Why does ConditionallySelectable require Copy? about subtle HOT 3 CLOSED

The Copy trait is a marker that the value can be duplicated by copying bits, whereas Clone can perform arbitrary logic (a "deep copy"). We require Copy because it doesn't really make sense to do constant-time operations on non-Copyable types.

Your example does use Copy, just not explicitly (e.g., in the line let t = mask & (*pi ^ *qi)).

As you note in the edit, the provided implementations are there for implementation convenience, but implementors can override them if they wish.

from subtle.

Yes, it's a marker trait, but it also (when implemented e.g. on newtypes) cause the compiler to change behaviour and implicitly copy things around, eliminating compile time checks. So not just "can" but also "does". (Yes, in my example copies are also done, but of individual entries, not the entire type.)

As noted, I assume all your current use cases that are Copy implement Clone also, in exactly the same way - no deep copy. Do they?

As it stands, I have the choice of making my types Copy or not using your library. Do you have some (heuristic...) argument on why I should do the former?

Also, do you foresee actual harm in replacing the Copy with a Clone bound (given it would trivially extend the applicability of the library)?

from subtle.

The heuristic reason why your types should be Copy is that constant-time operations implemented using bit operations should be performed on types which are bit-copyable.

You are of course free to not use this library if it doesn't suit your needs, but I don't think we will be relaxing the Copy bound.

from subtle.