compilation is OK - however "cargo test" fails

main.rs

fn main() {
    fn factorial(n: u64) -> u64 {
        if n == 0 || n == 1 {
            1
        } else {
            n * factorial(n - 1)
        }
    }
}
#[cfg(test)]
mod tests {
    use crate::factorial;

    #[test]
    fn first() {
        assert_eq!(factorial(0), 1);
    }
    #[test]
    fn fifth() {
        assert_eq!(factorial(4), 24);
    }
}