如何修复借来的值在' while '中存在的时间不够长

Question

enum TreeNode {
Child(Rc<RefCell<[TreeNode]>>),
Leaf(i32),
Nil,
}
struct Tree {
root: TreeNode,
}
impl Tree {
fn new() -> Tree {
Tree {
root: TreeNode::Nil,
}
}
fn traverse(&mut self) {
let mut node = &self.root;
let mut parent = None;
while let TreeNode::Child(child) = node {
let mut child_mut = child.borrow_mut();
child_mut[1] = TreeNode::Leaf(1);
parent = Some(node);
node = &child_mut[0]; // error: borrowed value does not live long enough
}
}
}

编译错误

error[E0597]: `child_mut` does not live long enough
--> src/lib.rs:59:21
|
59 |             node = &child_mut[0];
|                     ^^^^^^^^^ borrowed value does not live long enough
60 |         }
|         -
|         |
|         `child_mut` dropped here while still borrowed
|         borrow might be used here, when `child_mut` is dropped and runs the destructor for type `std::cell::RefMut<'_, [TreeNode]>`

我知道错误原因，因为child_mut是while范围内的临时变量，所以rust不允许将其引用分配给node，但我不知道如何修复它。帮助需要。

Answer 1

Rc与RefCell是相当棘手的。您必须相当自由地使用Rc::clone，以便拥有值而不是引用，因为引用不会存在足够长的时间。一般的经验法则:如果你有一个&T不能存活足够长的时间，你应该想办法把它改成&Rc<T>或&Rc<RefCell<T>>，这样你就可以把它改成Rc::clone来拥有它。

这应该可以工作(至少，它编译并且看起来是正确的):

fn traverse(&mut self) {
if let TreeNode::Child(root) = &self.root {
let mut node = Rc::clone(root);
let mut parent = None;
while let TreeNode::Child(child) = &Rc::clone(&node).borrow()[1] {
let child = Rc::clone(child);
child.borrow_mut()[1] = TreeNode::Leaf(1);
parent = Some(node);
node = child;
}
}
}