Add debug_assert comment

Co-authored-by: Camille Gillot <[email protected]>

Author: spastorino

Diff for: compiler/rustc_ast_lowering/src/lib.rs

@@ -479,6 +479,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         let item = f(self);
         debug_assert_eq!(def_id, item.def_id());
+        // `f` should have consumed all the elements in these vectors when constructing `item`.
         debug_assert!(self.impl_trait_defs.is_empty());
         debug_assert!(self.impl_trait_bounds.is_empty());
         let info = self.make_owner_info(item);