Macro declare_type 

macro_rules! declare_type {
    ($ty:ident) => { ... };
    ($datatype:path, $ty:ident , $( flags=[$($flag:ident),*], )?
    {
        $( fn to_usize($eusize:ident: usize) -> $eusize_self:ty $to_usize:block )?
        $( fn from_usize($eusize2:ident: $eusize_self2:ty) -> usize $from_usize:block )?
    }
        ) => { ... };
    ($datatype:path, $ty:ident<$lt:lifetime>, builder: $builder:ty, $( flags=[$($flag:ident),*], )?
    {
    }) => { ... };
}

This macro is used to declare serialization traits for a type.