0 mutations generated → MSI failure

Root cause

Adding a "mutators": {} key to infection.json without "@default": true turns the block into an explicit allowlist. Infection saw no selected mutators and generated 0 mutations, which caused the 50% MSI threshold to fail immediately.

What happened

The original infection.json had no "mutators" key at all, so Infection ran all default mutators. PR #261 added:

"mutators": {
    "global-ignoreSourceCodeByRegex": ["#\[TestInline"]
}

global-ignoreSourceCodeByRegex is a configuration option, not a mutator selection — but because the mutators key was now present with no mutator entries, Infection treated it as "run zero mutators plus apply this filter". Result: 0 mutations, 0% MSI.

Fix

Add "@default": true to re-enable the full default mutator set alongside the regex filter:

"mutators": {
    "@default": true,
    "global-ignoreSourceCodeByRegex": ["#\[TestInline"]
}

Also included in this branch

• tests/Application/Stub/EmptyRun/.placeholder.php — a no-op PHP file so that bin/build-phpunit.php creates tests/PhpUnit/Application/Stub/EmptyRun/ during the PHPUnit mirror build. Without it, EmptyRunTest throws InvalidArgumentException: File or directory not found when Infection runs the PHPUnit mirror suite, blocking Infection from starting at all (pre-existing CI failure on 1.x).

[roxblnfk]

We discussed the issue there infection/infection#3053 (comment)
The PR ignores mutations only for this repo.
But I'm not sure that it's possible to resolve the issue for all the Testo users right now.