Migrate from Doctrine annotations to PHP attributes

This can be part of the 3.0 release.

Plugin implementations should use PHP attributes instead of annotations →

Plugin types should use PHP attributes instead of annotations →

After struggling with this, I think we're blocked on ✨ Fatal error: Declaration of Drupal\Component\Plugin\Attribute\AttributeBase::setClass(string $class): void must be compatible with Drupal\Component\Plugin\Definition\PluginDefinitionInterface::setClass($class) Active .

Comment from fathershawn on Slack:

Both classes are essentially data objects. As I see it, they are related but not embodied in the same class.
Your instance of AttributeInterface powers discovery of your plugin classes and manages the metadata needed to instantiate them.
DefaultPluginManager::getDiscovery instantiates AttributeClassDiscovery
DefaultPluginManager extends PluginManagerBase and PluginManagerBase::getDefinitions passes it’s work to the discovery class. AttributeClassDiscovery::getDefinitions ultimately calls AttributeInterface::get to get the data from the attribute which has a return type of mixed and AttributeBase refines that as array|object . Which means that in the ::get method of your attribute you can return an instance of PluginDefinitionInterface
See \Drupal\Core\Layout\Attribute\Layout for an example.