Consider requiring all ULEs to be Copy

[sffc]

Part of #1082

We currently have the Copy bound on the impl ZeroVec. However, it may make more sense to move it to the trait ULE itself. I know there is a debate about when to put bounds on the impl and when to put them on the trait.

Motivating case: #[derive(Clone)] does not seem to work on structs containing a ZeroVec field. See e.g. #1161

Needs approval from:

• @Manishearth
• @iainireland

[Manishearth]

Yeah that works for me.

[Manishearth]

The debate is usually for when T itself is being used, when an associated type is used then people need to bound on a trait anyway. ZeroVec already requires ULE on the definition (it has to!) so adding a bound to ULE seems fine

[Manishearth]

Conceptually I think that AsULEs can also be Copy, and we can stop using references in their methods

[iainireland]

Sounds good to me