The obvious answer to that is then "any trait consisting only of methods which take a Self parameter". But given (a) how confusing Rust's orphan rules already a

The obvious answer to that is then "any trait consisting only of methods which take a Self parameter". But given (a) how confusing Rust's orphan rules already are to the average programmer, and (b) how easy it is to implement your own Never type if needed, I think it's best to just have Never implement the main built-in traits (Copy, Clone, etc.) and leave it at that.