Add support for ~const item bounds

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -100,6 +100,15 @@ where
         })
     }
 
+    /// Assemble additional assumptions for an alias that are not included
+    /// in the item bounds of the alias. For now, this is limited to the
+    /// `implied_const_bounds` for an associated type.
+    fn consider_additional_alias_assumptions(
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
+        alias_ty: ty::AliasTy<I>,
+    ) -> Vec<Candidate<I>>;
+
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
@@ -594,6 +603,8 @@ where
             ));
         }
 
+        candidates.extend(G::consider_additional_alias_assumptions(self, goal, alias_ty));
+
         if kind != ty::Projection {
             return;
         }