Tree graph algorithm without RefCell

use std::error::Error;
use refbox::{RefBox, Ref};

struct Node<T> {
    id: T,
    left: Option<RefBox<Node<T>>>,
    right: Option<RefBox<Node<T>>>,
}

trait MapRefBox<T> {
    fn map_to_ref(&self) -> Option<Ref<T>>;
}

impl<T> MapRefBox<T> for Option<RefBox<T>> {
    fn map_to_ref(&self) -> Option<Ref<T>> {
        self.as_ref().map(|rc| rc.create_ref())
    }
}

fn node(id: i32, l: RefBox<Node<i32>>, r: RefBox<Node<i32>>) -> RefBox<Node<i32>> {
    RefBox::new(Node { id, left: Some(l), right: Some(r) })
}
fn left(id: i32, l: RefBox<Node<i32>>) -> RefBox<Node<i32>> {
    RefBox::new(Node { id, left: Some(l), right: None })
}
fn right(id: i32, r: RefBox<Node<i32>>) -> RefBox<Node<i32>> {
    RefBox::new(Node { id, left: None, right: Some(r) })
}
fn leaf(id: i32) -> RefBox<Node<i32>> {
    RefBox::new(Node { id, left: None, right: None })
}

// immutable access to tree
fn print_tree(node: Option<Ref<Node<i32>>>, depth: usize) -> Result<(), Box<dyn Error>> {
    if let Some(n) = node {
        let n = n.try_borrow_mut()?;
        println!("{}{}", "  ".repeat(depth), n.id);
        print_tree(n.left.map_to_ref(), depth + 1)?;
        print_tree(n.right.map_to_ref(), depth + 1)?;
    }
    Ok(())
}


fn main() -> Result<(), Box<dyn Error>> {
    let tree = node(1,
                    node(2, leaf(4), leaf(5)),
                    node(3, leaf(6), leaf(7)));
    print_tree(Some(tree).map_to_ref(), 0)?;


    println!("Hello, world!");
    Ok(())
}