json decoding should fail with a more descriptive error

[erickt]

Right now decoding will do a fail ~"not a list" if it tries to deserialize the wrong type. We should give some hints as to the type of the value it found.

[catamorphism]

Seems non-critical for 0.7. Nominating for milestone 3 (feature complete).

[graydon]

declined, noncritical

[msullivan]

Still there, still not critical.

[metajack]

visiting for triage. I'm not sure exactly how to reproduce this but the fail! code is still present. It's in read_seq.

[huonw]

Triage: reproduced by

#[feature(managed_boxes)];

extern mod extra;
use extra::serialize::Decodable;
use extra::json;

#[deriving(Decodable)]
struct Foo {
    x: ~[Foo]
}

fn main() {
    let mut x = json::Decoder(json::from_str(r#"{"x": 1}"#).unwrap());
    let _: Foo = Decodable::decode(&mut x);
}

I'm going through extra::json with a goal to improve most of these error messages.