Refactor Fold to take by value

[detrumi]

Currently, the Folder trait takes folded values by reference. As part of the shared type library plan, the fold traits should take their folded values by value, to match the TypeFolder trait in rustc.

For example, this

fn fold_ty(&mut self, ty: &Ty<I>, outer_binder: DebruijnIndex) -> Fallible<Ty<TI>>

should be changed to this:

fn fold_ty(&mut self, ty: Ty<I>, outer_binder: DebruijnIndex) -> Fallible<Ty<TI>>

Same thing with the other folding functions.
Lastly, code that calls folding functions should pass the values by value, instead of by reference.

[lnicola]

Should the Fold implementations and trait be updated to also pass by value, or can they copy/clone?

impl<I: Interner, TI: TargetInterner<I>> Fold<I, TI> for Const<I> {
    type Result = Const<TI>;

    fn fold_with<'i>(
        &self,
        folder: &mut dyn Folder<'i, I, TI>,
        outer_binder: DebruijnIndex,
    ) -> Fallible<Self::Result>
    where
        I: 'i,
        TI: 'i,
    {
        folder.fold_const(self /* <-- this */, outer_binder)
    }
}

Clone works, since they all use interned values, but I guess the point is to make this change everywhere?

[detrumi]

Yes, because fold_with should also take self by value. The Fold and SuperFold traits should also be updated here.

[lnicola]

Sorry for the noob questions, but is it all right to add Clone bounds to the T in u_canonicalize and map_from_canonical? They seem to call fold_with on borrowed values.

[detrumi]

Sorry for the noob questions, but is it all right to add Clone bounds to the T in u_canonicalize and map_from_canonical? They seem to call fold_with on borrowed values.

You mean the calls like this?

            .fold_with(
                &mut UMapFromCanonical {
                    interner,
                    universes: self,
                },
                DebruijnIndex::INNERMOST,
            )

I think you can remove the &mut and have it work, no need to clone here.
Oh wait, that doesn't work, because it's the self that needs to be cloned. Nvm

[lnicola]

I tried to make this work, but couldn't manage. I left some notes in #644 (comment) and the branch is up, in case someone else wants to try.