rust - Borrow two mutable values from the same HashMap

Question

I have the following code:

use std::collections::{HashMap, HashSet};

fn populate_connections(
    start: i32,
    num: i32,
    conns: &mut HashMap<i32, HashSet<i32>>,
    ancs: &mut HashSet<i32>,
) {
    let mut orig_conns = conns.get_mut(&start).unwrap();
    let pipes = conns.get(&num).unwrap();

    for pipe in pipes.iter() {
        if !ancs.contains(pipe) && !orig_conns.contains(pipe) {
            ancs.insert(*pipe);
            orig_conns.insert(*pipe);
            populate_connections(start, num, conns, ancs);
        }
    }
}

fn main() {}

The logic is not very important, I'm trying to create a function which will itself and walk over pipes.

My issue is that this doesn't compile:

error[E0502]: cannot borrow `*conns` as immutable because it is also borrowed as mutable
  --> src/main.rs:10:17
   |
9  |     let mut orig_conns = conns.get_mut(&start).unwrap();
   |                          ----- mutable borrow occurs here
10 |     let pipes = conns.get(&num).unwrap();
   |                 ^^^^^ immutable borrow occurs here
...
19 | }
   | - mutable borrow ends here

error[E0499]: cannot borrow `*conns` as mutable more than once at a time
  --> src/main.rs:16:46
   |
9  |     let mut orig_conns = conns.get_mut(&start).unwrap();
   |                          ----- first mutable borrow occurs here
...
16 |             populate_connections(start, num, conns, ancs);
   |                                              ^^^^^ second mutable borrow occurs here
...
19 | }
   | - first borrow ends here

I don't know how to make it work. At the beginning, I'm trying to get two HashSets stored in a HashMap (orig_conns and pipes).

Rust won't let me have both mutable and immutable variables at the same time. I'm confused a bit because this will be completely different objects but I guess if &start == &num, then I would have two different references to the same object (one mutable, one immutable).

Thats ok, but then how can I achieve this? I want to iterate over one HashSet and read and modify other one. Let's assume that they won't be the same HashSet.

Answer 1

If you can change your datatypes and your function signature, you can use a RefCell to create interior mutability:

use std::cell::RefCell;
use std::collections::{HashMap, HashSet};

fn populate_connections(
    start: i32,
    num: i32,
    conns: &HashMap<i32, RefCell<HashSet<i32>>>,
    ancs: &mut HashSet<i32>,
) {
    let mut orig_conns = conns.get(&start).unwrap().borrow_mut();
    let pipes = conns.get(&num).unwrap().borrow();

    for pipe in pipes.iter() {
        if !ancs.contains(pipe) && !orig_conns.contains(pipe) {
            ancs.insert(*pipe);
            orig_conns.insert(*pipe);
            populate_connections(start, num, conns, ancs);
        }
    }
}

fn main() {}

Note that if start == num, the thread will panic because this is an attempt to have both mutable and immutable access to the same HashSet.

Safe alternatives to RefCell

Depending on your exact data and code needs, you can also use types like Cell or one of the atomics. These have lower memory overhead than a RefCell and only a small effect on codegen.

In multithreaded cases, you may wish to use a Mutex or RwLock.