I'm honestly not exactly sure what you're talking about, but the fundamental limit for atomic writes is usually the register-size for a CPU which is usually 64 or 32 bits. Considering enum variants are often larger than a single register in size, I don't see how atomic writes would ever be sane expectation.
I see. Yeah, you would either have to add the tagging to the upper bits of the pointer itself or concede that updates to a tagged type is not atomic. I feel like the latter is fine in most every scenario I've encountered in Rust (thanks to borrow checker rules) but in other languages the same cannot be said.
