both work OK (compilation and cargo test)

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);
    }
}