Result/Option layout guarantee clarifications
- It seems worth spelling out that this guarantee allows particular transmutes.
- After the `Result` section was written, the `Option` section it referenced gained *more* guarantees, saying that `None` is represented as `[0u8; N]`. Those guarantees were not meant to apply to `Result`. Make the `Result` section more self-contained to make this more clear.
- "Type has no fields" is unclear since there is no general definition of what the fields of some arbitrary type are. Replace that by a more accurate description of the actual check implemented [here](https://github.com/rust-lang/rust/blob/e379c7758667f900aaf5551c4553c7d4c121e3e1/compiler/rustc_lint/src/types.rs#L828-L838).
r? `@traviscross`
Matthias Krüger
32d21cb0b2