macro in rust

hashmap_macro.rs

macro_rules! hashmap {
    ($($key: expr => $val: expr), *) => {{
        let mut map = ::std::collections::HashMap::new();
        $(map.insert($key, $val);)*
        map
    }};
}

fn main() {
    let counts = hashmap!['A' => 0, 'C' => 1, 'G' => 2, 'T' => 3];

    for (k, v) in counts {
        println!("{:?} => {:?}", k, v);
    }
}