Count nucleotiedes using an array for the count values.

main.rs

use std::fmt;
use std::ops::{Index, IndexMut};

struct NucleotideCounts {
    counts: [u64; 4],
}

impl NucleotideCounts {
    fn new() -> Self {
        NucleotideCounts { counts: [0; 4] }
    }
    fn count(&mut self, strand: &str) {
        for c in strand.bytes() {
            self[c] += 1;
        }
    }
}

impl fmt::Debug for NucleotideCounts {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let c = self.counts;
        write!(f, "[{}, {}, {}, {}]", c[0], c[2], c[1], c[3])
    }
}

impl Index<u8> for NucleotideCounts {
    type Output = u64;
    fn index(&self, c: u8) -> &u64 {
        &self.counts[(((c as u8)/2) & 3) as usize]
    }
}
impl IndexMut<u8> for NucleotideCounts {
    fn index_mut(&mut self, c: u8) -> &mut u64 {
        &mut self.counts[(((c as u8)/2) & 3) as usize]
    }
}

fn main() {
    //let mut count : [u32; 4] = [0, 0, 0, 0];
    let mut counter = NucleotideCounts::new();
    counter.count("AATCGAAAGTTTCCTTTTTAATTTACCCTATTTATATATTAT");

    for c in "ATCG".bytes() {
        println!("{}: {}", c as char, counter[c]);
    }
    println!("{:?}", counter)
}