Extracting an attribute from a Box inside an enum

Question

#[derive(Clone, Debug)]
pub struct Node<T> {
    value: T,
    left: Addr<T>,
    right: Addr<T>,
}

#[derive(Clone, Debug)]
pub enum Addr<T> {
    Addr(Box<Node<T>>),
    None,
}

impl<T> Addr<T> {
    pub fn is_none(&self) -> bool {
        matches!(&*self, Addr::None)
    }
}
impl<T> Node<T> {
    fn node_insert(mut n: Addr<T>, val: T) {
        let nd = Box::new(Node {
            value: val,
            left: Addr::None,
            right: Addr::None,
        });

        if n.is_none() {
            n = Addr::Addr(nd);
        } else {
            node_insert(n.left, val);
        }
    }
}

I'm trying to implement a simple binary tree, and in the node_insert function I can't figure out how to access the left attribute of the Node struct when calling node_insert recursively.

Nothing I've done worked and I couldn't find any answer to help with it, probably because its somehow specific.

Answer 1

Use pattern matching:

fn node_insert(mut n: Addr<T>, val: T) {
    match n {
        Addr::None => {
            n = Addr::Addr(Box::new(Node {
                value: val,
                left: Addr::None,
                right: Addr::None,
            }))
        }
        Addr::Addr(n) => Self::node_insert(n.left, val),
    }
}

However, the compiler rightfully warns:

warning: value assigned to `n` is never read
  --> src/lib.rs:23:13
   |
23 |             n = Addr::Addr(Box::new(Node {
   |             ^
   |
   = help: maybe it is overwritten before being read?
   = note: `#[warn(unused_assignments)]` on by default

Since you took ownership of n, you're writing to a local variable and not to the tree. Instead, you should take &mut Addr:

fn node_insert(n: &mut Addr<T>, val: T) {
    match n {
        Addr::None => {
            *n = Addr::Addr(Box::new(Node {
                value: val,
                left: Addr::None,
                right: Addr::None,
            }))
        }
        Addr::Addr(n) => Self::node_insert(&mut n.left, val),
    }
}