Allow self in const generics

[bb1yd]

Fixes #149203

[fmease]

IINM you're now accepting this code despite N depending on T which shouldn't be allowed w/o enabling GCPT.

impl<T> Wrap<T> {
    fn f<const N: Self>() {}
}

struct Wrap<T>(T);

View changes since the review

[bb1yd]

Is there a way to check if Self depends on generics?

[fmease]

I'm not super well-versed in name resolution, so I don't know if there's a good way to do this in rustc_resolve that still nicely leverages the existing rib structure. Boxy probably has an idea.

Calling type_of here on the LocalDefId found in the SelfTyAlias is probably too prone to cycles or hangs; looking at hir_node corresp. to the LocalDefId might be an option but that might not integrate with the visitor.

Naively speaking, I would do it in HIR ty lowering by inspecting the type as returned by type_of and by reusing+generalizing check_assoc_const_binding_type but that's probably also not super ideal. I haven't read a lot of the new mGCA code in HIR ty lowering, maybe there's already a function for that (that's similar to check_assoc_const_binding_type?)?

[bb1yd]

@rustbot ready