如何返回对递归数据结构的一部分的引用？

Question

I'm trying to define a recursive data structure in Rust, but there are some pieces missing in my understanding of Rust and memory - the only thing that I manage to do is pick a fight with the borrow checker.

I have the following stub of a quad tree and want to project one of the quadrants as follows.

use CC::{Node, Leaf};

enum CC {
    Node(i32, bool, i32, Rc<CC>, Rc<CC>, Rc<CC>, Rc<CC>),
    Leaf(bool),
}

impl CC {
    fn nw(&self) -> CC {
        match *self {
            Node(_, _, _, ref nw, _, _, _) => *nw.clone(),
            _ => panic!()
        }
    }
}

But all I end up with is

src/hashlife.rs:34:47: 34:58 error: cannot move out of borrowed content
src/hashlife.rs:34             Node(_, _, _, ref nw, _, _, _) => *nw.clone(),
                                                                 ^~~~~~~~~~~

Answer 1

You have two options here.

First, you can return a reference to the subtree:

fn nw(&self) -> &CC {
    match *self {
        Node(_, _, _, ref nw, _, _, _) => &**nw,
        _ => panic!()
    }
}

Second, you can return a reference-counted pointer:

fn nw(&self) -> Rc<CC> {
    match *self {
        Node(_, _, _, ref nw, _, _, _) => nw.clone()
        _ => panic!()
    }
}

You can't return just CC , however, unless you are willing to clone the value itself. The reason is that this would mean moving out of Rc , leaving it in some undefined state, which is rightly prohibited.