Rust iterate over borrowed mutable reference

Question

I wanted a function to control and change a vector in rust. Here is a simplified version:

fn foo(vec: &mut Vec<i32>) {
    for (i, element) in vec.iter().enumerate() {
        // Some checks here
        vec[(*element) as usize] = i as i32;
    }
}

fn main() {
    let mut bar: Vec<i32> = vec![1, 0, 2];
    foo(&mut bar);
}

This code does not compile because there is both an immutable and a mutable borrow of vec in foo. I tried getting around this by copying vec to a separate copy, which didn't work and also wouldn't have been very pretty. What is the correct way to do this?

Answer 1

If you want to mutate the Vec, the correct way is to iterate over it mutably instead of immutably:

fn foo(vec: &mut Vec<i32>) {
    // note the `iter_mut` here:
    for element in vec.iter_mut() {
        // Some checks here
        // element now has type `&mut i32` and we can mutate it directly.
        *element *= 2;
    }
}

fn main() {
    let mut bar: Vec<i32> = vec![1, 2, 3];
    foo(&mut bar);
    println!("{:?}", bar); // [2, 4, 6]
}