Macro hashmap 

macro_rules! hashmap {
    ($($key:expr => $value:expr),*$(,)?) => { ... };
}

Handy macro to construct a hashmap

Example:

let _: HashMap<_, HashMap<_, _>> = hashmap! [
    "key" => hashmap![],
    "key2" => hashmap![
        "inner key" => "inner value",
    ],
];