Can we rely on NonNull<T> having the same representation as *mut T for all non-null values of that type? In particular, can we rely on e.g. NonNull<std::ffi::c_void> being ABI compatible with C void* for non-null pointers?

It seems pretty likely that this should work, but the documentation doesn't guarantee it as far as I can see. The representation documentation for Nonnull only says that an extra byte won't be used for the discriminant in Option<Nonnull<T>>.