A simple factorial function written in Rust

factorial.rs

#![feature(generic_const_exprs)]
#![allow(incomplete_features)]

trait Factorialable<const N : usize> {
  fn value() -> usize;
}

impl Factorialable<0> for () {
  fn value() -> usize { 1 }
}

impl<T, const N : usize> Factorialable<N> for (T,)
    where
    T : Factorialable<{ N - 1 }> {
  fn value() -> usize { N * T::value() }
}

macro_rules! count {
  () => (0);
  ($x:tt $($xs:tt)*) => (1 + count!($($xs)*));
}

macro_rules! number {
  () => (());
  ($x:tt $($xs:tt)*) => ((number!($($xs)*),));
}

macro_rules! factorial {
  ($($xs:tt)*) => (<number!($($xs)*) as Factorialable::<{count!($($xs)*)}>>::value());
}

fn main() {
  println!("{}", factorial!());           // 0!
  println!("{}", factorial!(#));          // 1!
  println!("{}", factorial!(##));         // 2!
  println!("{}", factorial!(###));        // 3!
  println!("{}", factorial!(##########)); // 10!
  println!("{}", factorial!(##########
                            ##########)); // 20!
}

playground