We can obsolete errata, we have the technology

In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n.

pub fn factorial_recursive(n: u64) -> u64 {
    match n {
        0 => 1,
        _ => n * factorial_recursive(n - 1),
    }
}

DITA

Figure 1. factorial.dita
<p>In mathematics, the factorial of a non-negative integer <i>n</i>,
  denoted by <i>n</i>!, is the product of all positive integers less than or
  equal to <i>n</i>.</p>
<codeblock><coderef
    href="src/lib.rs#token=start_fact_rec,end_fact_rec"/></codeblock>
Figure 2. src/lib.rs
// start_fact_rec
pub fn factorial_recursive(n: u64) -> u64 {
    match n {
        0 => 1,
        _ => n * factorial_recursive(n - 1),
    }
}
// end_fact_rec

#[test]
fn test() {
    assert_eq!(1, factorial_recursive(1));
    assert_eq!(2, factorial_recursive(2));
    assert_eq!(6, factorial_recursive(3));
    assert_eq!(24, factorial_recursive(4));
    assert_eq!(120, factorial_recursive(5));
    assert_eq!(720, factorial_recursive(6));
    assert_eq!(5040, factorial_recursive(7));
    assert_eq!(40320, factorial_recursive(8));
    assert_eq!(362880, factorial_recursive(9));
}