It is using rustc-serialize crate in an old version that doesn't work any more. The whole crate is deprecated, and the current version is 0.3.24.
That old version has a bug that was accidentally allowed by the compiler a couple of years ago. In traits functions are supposed to declare arguments as name: type or just type, but pattern: type used to work by chance, since trait declarations didn't have a body, so the pattern was never executed.