CTFE Undefined Behavior

[ehuss]

Some undefined behavior found during CTFE is now treated as an error as of https://github.com/rust-lang/rust/pull/86194. I think https://github.com/rust-lang/reference/blob/master/src/const_eval.md should detail which errors are detected, but Ralf indicated in https://github.com/rust-lang/rust/pull/86194#issuecomment-864803029 that they don't want to make any guarantees about which UB is detected. I can understand that, but it also leaves the language a little under specified. I'm not sure how to resolve that.