HashMap 出现结构错误：不能将“&”引用中的数据借用为可变

Question

I have the following code:

struct Node{
    node_map: HashMap<char, Node>,
    value: Option<i32>,
}

struct Trie {
    root: Node,
}

impl Trie {
    fn new() -> Trie {
        Trie {
            root: Node{
                node_map: HashMap::new(),
                value: None,
            },
        }
    }

    fn find(&self, key: &String) -> Option<&Node> {
       // Returning some Option<&Node>
    }

    fn delete(&mut self, key: &String) -> Option<i32> {
        // extract code snippet
        let mut search_node = self.find(key);
        if search_node.is_some() {
            search_node.unwrap().node_map.remove(& 'x');
        }
        None
    }
}

Rust complains the error under search_node.unwrap().chs part: cannot borrow data in a "&" reference as mutable

So I understand that the find function returns Option<&Node> so when unwrapping at the above line, I get the reference to Node .

Attempts :

• I tried to dereference the node by: *search_node.unwrap().node_map.remove(& 'x'); or *(search_node.unwrap()).node_map.remove(& 'x'); but it still throws error.
• I followed another answer here and tried to make node_map mutable like:

 struct Node<'a> {
     node_map: &'a mut HashMap<char, Node<'a>>,
     value: Option<i32>,
 }

But then I got complain about lacking lifetime several places. One particular place I dont know how to add is in the new function.

Please let me know how to solve the original issue or how to add the appropriate lifetime.

Answer 1

The problem is that find returns an (optional) immutable reference, but then you try to mutate it later. For that reason, you'll probably want to add a method find_mut with signature

fn find_mut(&mut self, key: &str) -> Option<&mut Node>

(I changed the key argument to &str because it's discouraged to take &String as an argument )

Another stylistic thing: you should use if let instead of checking that search_node is some and then unwrapping.

if let Some(search_node) = self.find_mut(key) {
    search_node.node_map.remove(&'x');
}