Replace weird handling of edge case with panic

Author: camelid

src/librustdoc/clean/mod.rs

@@ -1704,8 +1704,8 @@ fn clean_ty<'tcx>(this: Ty<'tcx>, cx: &mut DocContext<'tcx>, def_id: Option<DefI
             ImplTrait(bounds)
         }
 
-        ty::Closure(..) | ty::Generator(..) => Tuple(vec![]), // FIXME(pcwalton)
-
+        ty::Closure(..) => panic!("Closure"),
+        ty::Generator(..) => panic!("Generator"),
         ty::Bound(..) => panic!("Bound"),
         ty::Placeholder(..) => panic!("Placeholder"),
         ty::GeneratorWitness(..) => panic!("GeneratorWitness"),