Rust: hash_map!()

hash_map_macro.rs

#[macro_export]
macro_rules! hash_map {
    ($($key:expr => $val:expr),* ,) => (
        $crate::hash_map!($($key => $val),*)
    );
    ($($key:expr => $val:expr),*) => {
        {
            let mut dict = ::std::collections::HashMap::new();
            $( dict.insert($key, $val); )*
            dict
        }
    };
}

hash_map_macro_2.rs

#[macro_export]
macro_rules! expr_count {
    () => (0);

    ($e:expr) => (1);

    ($e:expr; $($other_e:expr);*) => ({
        1 $(+ $crate::expr_count!($other_e) )*
    });

    ($e:expr; $($other_e:expr);* ; ) => (
        $crate::expr_count! { $e; $($other_e);* }
    );
}

#[macro_export]
macro_rules! hash_map {
    ($($key:expr => $val:expr),* ,) => (
        $crate::hash_map!($($key => $val),*)
    );

    ($($key:expr => $val:expr),*) => {
        {
            let capacity = $crate::expr_count!($($key);*);
            let mut dict = ::std::collections::HashMap::with_capacity(capacity);
            $( dict.insert($key, $val); )*
            dict
        }
    };
}

hash_map_macro_test.rs

#[test]
fn hashmap_macro() {
    let dict: HashMap<u64, u64> = hash_map!(0 => 0, 1 => 2);
    assert_eq!(dict.get(&0u64), Some(&0u64));
    assert_eq!(dict.get(&1u64), Some(&2u64));

    let dict: HashMap<u64, u64> = hash_map!{
        0 => 0,
        1 => 2,
    };
    assert_eq!(dict.get(&0u64), Some(&0u64));
    assert_eq!(dict.get(&1u64), Some(&2u64));
}