Rust:“因为借用而不能移出“自我"".错误 [英] Rust: "cannot move out of `self` because it is borrowed" error

问题描述

我正在尝试编写一种递归方法，该方法将一个项目添加到树中并返回与该项目相对应的树节点.

I'm trying to write a recursive method that adds an item to a tree and returns the tree node corresponding to that item.

enum BstNode {
    Node(int, ~BstNode, ~BstNode),
    Leaf
}

impl BstNode {
    fn insert<'a>(&'a mut self, item: int) -> &'a mut BstNode {
        match *self {
            Leaf => {
                *self = Node(item, ~Leaf, ~Leaf);
                self
            },
            Node(ref node_item, ref mut left, ref mut right) =>
                match item.cmp(node_item) {
                    Less => left.insert(item),
                    Equal => self,
                    Greater => right.insert(item)
                }
        }
    }
}

我被以下错误所咬住:

bst.rs:19:30: 19:34 error: cannot move out of `self` because it is borrowed
bst.rs:19                     Equal => self,
                                       ^~~~
bst.rs:16:18: 16:31 note: borrow of `self#0` occurs here
bst.rs:16             Node(ref node_item, ref mut left, ref mut right) =>
                           ^~~~~~~~~~~~~

移出something"是什么意思?如何解决此错误?

What does "moving out of something" mean? How do I fix this error?

我正在使用Rust 0.10.

I'm using Rust 0.10.

推荐答案

在您的示例中，node_item，left和right由self变量拥有.借用检查器在

In your example node_item, left and right are owned by the self variable. The borrow checker doesn't know that in the Equal branch of

match item.cmp(node_item) {
    Less => left.insert(item),
    Equal => self,
    Greater => right.insert(item)
}

既没有使用node_item，left也没有使用right，但是它看到self正在移动(正在返回)，而这3个变量仍在借用(您仍然在匹配(在哪里借用).我认为这是一个已知的错误，表明此行为过于严格，请参见 issue#6993 .

neither node_item, left nor right is used, but it sees that self is moving (you are returning it) while those 3 variables are still borrowed (you are still in the lexical scope of the match, where they are borrowed). I think this is a known bug that this behavior is too strict, see issue #6993.

关于修复代码的最佳方法，老实说，我不确定.我会使用完全不同的结构(至少在修复之前的错误之前):

As for the best way to fix the code, honestly I'm not sure. I would go with using a completely different structure (at least until the previous bug is fixed) :

pub struct BstNode {
  item: int,
  left: Option<~BstNode>,
  right: Option<~BstNode>
}

impl BstNode {
    pub fn insert<'a>(&'a mut self, item: int) -> &'a mut BstNode {
        match item.cmp(&self.item) {
            Less => match self.left {
              Some(ref mut lnode) => lnode.insert(item),
              None => {
                self.left = Some(~BstNode {item: item, left: None, right: None});
                &mut **self.left.as_mut().unwrap()
              }
            },
            Equal => self,
            Greater => match self.right {
              Some(ref mut rnode) => rnode.insert(item),
              None => {
                self.right = Some(~BstNode {item: item, left: None, right: None});
                &mut **self.right.as_mut().unwrap()
              }
            }
        }
    }
}

这样，当您返回节点时，就永远不会再借用它的任何成员.

This way when you return your node, you never have any of its members still borrowed.

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