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KCFI needs to be able to tell which kind of `ReifyShim` it is examining in order to decide whether to use a concrete type (`FnPtr` case) or an abstract case (`Vtable` case). You can *almost* tell this from context, but there is one case where you can't - if a trait has a method which is *not* `#[track_caller]`, with an impl that *is* `#[track_caller]`, both the vtable and a function pointer created from that method will be `ReifyShim(def_id)`. Currently, the reason is optional to ensure no additional unique `ReifyShim`s are added without KCFI on. However, the case in which an extra `ReifyShim` is created is sufficiently rare that this may be worth revisiting to reduce complexity.