Macro json 

macro_rules! json {
    ($($json:tt)+) => { ... };
}

Construct a serde_json::Value from a JSON literal.

let value = json!({
    "code": 200,
    "success": true,
    "payload": {
        "features": [
            "serde",
            "json"
        ],
        "homepage": null
    }
});

Variables or expressions can be interpolated into the JSON literal. Any type interpolated into an array element or object value must implement Serde’s Serialize trait, while any type interpolated into a object key must implement Into<String>. If the Serialize implementation of the interpolated type decides to fail, or if the interpolated type contains a map with non-string keys, the json! macro will panic.

let code = 200;
let features = vec!["serde", "json"];

let value = json!({
    "code": code,
    "success": code == 200,
    "payload": {
        features[0]: features[1]
    }
});

Trailing commas are allowed inside both arrays and objects.

let value = json!([
    "notice",
    "the",
    "trailing",
    "comma -->",
]);