pyclass: move flags to PyClassImpl

[davidhewitt]

This is a (mostly internal) refactoring to move some pieces of PyTypeInfo which aren't needed for anything other than #[pyclass] onto the PyClassImpl trait.

At the same time I cleaned up the old FLAGS constant which still had unneeded entries for e.g. weakref.

[kngwyu]

Thanks!
Looks like reasonable changes, but why PyClassImpl instead of PyClass?
I understand that PyClassImpl is for collecting methods by pseudo-specialization trick, so I don't see the necessity to place these flags under PyClassImpl.

[davidhewitt]

Looks like reasonable changes, but why PyClassImpl instead of PyClass?

Overall I see PyClassImpl as the pieces of PyClass which we don't consider stable API (it's #[doc(hidden)]). So unless we are sure this is useful for users and it's not going to need to change, it should go on PyClassImpl.

To reduce the complex trait hierarchy I was thinking to merge PyClassAlloc into PyClassImpl also.

[kngwyu]

How about moving some flags under PyClassAlloc?
They are actually related only to allocation.

[davidhewitt]

How about moving some flags under PyClassAlloc? They are actually related only to allocation.

Only the IS_SUBCLASS flag is used during the default_new implementation - the others are used in py_class_flags which isn't anything to do with allocation.

I'll have a go with a follow-up commit to try refactoring PyClassAlloc a bit. 👍

[davidhewitt]

I've pushed a commit which removes the #[inline] and puts PyTypeInfo bound onto PyClassAlloc instead of PyClassImpl. Also renamed T::DESCRIPTION to T::DOC for simplification.

I looked at doing further work to PyClassAlloc in this PR but realised there are a lot of complications with that trait as-is, especially with #991.

So I will try and push something experimental in a separate PR later.

[kngwyu]

Nice, thank you!