You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.rs
    path of file: ./repos/rust-bible/part1/link_demo/src
    
    the code of the file until where you have to start completion: use std::cell::RefCell;
use std::rc::Rc;
use std::rc::Weak;

// 单向引用计数链表实现
// 这是一种相当实用的结构体应用形式，每个 append 方法通过将数据添加到头部的方式
// 运行，这意味着我们不必使用引用和实际的列表引用就可以确保不变性。
#[derive(Debug)]
struct Node<T> {
    next: Option<Rc<Node<T>>>,
    data: T,
}

#[derive(Debug)]
struct LinkedList<T> {
    head: Option<Rc<Node<T>>>,
}
impl<T> LinkedLi
    
//     fn append(&self, data: T) -> Self {
//         let new_node = Rc::new(LinkedListNode {
//             data: data,
//             next: self.head.clone(),
//             prev: None,
//         });
//
//         match self.head.clone() {
//             Some(node) => {
//                 // 这一行是问题的^^^^^^^^^ cannot assign
//                 node.prev = Some(Rc::downgrade(&new_node));
//             }
//             None => {}
//         }
//
//         Self {
//             head: Some(new_node),
//         }
//     }