If you run core's tests with code coverage on, 25 test classes trigger a PHPUnit warning. Each of them uses #[CoversClass] to name a trait. PHPUnit 12 wants us to use #[CoversTrait] for that.

I did some research, and it appears that PHPUnit changed this in a few steps. PHPUnit 11.2 added #[CoversTrait] (phpunit#5799) and deprecated #[CoversClass] (phpunit#5800). Then it was dropped in PHPUnit 12.0 (phpunit#5801).

PHPUnit only checks these attributes when it collects coverage, so neither the deprecation nor today's warning shows up in a normal test run. You can try it with PCOV or Xdebug in coverage mode and running:

$ vendor/bin/phpunit -c core --coverage-text core/tests/Drupal/Tests/Core/StringTranslation/StringTranslationTraitTest.php

You'll get warnings like:

2 tests triggered 2 PHPUnit warnings:

1) Drupal\Tests\Core\StringTranslation\StringTranslationTraitTest::testT
Class Drupal\Core\StringTranslation\StringTranslationTrait is not a valid target for code coverage

2) Drupal\Tests\Core\StringTranslation\StringTranslationTraitTest::testFormatPlural
Class Drupal\Core\StringTranslation\StringTranslationTrait is not a valid target for code coverage

OK, but there were issues!
Tests: 2, Assertions: 6, PHPUnit Warnings: 2.

The solution is pretty simple: switch from #[CoversClass] to #[CoversTrait]. I'd also recommend adding a PHPStan rule to reject #[CoversClass]/#[UsesClass].

Core already uses #[CoversTrait] in some tests, so this would make Core more consistent.

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dries created an issue. See original summary.

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dries commented
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dries commented
Issue summary: View changes

dries opened merge request !17443

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dries commented

I converted the #[CoversClass]'s with the help of Claude Code and opened an MR. The MR switches 25 tests to #[CoversTrait], and adds a test to the TestClassClassMetadata PHPStan rule.

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dries commented
Status: Active » Needs review
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mondrake
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Do we really need to add a PHPStan rule too? PHPUnit will be already flagging the error itself (it's 'PHPUnit warning'). It's not like test annotations, where PHPUnit removed the annotation parsing code and we would be left out with crufty annotations without the PHPStan check.

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dries commented

Fair question and either works for me. I added the rule because PHPUnit only flags it when running with --coverage-text and Drupal.org's CI/CD pipelines don't. So it would go unnoticed until someone runs the code coverage manually.

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mondrake
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Status: Needs review » Reviewed & tested by the community

Oh I missed that happens only when processing code coverage. Fair enough then. I hope we could soon enable code coverage for core unit testing, with PHPUnit native parallelization coming hopefully in 13.5, https://github.com/sebastianbergmann/phpunit/pull/6784. But that’d be another issue.

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mondrake
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Title: PHPUnit 12 deprecated #[CoversClass] » PHPUnit 12 deprecated using #[CoversClass] to target traits