Macro map_append 

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

append items to a map

let mut m = HashMap::new();
map_append! { m;
    ka => va,
    kb => vb,
}

equivalent to

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