Symfony Hardening Rule

Turns a recurring hardening invariant into a CI gate. The mechanics are obvious from the three shipped rules (HardenedComparisonRule, UnserializeToStringTrampolineRule, UnserializeMissingAllowedClassesRule): copy the class shape and the registration. This skill is the part the example files cannot teach: when a rule is worth building, and the traps that cost an afternoon.

When to build (and when not)

A PHPStan rule earns its place only if it catches a broad, recurring shape across many sites. The shipped rules each span dozens (every webhook compare; 60+ __unserialize; every unserialize()). If your rule effectively targets one class or one call site, it is too narrow: it carries CI weight for almost no coverage. Use a regression test instead. (A verify-MAC-before-deserialize rule was built and dropped for exactly this reason: it matched a single serializer.)

Pick the mechanism by what the invariant is about:

Do not force a taint problem into a syntactic rule: if safe and unsafe look identical at the sink and differ only by where a value came from, the rule is all false positives.

The shape (copy, do not reinvent)

Model the new rule on the existing .github/sa-tools/rules/*Rule.php: global namespace, Symfony license header, implements Rule with getNodeType()/processNode(), a stable identifier('symfony.<camelCase>'), and the narrowest node type that sees the shape.

The traps

1. Registration is one line plus the file. Drop <Name>Rule.php into .github/sa-tools/rules/ and add its class to the rules: list in .github/sa-tools/rules.neon (which phpstan.dist.neon includes). The folder's autoloader (rules/bootstrap.php, wired once via --autoload-file in static-analysis.yml) and the .gitignore folder whitelist (!rules, !rules/*) pick the file up with no further edits. A rule that needs constructor arguments takes a services: entry tagged phpstan.rules.rule in that same rules.neon instead of the rules: shorthand. (PHPStan only autoloads a project-local rule class early enough via --autoload-file or composer PSR-4 — a neon scanDirectories/bootstrapFiles runs too late — hence the stable bootstrap, which you never touch per rule.)
2. phpstan analyse counts are nondeterministic (result cache + parallel workers report 0 then N for the same input). Verify the logic with PHPStan\Testing\RuleTestCase in a throwaway composer require phpstan/phpstan phpunit/phpunit project, with fixtures for the unsafe shape (must fire), the fixed shape (silent), and the nearest safe look-alike from the decision boundary (silent). Never gate on a local analyse count.
3. The diff-lint only flags new errors. phpstan-diff.php compares the PR against the base branch and fails only on errors new to the PR, so standing violations are absorbed. Design target: zero findings on the current tree at introduction (no baseline entry).

Convention path

If the invariant is "this boundary class must ship its regression test", extend check-hardening-tests.php (pure tokenizer, no autoload) instead of writing a rule. Add the boundary predicate and the required-test predicate in its existing style; accepted gaps go in the ALLOWLIST const as a TODO with a reason, never a permanent waiver.

Error handling

• If the invariant needs data-flow, do not ship a syntactic rule that fakes it. Say it is a taint or checklist item and wait for confirmation on the fallback.
• Do not edit phpstan-diff.php or the workflow to make a rule pass. Fix the rule.
• House conventions: Symfony license header, no em-dashes, no Claude/Anthropic credit, comments sparingly, no issue numbers in code.