add FIXME's

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -149,6 +149,13 @@ fn satisfied_from_param_env<'tcx>(
             } else if let ty::ConstKind::Expr(e) = c.kind() {
                 e.visit_with(self)
             } else {
+                // FIXME(generic_const_exprs): This doesn't recurse into `<T as Trait<U>>::ASSOC`'s substs.
+                // This is currently unobservable as `<T as Trait<{ U + 1 }>>::ASSOC` creates an anon const
+                // with its own `ConstEvaluatable` bound in the param env which we will visit separately.
+                //
+                // If we start allowing directly writing `ConstKind::Expr` without an intermediate anon const
+                // this will be incorrect. It might be worth investigating making `predicates_of` elaborate
+                // all of the `ConstEvaluatable` bounds rather than having a visitor here.
                 ControlFlow::CONTINUE
             }
         }