rust学习中遇到的问题

Question

#[derive(Debug, Ord, PartialOrd, Eq, PartialEq)]
pub struct TreeNode {
    pub val: i32,
    pub left: Option<Rc<RefCell<TreeNode>>>,
    pub right: Option<Rc<RefCell<TreeNode>>>,
}

impl TreeNode {
    pub fn new(val: i32) -> TreeNode {
        TreeNode {
            val,
            left: None,
            right: None,
        }
    }
}

pub fn traverse_post(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
    let mut vec = vec![];
    dfs(root, &mut vec);
    return vec;
    fn dfs(node: Option<Rc<RefCell<TreeNode>>>, list: &mut Vec<i32>) {
       if  let Some(x)=node {
           dfs(x.borrow().left.clone(), list);
           list.push(x.borrow().val);
           dfs(x.borrow().right.clone(),list);
       }
    }
}

这段代码在dfs中，x.borrow()那里一直报错，让我指定type annotation ，有人知道这该怎么解决吗，谢谢~