I feel like I'm getting closer, but I cannot figure out how to use the apply_to field correctly.
Both of these produce a warning:
apply_to = block
apply_to = any(block)
Unused attribute 'swift_attr' in '#pragma clang attribute push' region
If I combine this with objc_interface, I get no warning, but also don't get the bahavior I want. The attribute is correctly applied to the ObjC interfaces, but not to imported blocks.
apply_to = any(objc_interface, block)