"cannot borrow `graph` as immutable because it is also borrowed as mutable." 我怎样才能让 Rust 知道我已经完成了那个可变借用？

Question

我正在尝试用 Rust 制作一个类似图形的结构。我的第一个实现编译得很好：

fn main() {
    let mut graph: Graph = Graph::new(); // Contains a vector of all nodes added to the graph.  The graph owns the nodes.

    // Create a node
    let parent: usize = graph.add_node(ParentNode::new()); // Returns the ID of the node.
    let parent: &Node = graph.get_node_with_id(parent); // Returns a borrowed reference to the node with the given ID

    // Print the number of nodes
    println!("Num nodes: {}", graph.count_nodes());
}

我不喜欢我必须先调用 add_node 然后再调用 get_node_with_id 的方式，因此我编写了另一种将这两个步骤合二为一的方法：

fn main() {
    let mut graph: Graph = Graph::new();

    // Create a node
    let parent: &Node = graph.add_node_and_borrow(ParentNode::new());

    // Print the number of nodes
    println!("Num nodes: {}", graph.count_nodes());
}

add_node_and_borrow 只是一个 shorthand:

/// Like add_node, but returns a borrowed reference
/// instead of the id
pub fn add_node_and_borrow(&mut self, node: Box<Node>) -> &Node {
    let id = self.add_node(node);
    return self.get_node_with_id(id);
}

当我尝试编译它时，出现错误：

error[E0502]: cannot borrow `graph` as immutable because it is also borrowed as mutable
  --> src/main.rs:23:31
   |
20 |     let parent: &Node = graph.add_node_and_borrow(ParentNode::new());
   |                         ----- mutable borrow occurs here
...
23 |     println!("Num nodes: {}", graph.count_nodes());
   |                               ^^^^^ immutable borrow occurs here
24 | }
   | - mutable borrow ends here

奇怪！在这两个示例中，我都在做完全相同的事情……不是吗？为什么 Rust 认为我从未停止在第二个示例中可变借用 graph？

这是完整的源文件，去掉了不重要的部分，这样您就可以看到完整的图片：

fn main() {
    does_not_compike();
}

fn compiles() {
    let mut graph: Graph = Graph::new();

    // Create a node
    let parent: usize = graph.add_node(ParentNode::new());
    let parent: &Node = graph.get_node_with_id(parent);

    // Print the number of nodes
    println!("Num nodes: {}", graph.count_nodes());
}

fn does_not_compike() {
    let mut graph: Graph = Graph::new();

    // Create a node
    let parent: &Node = graph.add_node_and_borrow(ParentNode::new());

    // Print the number of nodes
    println!("Num nodes: {}", graph.count_nodes());
}

struct Graph {
    nodes: Vec<Box<Node>>,
    next_node_id: usize,
}

impl Graph {
    pub fn new() -> Graph {
        // Construct a new graph with no nodes.
        let new_graph = Graph {
            nodes: Vec::new(),
            next_node_id: 0,
        };

        return new_graph;
    }

    /// Adds a newly-created node to graph.
    /// The graph becomes the new owner of the node.
    /// Returns the node id of the node.
    pub fn add_node(&mut self, node: Box<Node>) -> usize {
        // Add the node
        self.nodes.push(node);

        // Return the id
        let id = self.next_node_id;
        self.next_node_id += 1;

        return id;
    }

    /// Like add_node, but returns a borrowed reference
    /// instead of the id
    pub fn add_node_and_borrow(&mut self, node: Box<Node>) -> &Node {
        let id = self.add_node(node);
        return self.get_node_with_id(id);
    }

    /// Returns a borrowed reference to the node with the given id
    pub fn get_node_with_id(&self, id: usize) -> &Node {
        return &*self.nodes[id];
    }

    pub fn count_nodes(&self) -> usize {
        return self.nodes.len();
    }
}

trait Node {
    // Not important
}

struct ParentNode {
    // Not important
}

impl ParentNode {
    pub fn new() -> Box<Node> {
        Box::new(ParentNode {
            // lol empty struct
        })
    }
}

impl Node for ParentNode {
    // Not important
}

Answer 1

在你的第一个例子中：

let parent: usize = graph.add_node(ParentNode::new());

为调用 add_node 可变地借用 graph 然后释放借用。

let parent: &Node = graph.get_node_with_id(parent);

不可变地借用 graph 并且保留借用，因为 parent 是对属于该图的节点的引用。此时，，您将无法向图中添加新节点，因为不可变借用阻止您创建新的可变借用。但是，您可以不可变地重新借用 graph 来打印它。

在你的第二个例子中：

let parent: &Node = graph.add_node_and_borrow(ParentNode::new());

可变借用graph（因为函数签名）并保留借用（因为parent引用）。在此之后，您根本无法再借用图表，因为您已经有一个活动的可变借用。

"cannot borrow `graph` as immutable because it is also borrowed as mutable." 我怎样才能让 Rust 知道我已经完成了那个可变借用？

"cannot borrow `graph` as immutable because it is also borrowed as mutable." How can I let Rust know that I'm done with that mutable borrow?

graph

mutable

immutability

rust

borrow-checker

在你的第一个例子中：

在你的第二个例子中：