b263981fd7
improve error message for disallowed ptr-to-int casts in const eval Improves an error message as [suggested](https://github.com/rust-lang/rust/issues/80875#issuecomment-762754580) in #80875. Does the wording make enough sense? I tried to follow precedent for error message style while maintaining brevity. It seems like the rest of the `ConstEvalErrKind::NeedsRfc` error messages could be improved as well. I could give that a go if this approach works. Closes #80875