How should I restructure my graph code to avoid an “Cannot borrow variable as mutable more than once at a time” error?

Question

I have a simple graph that successfully compiles:

use std::collections::HashMap;

type Key = usize;
type Weight = usize;

#[derive(Debug)]
pub struct Node<         


        

Answer 1

I solved problem by using std::rc::Rc:

use std::collections::HashMap;
use std::rc::Rc;

type Key = usize;
type Weight = usize;

#[derive(Debug)]
pub struct Node {
    key: Key,
    value: T,
}
impl Node {
    fn new(key: Key, value: T) -> Self {
        Node {
            key: key,
            value: value,
        }
    }
}

#[derive(Debug)]
pub struct Graph {
    map: HashMap>,
    list: HashMap>>, // <-- Changed
    next_key: Key,
}
impl Graph {
    pub fn new() -> Self {
        Graph {
            map: HashMap::new(),
            list: HashMap::new(),
            next_key: 0,
        }
    }

    pub fn add_node(&mut self, value: T) -> Rc> {
        // <-- Changed
        let key = self.get_next_key();
        let node = Rc::new(Node::new(key, value)); // <-- Changed
        self.list.insert(key, node.clone()); // <-- Changed
        self.map.insert(key, HashMap::new());
        node
    }

    fn get_next_key(&mut self) -> Key {
        let key = self.next_key;
        self.next_key += 1;
        key
    }
}

fn main() {
    let mut graph = Graph::::new();
    let n1 = graph.add_node(111);
    let n2 = graph.add_node(222);
}