cannot assign to self.cart.parent_order_sign, which is behind a & reference self is a & reference, so the data it refers to cannot be written.

    1. fn main() {
    2.     let session = Session::new();
    3.     let _ = get_effprod();
    4. }
    6. pub fn get_effprod(& self) -> bool {
    7.     ...
    8.     self.cart.product_list.push(...); // 报错
    9.     ...
    10. }
    11. 正确的写法：&后面加上mut
    12. pub fn get_effprod(&mut self) -> bool {
    13.     ...
    14.     self.cart.product_list.push(...); // 报错
    15.     ...
    16. }