Adding any trait to any type is a breaking change

While reading through When matching on enums, read the discriminant even if there's only a single variant by meithecatte · Pull Request #154756 · rust-lang/rust · GitHub, I noticed that it mentioned that in current Rust, there is a semver compatibility hazard in which removing #[non_exhaustive] could cause a downstream library to stop compiling. The intent of that pull request is to remove the hazard. However, it made me realise that the same principle could be used to determine during const-ev...