Proposal hashmap! macro with options

maplit-hashmap-options.rs

macro_rules! hashmap {
    (@opts nocap $hash:tt capacity: $cap:expr $(, $($tail:tt)*)?) => (hashmap!(@opts $cap $hash $($($tail)*)?));
    (@opts $cap:tt nohash S: $hash:ty $(, $($tail:tt)*)?) => (hashmap!(@opts $cap (<$hash>::default()) $($($tail)*)?));
    (@opts $cap:tt nohash hasher: $hash:expr $(, $($tail:tt)*)?) => (hashmap!(@opts $cap $hash $($($tail)*)?));
    (@opts $cap:tt $hash:tt $name:ident: $($tail:tt)*) => (compile_error!(concat!("Invalid option: ", stringify!($name))));

    (@opts $cap:tt $hash:tt) => (hashmap!(@init $cap $hash));
    (@opts $cap:tt $hash:tt $($key:expr => $value:expr),+ $(,)?) => {
        {
            let mut map = hashmap!(@init $cap $hash $($key)*);
            $(
                let _ = map.insert($key, $value);
            )*
            map
        }
    };
    (@opts $($tail:tt)*) => (compile_error!("Invalid syntax"));

    (@init nocap nohash) => (::std::collections::HashMap::new());
    (@init nocap $hash:tt) => (::std::collections::HashMap::with_hasher($hash));
    (@init nocap $hash:tt $($t:expr)+) => {
        {
            let capacity = hashmap!(@count $($t)*);
            hashmap!(@init capacity $hash)
        }
    };
    (@init $cap:tt nohash $($tail:tt)*) => (::std::collections::HashMap::with_capacity($cap));
    (@init $cap:tt $hash:tt $($tail:tt)*) => (::std::collections::HashMap::with_capacity_and_hasher($cap, $hash));

    (@count $($t:tt)*) => (<[()]>::len(&[$(hashmap!(@unit $t)),*]));
    (@unit $t:tt) => (());

    ($($t:tt)*) => (hashmap!(@opts nocap nohash $($t)*));
}