Macro const_map 

macro_rules! const_map {
    ($V:ty) => { ... };
    ($V:ty, fixed) => { ... };
    ($V:ty, $H:ty) => { ... };
    ($V:ty, $H:ty, fixed) => { ... };
}

Expands to ConstMap<<V as CollectionMeta>::SelfId, { size_of::<V>() } [, H, D]>.