AttributeDiscoveryTestMissingTrait.php

<?php

declare (strict_types=1);
namespace com\example\PluginNamespace;

use Drupal\a_module_that_does_not_exist\Plugin\CustomTrait;

/**
 * Provides a custom test plugin that uses a missing trait.
 */
class AttributeDiscoveryTestMissingTrait {
  use CustomTrait;

}