const SIZE: usize = 3;

fn letter(n: u8) -> char {
    assert!(n < 26);
    (n + b'A') as char
}

fn main() {
    use rand::Rng;
    let mut rng = rand::thread_rng();

    let vec: Vec<char> = (0..SIZE * SIZE)
        .map(|_| letter(rng.gen_range(0, 26)))
        .collect();
    let vec = dbg!(trivial::Trivial2dArr::new(vec));
    dbg!(vec[2][1]);
}

mod trivial {
    use super::SIZE;

    #[derive(Debug, Clone)]
    pub(crate) struct Trivial2dArr<T>(Box<[T]>); // even better would be Box<[T; SIZE*SIZE]> but we'd need unsafe for the array cast, still, in 2019 :(

    impl<T> Trivial2dArr<T> {
        pub(crate) fn new(vec: Vec<T>) -> Self {
            assert_eq!(vec.len(), SIZE * SIZE);
            Self(vec.into())
        }
    }

    impl<T> std::ops::Index<usize> for Trivial2dArr<T> {
        type Output = [T];
        fn index(&self, x: usize) -> &[T] {
            let base = x * SIZE;
            &self.0[base..base + SIZE]
        }
    }
    impl<T> std::ops::IndexMut<usize> for Trivial2dArr<T> {
        fn index_mut(&mut self, x: usize) -> &mut [T] {
            let base = x * SIZE;
            &mut self.0[base..base + SIZE]
        }
    }
}