Code shared from the Rust Playground

playground.rs

enum RecursionResult<C, R> {
    Continue(C),
    Return(R),
}

fn tail_recurse<C, R>(mut init: C, mut f: impl FnMut(C) -> RecursionResult<C, R>) -> R {
    loop {
        match f(init) {
            RecursionResult::Continue(c) => init = c,
            RecursionResult::Return(r) => return r,
        }
    }
}

pub fn factorial(x: usize) -> usize {
    tail_recurse((x, 1), |(a, b)| {
        if a == 0 {
            RecursionResult::Return(b)
        } else {
            RecursionResult::Continue((a - 1, a * b))
        }
    })
}

pub fn factorial2(x: usize) -> usize {
    (1..=x).product()
}