Ultimately, at least at the time, there was no other option that had better compatibility. Now I suppose you could use MaybeUninit<#[repr(C)] enum> or something. Realistically I don't have much confidence in rustc properly matching enum sizes with C implementations that don't follow the typical behaviour here, so I doubt that would be worth it.

I guess the linux kernel is generic enough that it might end up on some such platforms, I guess in that context I'd just bite the bullet and accept undefined behavior if passed an unexpected enum variant, it's not like that should happen in a coherent system like the linux kernel (vs my use case which was wrapping some library that definitely might chose to add enum variants).