add fixme

compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs

@@ -410,7 +410,10 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
                 }
 
                 ty::Alias(_, _) | ty::Param(_) | ty::Placeholder(..) => {
-                    // FIXME(ptr_metadata): It would also be possible to return a `Ok(Ambig)` with no constraints.
+                    // This is the "fallback impl" for type parameters, unnormalizable projections
+                    // and opaque types: If the `self_ty` is `Sized`, then the metadata is `()`.
+                    // FIXME(ptr_metadata): This impl overlaps with the other impls and shouldn't
+                    // exist. Instead, `Pointee<Metadata = ()>` should be a supertrait of `Sized`.
                     let sized_predicate = ty::TraitRef::from_lang_item(
                         tcx,
                         LangItem::Sized,