Macro declare_structs 

macro_rules! declare_structs {
    (
        vis: [ $( $VIS:ident )* ],
        meta: [ $( #[$META:meta] )* ],
        spec: $BUILDER:ident $MODE:tt $STRUCT:ident,
        fields: {
            $(
                {
                    vis: [ $( $FIELD_VIS:ident )* ],
                    meta: [ $( #[$F_META:meta] )* ],
                    spec: $F_NAME:ident: $F_TY:ty
                } $(,)*
            )*
        }
    ) => { ... };
}

Declares the type struct and its corresponding builder struct.