The PHP attribute that yells at you for ignoring a return value: #[\NoDiscard] in 8.5
PHP 8.5's #[\NoDiscard] makes the engine warn you when you call a function and throw away its result — the classic immutable-API footgun ($url->withScheme('https') that quietly did nothing). Why it's the rare attribute that improves your code by nagging you, where it fits in Drupal (Url, cache metadata, fluent withX() setters), the (void)-cast escape hatch, and why it only helps if your CI fails on warnings.