Small program that finds all primes for which all truncations from the right are prime

trunc_primes.rs

/// Naive primality check, assumes the input is odd
fn is_prime(n: i32) -> bool {
    let max = (n as f64).sqrt() as i32 + 1;

    for i in (3..max).step_by(2) {
        if n % i == 0 {
            return false
        }
    }
    true
}

fn main() {
    let mut generations = vec![vec![2, 3, 5, 7]];

    loop {
        let mut next_gen = vec![];

        for p in generations.last().unwrap().iter() {
            for d in [1, 3, 7, 9].iter() {
                let n = p * 10 + d;
                if is_prime(n) {
                    next_gen.push(n);
                }
            }
        }

        if next_gen.is_empty() {
            break
        }
        generations.push(next_gen);
    }

    for (i, gen) in generations.iter().enumerate() {
        if i == 0 {
            println!("Single digit ({} results):", gen.len());
        } else {
            println!("{} digits ({} results):", i + 1, gen.len());
        }

        for n in gen.iter() {
            println!("    {}", n)
        }
    }
}

trunc_primes.txt

Single digit (4 results):
    2
    3
    5
    7
2 digits (9 results):
    23
    29
    31
    37
    53
    59
    71
    73
    79
3 digits (14 results):
    233
    239
    293
    311
    313
    317
    373
    379
    593
    599
    719
    733
    739
    797
4 digits (16 results):
    2333
    2339
    2393
    2399
    2939
    3119
    3137
    3733
    3739
    3793
    3797
    5939
    7193
    7331
    7333
    7393
5 digits (15 results):
    23333
    23339
    23399
    23993
    29399
    31193
    31379
    37337
    37339
    37397
    59393
    59399
    71933
    73331
    73939
6 digits (12 results):
    233993
    239933
    293999
    373379
    373393
    593933
    593993
    719333
    739391
    739393
    739397
    739399
7 digits (8 results):
    2339933
    2399333
    2939999
    3733799
    5939333
    7393913
    7393931
    7393933
8 digits (5 results):
    23399339
    29399999
    37337999
    59393339
    73939133