Rust: Erathosthenes prime sieve

primes.rs

fn is_prime(n: usize, primes: &Vec<usize>) -> bool {
    for &p in primes {
        let q = n / p;
        if q < p { return true };
        let r = n - q * p;
        if r == 0 { return false };
    }
    panic!("too few primes")
}

fn primes_lt(bound: usize) -> Vec<usize> {
    let mut primes: Vec<bool> = (0..bound + 1).map(|num| num == 2 || num & 1 != 0).collect();
    let mut num = 3usize;
    while num * num <= bound {        
        let mut j = num * num;
        while j <= bound {
            primes[j] = false;
            j += num;
        }
        num += 2;
    }
    primes.into_iter().enumerate().skip(2).filter_map(|(i, p)| if p {Some(i)} else {None}).collect::<Vec<usize>>()
}

// primes -- an ordered vector of prime numbers.
fn is_prime(n: usize, primes: &Vec) -> bool {
primes.binary_search(&n).is_ok()
}