Attributes are not rendered when showing the source code

The question we need to first to answer is: what is the desired outcome here? I think the attribute class itself already has an api page because it is a class, after all. Everything marked with that attribute should have a reference to this page and vice versa much like it happens with annotations. I imagine there would be an object-type of attribute, and an attributes reference type for buildReferencesSection / findReferences. This is a fairly trivial affair, ✨ support for OO hooks in classes Active shows how to parse attributes.

A much more difficult question would be: what happens to the arguments of the attribute? The issue linked above definitely needs it -- perhaps it should special case for now and this issue should just store attributes as separate docblock entries and punt on argument processing. A followup would probably parse the arguments, use https://github.com/nikic/PHP-Parser/blob/master/doc/component/Constant_expression_evaluation.markdown and reference any classes and constants used here.

So this code parses and stores an attribute reference with less than 30 lines of code. This is a good start. Needs display and test.

This works for classes, function, methods, class constants, class properties. The next issue will be parameter attributes as used for autowiring for example. That's a bit more work because there's no docblock entry to piggyback on to currently.

Trying to add some tests.

The failing tests in the CI are mostly unrelated to the changes here, and defo not a blocker.

This is how it's rendered in the tests:

The code looks good and it opens the door to some of the other critical issues.
RTBC.

Merged as this will unblock some other critical issues. We can create follow-ups if needed.

This will go with the next deployment round to api.drupal.org.

Thanks so much!