Clear some more todos

Author: tiif

compiler/rustc_privacy/src/lib.rs

@@ -156,7 +156,7 @@ where
             }
             ty::ClauseKind::ConstEvaluatable(ct) => ct.visit_with(self),
             ty::ClauseKind::WellFormed(term) => term.visit_with(self),
-            ty::ClauseKind::UnstableFeature(_) => todo!(), // TODO: maybe we need a visit with function here too?
+            ty::ClauseKind::UnstableFeature(_) => V::Result::output(),
         }
     }
 

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -639,7 +639,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         }
                     }
 
-                    ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(_)) => todo!(), // TODO: is this for error reporting?
+                    ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(symbol)) => {
+                        // TODO: write a test that can trigger this path.
+                        span_bug!(span, "unstable feature {:?} not enabled", symbol);
+                    } 
 
                     // Errors for `ConstEvaluatable` predicates show up as
                     // `SelectionError::ConstEvalFailure`,