Various prime number implementations, including a buggy one

badprime.rs

// https://www.reddit.com/r/rust/comments/a0g730/new_to_rust_discouraged_by_slowrunning_code/

fn nth_prime_bad(initial: u64) -> u64 {
    let mut prime_count = 0;
    let mut curr = 2;
    let mut prime = None;
    while prime_count < initial {
        let mut denom = 2;
        while denom < curr {
            if curr % denom == 0 {
                curr = curr + 1;
                continue;
            }
            denom += 1;
        }
        prime = Some(curr);
        curr = curr + 1;
        prime_count = prime_count + 1;
    }

    return prime.unwrap();
}

fn nth_prime(initial: u64) -> u64 {
    let mut prime_count = 0;
    let mut curr = 2;
    let mut prime = None;
    'next_candidate: while prime_count < initial {
        let mut denom = 2;
        while denom < curr {
            if curr % denom == 0 {
                curr = curr + 1;
                continue 'next_candidate;
            }
            denom += 1;
        }
        prime = Some(curr);
        curr = curr + 1;
        prime_count = prime_count + 1;
    }

    return prime.unwrap();
}

fn nth_prime_clean(n: u64) -> u64 {
    fn is_prime(p: u64) -> bool {
        for d in 2..p {
            if p % d == 0 {
                return false;
            }
        }
        true
    }

    let mut count = 0;
    for candidate in 2.. {
        if is_prime(candidate) {
            count += 1;
            if count == n {
                return candidate;
            }
        }
    }
    panic!("ran out of candidates");
}

fn nth_prime_functional(n: u64) -> u64 {
    let is_prime = |&c: &u64| (2..c).all(|d| c % d != 0);
    (2..).filter(is_prime).nth((n - 1) as usize)
        .expect("ran out of candidates")
}

const N: u64 = 100;

fn main() {
    println!("{}", nth_prime_bad(N));
    println!("{}", nth_prime(N));
    println!("{}", nth_prime_clean(N));
    println!("{}", nth_prime_functional(N));
}