Introduce PHPUNIT_FAIL_ON_PHPUNIT_DEPRECATION

This is a good idea. I saw this issue a few days ago, and thought 'yes nice'. But now that I am hitting the PHPUnit deprecations in my work to add attributes, it has become 'yes must' !

Are you saying that we just need to set the environment variable PHPUNIT_FAIL_ON_PHPUNIT_DEPRECATION to 1 or 0 and the phpunit binary will use that value? I've just looked in run-tests.sh and there is no reference to that variable or the --fail-on-phpunit-deprecation option.

I will try it and see what happens.

No it's a Drupal thing, not PHPUnit.

Drupal's PHPUnit bootstrap script activates DeprecationHandler, that introspects PHPUNIT_FAIL_ON_PHPUNIT_DEPRECATION in its ::getConfiguration() method. That method is invoked by PhpUnitTestRunner::runCommand that does this

    // If the deprecation handler bridge is active, we need to fail when there
    // are deprecations that get reported (i.e. not ignored or expected).
    $deprecationConfiguration = DeprecationHandler::getConfiguration();
    if ($deprecationConfiguration !== FALSE) {
      $command[] = '--fail-on-deprecation';
      if ($deprecationConfiguration['failOnPhpunitDeprecation']) {
        $command[] = '--fail-on-phpunit-deprecation';
      }
    }

that is, it adds (or not) --fail-on-phpunit-deprecation to the PHPUnit CLI sub-process spawned by run-tests.sh.

Thanks for that. In #3527579-9: Add #[Group] attributes for Core 11.2 and PHPUnit 11 → I used your suggestion of setting the plain variable PHPUNIT_FAIL_ON_PHPUNIT_DEPRECATION: 0 and this does prevent the internal phpunit deprecation log messages. I am also working on making some changes in our downstream test branches to demonstrate the issue.

To implement this, we could just add the new variable to the hidden-variables file and document how to use it. But I think it may be better to add our own differently-named variable that we set, and the users can customise, or enter in the pipeline form, then we convert that to the actual value for PHPUNIT_FAIL_ON_PHPUNIT_DEPRECATION: 0 in the script. This will give us the ability to add further options or enhancements in future, if they become available. I will add that to the MR and see how it looks.