Is it possible to make a recursive closure in Rust?

Question

This is a very simple example, but how would I do something similar to:

let fact = |x: u32| {
    match x {
        0 => 1,
        _ => x * fact(x - 1         


        

Answer 1

Here's a really ugly and verbose solution I came up with:

use std::{
    cell::RefCell,
    rc::{Rc, Weak},
};

fn main() {
    let weak_holder: Rc u32>>> =
        Rc::new(RefCell::new(Weak:: u32>::new()));
    let weak_holder2 = weak_holder.clone();
    let fact: Rc u32> = Rc::new(move |x| {
        let fact = weak_holder2.borrow().upgrade().unwrap();
        if x == 0 {
            1
        } else {
            x * fact(x - 1)
        }
    });
    weak_holder.replace(Rc::downgrade(&fact));

    println!("{}", fact(5)); // prints "120"
    println!("{}", fact(6)); // prints "720"
}

The advantages of this are that you call the function with the expected signature (no extra arguments needed), it's a closure that can capture variables (by move), it doesn't require defining any new structs, and the closure can be returned from the function or otherwise stored in a place that outlives the scope where it was created (as an Rc) and it still works.