Rust trying to maintain self-defined list: cannot borrow `self.array` as mutable more than once at a time

Question

I'm new to rust and trying to maintain 2 linked-list inside an array, my code goes like:

#[derive(Copy, Clone)]
struct Node {
    next: i32,
    prev: i32,
    val: i32
}

impl Node {

    pub fn new()->Self{
        Self{
            next: 0,
            val: 0,
            prev: 0
        }
    }
    pub fn get_prev(&self) -> i32{
        self.prev
    }

    pub fn get_next(&self) -> i32{
        self.next
    }

    pub fn set_prev(& mut self, idx: i32){
        self.prev = idx;
    }

    pub fn set_next(& mut self, idx: i32){
        self.next = idx;
    }

    pub fn get_val(&self)->i32{
        self.val
    }

    pub fn set_val(& mut self, v: i32){
        self.val = v;
    }
}

struct MyVec{
    red_start: i32,
    black_start: i32,
    data: [Node; 48]
}

impl MyVec{
    pub fn new()->Self{
        let mut ans = Self{
            red_start : -1,
            black_start: 0,
            data: [Node::new(); 48]
        };
        let len = ans.data.len();
        for i in 0..ans.data.len(){
            let n: & mut Node = & mut ans.data[i];
            n.set_prev(((i + len-1) % len) as i32);
            n.set_next(((i+1) % len) as i32);
        }
        ans
    }

    pub fn move_black_to_red(& mut self, idx: i32){
        let n: & mut Node = & mut self.data[idx as usize];
        let prev: & mut Node = & mut self.data[n.get_prev() as usize];
        let next: & mut Node = & mut self.data[n.get_next() as usize];
        prev.set_next(n.get_next());
        next.set_prev(n.get_prev());
        //other stuff...
    }
}

The difficulties are in move_black_to_red fucntion, I need to maintain this list, so I need to get 3 mut references at the same time and update them. However, there is at most 1 mut borrow from self, so what should I do?

Answer 1

Rust doesn't allow multiple mutable references to the same variable at the same time. The easiest way to fix this would be to interleave the borrows, and borrow n immutably, since you never mutate it:

pub fn move_black_to_red(& mut self, idx: i32){
    let n: Node = self.data[idx as usize];

    let prev: & mut Node = & mut self.data[n.get_prev() as usize];
    prev.set_next(n.get_next());

    let next: & mut Node = & mut self.data[n.get_next() as usize];
    next.set_prev(n.get_prev());

    //other stuff...
}