Cannot move out of borrowed content when matching

问题:

I'm trying to print out a tree (it's a LinkedList right now, but that will be fixed):

use std::io;
use std::rc::Rc;

enum NodeKind {
    Branch(Rc<Node>),
    Leaf,
}

struct Node {
    value: i32,
    kind: NodeKind,
}

fn main() {
    let leaf = Node { value: 10, kind: NodeKind::Leaf };
    let branch = Node { value: 50, kind: NodeKind::Branch(Rc::new(leaf)) };
    let root = Node { value: 100, kind: NodeKind::Branch(Rc::new(branch)) };

    let mut current = root;
    while true {
        println!("{}", current.value);
        match current.kind {
            NodeKind::Branch(next) => {
                current = *next;
            }
            NodeKind::Leaf => {
                break;
            }
        }
    }

    let mut reader = io::stdin();
    let buff = &mut String::new();
    let read = reader.read_line(buff);
}

The compiler says:

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:24:27
   |
24 |                 current = *next;
   |                           ^^^^^ cannot move out of borrowed content

I'm reading the value only, not changing anything. I'm assigning a value from a reference to another value, trying to dereference an Rc<T> value and store it in a local mut variable.

maybe something like this may work:

while true {
    println!("{}", current.value);
    match &current.kind {
        &NodeKind::Branch(next) => {
            current = next;
        }
        &NodeKind::Leaf => {
            break;
        }
    }
}

or maybe

let mut current = &Rc::new(root);
while true {
    println!("{}", current.value);
    match current.kind {
        NodeKind::Branch(next) => {
            current = &next;
        }
        NodeKind::Leaf => {
            break;
        }
    }
}

but I get the same error plus 'next' does not live long enough

回答1:

There is no need to clone here, it is absolutely possible to do what you want to achieve with references:

use std::rc::Rc;

enum NodeKind {
    Branch(Rc<Node>),
    Leaf,
}

struct Node {
    value: i32,
    kind: NodeKind,
}

fn main() {
    let leaf = Node { value: 10, kind: NodeKind::Leaf };
    let branch = Node { value: 50, kind: NodeKind::Branch(Rc::new(leaf)) };
    let root = Node { value: 100, kind: NodeKind::Branch(Rc::new(branch)) };

    let mut current = &root;
    loop {
        println!("{}", current.value);
        match current.kind {
            NodeKind::Branch(ref next) => {
                current = &**next;
            }
            NodeKind::Leaf => break,
        }
    }
}

The only important changes from your code is that the pattern in the match is ref next and current is of type &Node.

ref patterns bind their variables by reference, that is, next has type &Rc<Node>. To get &Node from it, you need to dereference it two times to get Node and then reference again to get &Node. Due to Deref coercions, it is also possible to write current = &next, and the compiler will insert an appropriate number of *s for you automatically.

I also changed from while (true) to loop because it is more idiomatic and it helps the compiler to reason about your code.

All traversals of tree-like structures are done like this in Rust. ref patterns allow not to move out of variables, which is absolutely necessary when you only need to read data. You can find more about patterns and how they interact with ownership and borrowing here.

回答2:

The error is displayed because by default match will perform a move.

After a value is moved (i.e. wasn't taken by reference or method that takes self was called) subsequent calls fail. You'll probably need to clone, which is a property both of your struct and enum lack. Once you add those (#[derive(Clone)) and change current = *next; into current = (*next).clone();, your program will work again!

use std::io;
use std::rc::Rc;

#[derive(Clone)]
enum NodeKind {
    Branch(Rc<Node>),
    Leaf,
}

#[derive(Clone)]
struct Node {
    value: i32,
    kind: NodeKind,
}

fn main() {
    let leaf = Node { value: 10, kind: NodeKind::Leaf };
    let branch = Node { value: 50, kind: NodeKind::Branch(std::rc::Rc::new(leaf)) };
    let root = Node { value: 100, kind: NodeKind::Branch(std::rc::Rc::new(branch)) };

    let mut current = root;
    while true {
        println!("{}", current.value);
        match current.kind {
            NodeKind::Branch(next) => {
                current = (*next).clone();
            }
            NodeKind::Leaf => {
                break;
            }
        }
    }

    let reader = io::stdin();
    let buff = &mut String::new();
    let read = reader.read_line(buff);
}

Playground

If you let mut current = &root then you can avoid clone() as per Vladimir's response below (playpen of Vladimir's version).

回答3:

I can't figure out the issue with 1) yet, but I did find an answer for 2).

At the top, you need to use:

use std::rc::Rc;

instead of

use std::rc;