Yokeable Derive: require Sized on certain types

[sffc]

Similar to #4657

For now only on structs with no explicit lifetimes

[Manishearth]

Is this really necessary? We should be able to support this, yes?

#[derive(Yokeable)]
struct MultiVec<'a, T: ?Sized> {
   x: VarZeroVec<'a, T>,
   y: VarZeroVec<'a, T>,
}

It seems you actually want where Self: Sized?

But really it's unclear why this is necessary in the first place: generic parameters are by default Sized and need to be opted out of, and if someone chooses to opt out of it we should respect that. It's extremely rare that T: Sized is necessary on an impl.

[Manishearth]

Okay, yeah, for now please generate where Self: ?Sized or where <output type>: Sized. In the long run we should integrate this into yoke(may_borrow) from #3771

[Manishearth]

Either that, or enforce FieldThatUsesGenerics: Sized for all fields that involve generics.