A clarification: Rust only makes this guarantee for certain types[1] which you would expect to be represented by a simple pointer under the hood (e.g., Box<U>, &T, function pointers) and for other types which can never be all 0.

Making this optimization for something like Option<u8> is, naturally, impossible.

(I assume you are aware of this and were implicitly referring to this optimization.)

[1]: https://doc.rust-lang.org/std/option/#representation