I have a code that counts number of elements in vector that contains only 0, 1, 2 values

    pub fn sort_colors(nums: &mut Vec<i32>) {
        let mut cntColors = vec![0; 3];
        
        for num in nums {
            cntColors[(num) as usize] += 1; // ERROR: casting `&mut i32` as `usize` is invalid
        }

I fixed the error with help of this code

        for idx in 0..nums.len() {
            cntColors[nums[idx] as usize] += 1;
        }

What is the proper way to cast &mut i32 to usize?