Implement `FromZeroes` for `Option` types?

[joshlf]

Some Option types have known layouts/bit validity (e.g., Option<&T> where T: Sized). Are these actually guaranteed, or just "in practice everyone knows that's how the compiler does it"? If it's the former, and we know that None is represented as a sequence of zeroes, then it should be sound to implement FromZeroes for these types.

Task:

• Find types, T, for which we can implement FromZeroes for Option<T> (EDIT: Here's a breadcrumb)
• For each such type, ensure that we can make a bulletproof soundness argument based on the Rust reference and the standard library documentation
• Put up a PR to emit the appropriate impl.

I've submitted rust-lang/rust#115333 to clarify the representation of Options and hopefully unblock more of this issue.