extra::json: Remove or rework ToJson

[bluss]

There are two ways to encode values into json in string form, and they can provide different results or be wrong:

• ToJson transforms a rust value to Json, which can be encoded by a serialize::Encoder
• json::Encoder implements the Encoder trait, so you can give it arbitrary values, as long as they implement Encodable, and it will output json, or json-like text.

We need to provide a single method of encoding to Json, and make sure we use serialize correctly if we use it.

The ToJson trait expresses the restriction that only maps with string keys can be encoded to Json. This is easy to step around by using the other json encoding method, which results in invalid json.

[JeffMurray]

I use to_json() a lot and find it quite handy when packaging up rust native data types for Json streaming.

I think if extra::json::Encoder ( and PrettyEncoder ) could be limited to encoding Json values the problem would be solved.

Other than that I find both the encoders pretty useful.

Thanks.

[brandonson]

I think this may just point to a need for clarification in the docs. I'd expect ToJson to perform a conversion to json that creates a very specific representation of whatever type implements it. On the other hand, I expect Encoder to perform a conversion that creates a more general representation which is therefore available to more types.

Unfortunately, the difference is not very clear in the docs, and ToJson and Encoder/Decoder were indeed very intertwined, and may still be.

[bluss]

I think extra::json is abusing the serialization API. Json implements Encodable, but not Decodable. The impl of Encodable is not valid in serialization context, since it does not emit the enum tag. This abuse must be eith reworked, or hidden inside the module (by wrapping it in a private newtype?)

[flaper87]

Triage bump. By checking the code, this is still the case. Decodable doesn't seem to be implemented and ToJson hasn't been reworked.

This issues seems legit to me. I don't think removing ToJson is the way to go, I'd opt for re-factoring it.