Macro maybe_default 

macro_rules! maybe_default {
    (
        ($return_mode:ident, default),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
    (
        ($return_mode:ident, required),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
}

Generate either field_ref_expr or field_ty::default

Used when generating an input’s builder.