Not a zig person, but the post you linked has a good counter example on why the proposal for adding cancellation to the interface is bad: You would be forcing cancellation semantics on interfaces that might not want nor be able to provide one. Forcing a new method because the libraries are not well designed does not seem right for me. Maybe another solutions is available, but that is not one.

I get the pain, but the answer from Scott Redig totally makes sense. I am not sure what is worse: Libraries swallowing cancelled() or libraries lying that they support cancelled.