This is way more complicated that it needs to be. Use code.google.com/p/go.tools/go/types to process the AST and you'll get basically the same information that the compiler sees. With that you can generate the code pretty easily. For comparison, our JSON code generator implementation is just ~350 lines and supports having different JSON representations for the same type, which vary depending on the container type.

Also, if you want to make the serialization faster you need to understand exactly what makes encoding/json slow (hint: is not only reflect) and remove all reasonable bottlenecks. You state that megajson does not support the MarshallJSON interface like that's a bad thing, but I'm pretty sure that's deliberate because it's indeed a feature. When encoding/json encounters a type which implements MarshalJSON it does the following:

1 - Call MarshallJSON to obtain its JSON representation as []byte 2 - Validate the produced JSON using a slower-than-the-bad's-guy-horse function based state machine which processes each character individually 3 - Copy the []byte returned by MarshallJSON to its own buffer

Unsurprisingly (after reading encoding/json's code, of course) having a MarshalJSON method is way slower than letting encoding/json use reflection if the JSON you're generating is anything but trivial and without almost any nesting, because it avoids extra allocations, copies and the validation step.