I am giving C++ a Box containing a trait object, and then when I get that Box<dyn Trait> back from C++, I want to try to downcast it into a concrete type. However, I can't do it safely because the types I'm using are not (and cannot be) 'static, so I can't use Any::downcast_ref. Is there another way to assert that the cast that I'm doing is valid (ex.: is there a hack I can use to generate something like a TypeId that ignores lifetime parameters)?