use serde_json::{json, Value};

fn main() {
    let def = json!({ "a": 1, "b": "bad" });
    let new = json!({ "b": "good" });

    if let (Value::Object(new), Value::Object(mut def)) = (new, def.clone()) {
        def.extend(new);
        println!("{}", Value::Object(def).to_string());
    }
}