allow deserialization to work with missing attributes

[nicolai86]

When parsing JSON I'd like to only parse parts of the payload, e.g.

given JSON like { 'x': 32, 'y': 12 } I'd expect serde to deserialize it using the following struct:

#[derive(Serialize, Deserialize)]
#[allow(non_snake_case)]
struct Point {
    y: f32,
}

However, this fails with an UnknownField("x") error.

This is important if you are only interested in parts of the payload, or if working with dynamic payload.

[jgallagher]

I think this is a dupe of #44, although that issue mentions adding an annotation to allow skipping unknown fields. I think this issue's request should be the default; that's how most JSON parsers in other languages (and rustc-serialize) work.

[softprops]

+1

[dovahcrow]

+1

[erickt]

This was implemented in #204, and will be a part of serde 0.7.0 when it comes out, hopefully within the week!