Macro codec_struct 

macro_rules! codec_struct {
    ($ty:ident { $($field:ident),* $(,)? }) => { ... };
    ($ty:ident { $($field:ident),* $(,)? } check $check:path) => { ... };
    ($ty:ident { $($field:ident),* $(,)? } default { $($left:ident),* $(,)? } check $check:path) => { ... };
}

Encode and Decode from one field list; default names fields left out of the encoding and decoded as their type’s default; check names a fn(&T) -> Result<(), String>.