Macro derive_merge 

macro_rules! derive_merge {
    (
        $(#[$attr:meta])*
        $vis:vis struct $struct_name:ident {
            $(
                $(#[$field_attr:meta])*
                $field_id:ident: $field_type:ty,
            )+
        }
    ) => { ... };
}

Derive the Mergeable trait for structs composed entirely of Mergeable fields.