Add roundtrip testing and bump format version

src/librustdoc/json/mod.rs

@@ -242,7 +242,7 @@ impl<'tcx> FormatRenderer<'tcx> for JsonRenderer<'tcx> {
                     )
                 })
                 .collect(),
-            format_version: 4,
+            format_version: 5,
         };
         let mut p = self.out_path.clone();
         p.push(output.index.get(&output.root).unwrap().name.clone().unwrap());