extra::json doesn't support using Encodable to encode straight to a json value

[huonw]

If one wants to convert an Encodable type to a Json value, then one has to do something like (i.e. encode as JSON to a string and then parse that string):

let foo = something_to_convert_to_json;

let s = do std::io::with_str_writer |w| {
    foo.encode(&mut extra::json::Encoder(w));
};
let json = match extra::json::from_str(s) {
    Ok(j) => j,
    Err(_) => fail!("Rust output some json it doesn't understand itself.")
};

[graydon]

Yes, this struck me as very weird

[catamorphism]

Triage bump

[seanmonstar]

So, I started to work on this, trying to do something like this:

impl <A:Encodable> ToJson for A {
  fn to_json(&self) -> ~Json {
    ToJsonEncoder::new().json_encode(self)
  }
}

struct ToJsonEncoder {
// ...
}
impl serialize::Encoder for ToJsonEncoder {
// ...
}

The ToJsonEncoder would convert to a Json object, instead of to strings. However, since a user can write their own implementation of Encodable, it's not guaranteed that an Encodable will emit all that is needed to make JSON.

For instance:

struct Foo {
  bar: ~str,
  baz: uint
}
impl<E: Encoder> Encodable<E> for Foo {
  fn encode(&self, e: &mut E) {
    e.emit_str(self.bar);
    e.emit_uint(self.baz);
  }
}

This Foo has defined its own way to encode itself, and a ToJsonEncoder will not be able to format a String + Number as Json. The ToJsonEncoder would need to keep some state, and fail!() if the Encodable emits values in a JSON-illegal order.

This leaves me wondering if extra::serialize gives too much freedom in how it can be implemented.

[alexcrichton]

cc @nick29581, could this move to rust-lang/rustc-serialize?