json::to_pretty_writer output should be consistent

[ozten]

Currently when you build up a Json object and write it to a file using json::to_pretty_writer, the order of the keys in objects is random.

There are two possible improvements that come to mind:

1. Write objects based on the natural sort order of the keys

2. Write objects based on the order in which keys were added to the object

Most mature JSON libraries I've used do #2, but even #1 would be an acceptable first step to getting deterministic serialized output.

[alexcrichton]

Are you using rust master? Internally json now uses a TreeMap instead of a HashMap so I figured they'd be deterministic unless they're getting purposefully scrambled...

[ozten]

Thanks! (No I was working on 0.7)

[alexcrichton]

Regardless, thanks for the report anyway! If you're trying out rust, I would recommend using the master branch anyway. It's got boatloads of fixes from 0.7 and it's almost always stable.

[erickt]

@ozten: I caution you on relying too heavily on this feature though. The json spec explicitly states that the key/value pairs in an object are unsorted. Right now Rust's json.rs library sorts all Maps before generating a json string, but we may skip that step in the future to speed up json::to_writer. However if we do do that, I believe we'll be able to keep json::to_pretty_writer sorting the maps. That function shouldn't be used on a critical path, so we can afford to spend some cycles making the output human readable.