Trying to return a union - Help needed about capnpc-rust HOT 1 OPEN

enum SquareOrCircle<'a> {
   Square(Square::Builder<'a>),
   Circle(Circle::Builder<'a>),
}

fn get_square<'a>(messge: &'a mut capnp::message::Builder) -> capnp::Result<SquareOrCircle<'a>> {
   let mut root: Shape::Builder = message.get_root()?;
   match root.which()? {
     Shape::Square(square) => SquareOrCircle::Square(square?),
     Shape::Circle(circl) => SquareOrCircle::Circle(circle?),
  }
}