rust - 返回由闭包借用 self 产生的迭代器的函数的正确返回类型是什么

Question

考虑以下代码，重点是children_iter():

use std::collections::HashMap;

type NodeMap = HashMap<i32, Node>;

struct Node {
    id: i32,
    parent: Option<i32>,
    sibling: Option<i32>,
    children: Vec<i32>,
    content: String,
}

pub struct NodeIterator<'a> {
    nodes: &'a NodeMap,
    current: &'a Node,
}

impl<'a> NodeIterator<'a> {
    fn new(node: &'a Node, nodes: &'a NodeMap) -> NodeIterator<'a> {
        NodeIterator {
            nodes,
            current: node,
        }
    }

    pub fn children_iter(&self) -> impl Iterator<Item = NodeIterator> {
        self.current
            .children
            .iter()
            .map(|i| self.nodes.get(i).unwrap())
            .map(|n| Self::new(n, self.nodes))
    }
}

该代码的编译失败，并且:

error[E0373]: closure may outlive the current function, but it borrows `self`, which is owned by the current function
  --> src/lib.rs:30:18
   |
30 |             .map(|i| self.nodes.get(i).unwrap())
   |                  ^^^ ---- `self` is borrowed here
   |                  |
   |                  may outlive borrowed value `self`
   |
note: closure is returned here
  --> src/lib.rs:26:36
   |
26 |     pub fn children_iter(&self) -> impl Iterator<Item = NodeIterator> {
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to force the closure to take ownership of `self` (and any other referenced variables), use the `move` keyword
   |
30 |             .map(move |i| self.nodes.get(i).unwrap())
   |                  ^^^^^^^^

error[E0373]: closure may outlive the current function, but it borrows `self`, which is owned by the current function
  --> src/lib.rs:31:18
   |
31 |             .map(|n| Self::new(n, self.nodes))
   |                  ^^^              ---- `self` is borrowed here
   |                  |
   |                  may outlive borrowed value `self`
   |
note: closure is returned here
  --> src/lib.rs:26:36
   |
26 |     pub fn children_iter(&self) -> impl Iterator<Item = NodeIterator> {
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to force the closure to take ownership of `self` (and any other referenced variables), use the `move` keyword
   |
31 |             .map(move |n| Self::new(n, self.nodes))
   |                  ^^^^^^^^

有没有一种方法可以正确地指定生存期，以便很明显返回的迭代器仅在当前 NodeIterator的生存期内有效？

Answer 1

您收到此错误的原因是因为在Rust中，迭代器是延迟计算的。看起来您的闭包将在children_iter函数内执行，但实际上直到调用者在返回的迭代器上调用next方法后，闭包才真正执行。
编译器建议使用move关键字将借用的引用移至闭包中，从而解决了该问题。但是，您也可以通过热切评估迭代器并将结果收集在Vec<NodeIterator>中来解决该问题。这是固定和编译示例中的两种方法:

use std::collections::HashMap;

type NodeMap = HashMap<i32, Node>;

struct Node {
    id: i32,
    parent: Option<i32>,
    sibling: Option<i32>,
    children: Vec<i32>,
    content: String,
}

pub struct NodeIterator<'a> {
    nodes: &'a NodeMap,
    current: &'a Node,
}

impl<'a> NodeIterator<'a> {
    fn new(node: &'a Node, nodes: &'a NodeMap) -> NodeIterator<'a> {
        NodeIterator {
            nodes,
            current: node,
        }
    }

    pub fn children_iter(&self) -> impl Iterator<Item = NodeIterator> {
        self.current
            .children
            .iter()
            // lazily-evaluted closures
            // they run in the caller's scope
            .map(move |i| self.nodes.get(i).unwrap())
            .map(move |n| Self::new(n, self.nodes))
    }
    
    pub fn children_iter_vec(&self) -> Vec<NodeIterator> {
        self.current
            .children
            .iter()
            // eagerly-evaluted closures
            // we run them in the current scope
            .map(|i| self.nodes.get(i).unwrap())
            .map(|n| Self::new(n, self.nodes))
            .collect()
    }
}

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