Allow extending AttributeExtension to declare callables with attributes

[GromNaN]

Follows #4989.

An existing extension can declare its filters, functions, and tests with PHP attributes instead of implementing getFilters(), getFunctions(), and getTests(). Add the #[AsTwigFilter], #[AsTwigFunction], and #[AsTwigTest] attributes on the methods, extend AttributeExtension, and remove the three getters:

use Twig\Attribute\AsTwigFunction;
use Twig\Extension\AttributeExtension;

class ProjectExtension extends AttributeExtension
{
    public function __construct(private Mailer $mailer) {}

    #[AsTwigFunction('lipsum')]
    public function lipsum(int $count): string { ... }
}

The class is still a regular extension, so it is registered as usual:

$twig->addExtension(new ProjectExtension($mailer));

It also works with lazy registration: AttributeExtension is registered with the class that holds the attributes, and a runtime loader builds that class on demand:

$twig->addExtension(new AttributeExtension(ProjectExtension::class));
$twig->addRuntimeLoader(new FactoryRuntimeLoader([
    ProjectExtension::class => fn () => new ProjectExtension($mailer),
]));

All the extension methods are final, so a subclass declares nothing else than the attributes to read. Globals cannot be declared with attributes: getGlobals() is final and throws, so a subclass cannot provide globals of its own.

Symfony counterpart: symfony/symfony#67135.

[stof]

The PR title and description don't reflect the implementation.

[GromNaN]

The PR title and description don't reflect the implementation.

Sorry, I forgot to push.
I'm think extending AttributeExtension is better for the DX: no @internal, only a class to extend.

The only disadvantage is the exposed getClass() method.

[fabpot]

The commit message (description) is not accurate anymore :)

[fabpot]

Note that Symfony's LazyExtensionPass won't register subclasses of AttributeExtension lazily, because AttributeExtension::getLastModified() redeclares an AbstractExtension method that its check doesn't allow; it'll need a follow-up PR on Symfony as well.

[GromNaN]

All the extension methods are now final: getTokenParsers(), getNodeVisitors(), getOperators(), getExpressionParsers(), getGlobals(), and getLastModified(), in addition to getClass(), getFilters(), getFunctions(), and getTests(). AttributeExtension extends AbstractExtension again, since dropping it would be a BC break.

getGlobals() is final and throws, so a subclass cannot declare globals of its own. Since the method is final, the check does not need to reflect or load anything.

Environment::getExtension() now checks that the registered extension is AttributeExtension itself, as suggested.

The Symfony PR that relies on this change is open: symfony/symfony#67135.

[fabpot]

Dropping extends AbstractExtension is a BC break. I would add it back as removing it is not needed.

[fabpot]

This is bypassed if a subclass implements GlobalsInterface and has its own constructor.
What about checking is_a($extension->getClass(), GlobalsInterface::class, true) in ExtensionSet::addExtension() for AttributeExtension instances?

[GromNaN]

Updated: getGlobals() is now final and throws, so a subclass cannot declare globals of its own. Being final, the guard needs no reflection and does not load the class.

I first tried a check in ExtensionSet::addExtension(), but that would have forced loading the extension class even when it is never used.