Derive Macro BuilderBits 

#[derive(BuilderBits)]
{
    // Attributes available to this derive:
    #[bitsize_internal]
    #[at]
    #[default]
}

Generate a compile-time builder (typed-builder style: each field exactly once).

Required fields have no #[default] and must be set before build. #[default(expr)] makes a field optional (same as in DefaultBits). reserved / padding are omitted, same as in new.