Macro map 

macro_rules! map {
    { } => { ... };
    { $($k:expr => $v:expr),* $(,)? } => { ... };
    { let $m:expr; $($k:expr => $v:expr),* $(,)? } => { ... };
}

new a HashMap<K, V>

map! {
    ka => va,
    kb => vb,
}

equivalent to

{
    let mut m = HashMap::new();
    m.insert(ka, va);
    m.insert(kb, vb);
    m
}

---

map like

map! { let BTreeMap::new();
    ka => va,
    kb => vb,
}

equivalent to

{
    let mut m = BTreeMap::new();
    m.insert(ka, va);
    m.insert(kb, vb);
    m
}