我正在学习 rust ，我不太明白为什么这是行不通的。

#[derive(Debug)]
struct Node {
    value: String,
}

#[derive(Debug)]
pub struct Graph {
    nodes: Vec<Box<Node>>,
}

fn mk_node(value: String) -> Node {
    Node { value }
}

pub fn mk_graph() -> Graph {
    Graph { nodes: vec![] }
}

impl Graph {
    fn add_node(&mut self, value: String) {
        if let None = self.nodes.iter().position(|node| node.value == value) {
            let node = Box::new(mk_node(value));
            self.nodes.push(node);
        };
    }

    fn get_node_by_value(&self, value: &str) -> Option<&Node> {
        match self.nodes.iter().position(|node| node.value == *value) {
            None => None,
            Some(idx) => self.nodes.get(idx).map(|n| &**n),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn some_test() {
        let mut graph = mk_graph();

        graph.add_node("source".to_string());
        graph.add_node("destination".to_string());

        let source = graph.get_node_by_value("source").unwrap();
        let dest = graph.get_node_by_value("destination").unwrap();

        graph.add_node("destination".to_string());
    }
}

（playground）
这有错误

error[E0502]: cannot borrow `graph` as mutable because it is also borrowed as immutable
  --> src/main.rs:50:9
   |
47 |         let source = graph.get_node_by_value("source").unwrap();
   |                      ----- immutable borrow occurs here
...
50 |         graph.add_node("destination".to_string());
   |         ^^^^^ mutable borrow occurs here
51 |     }
   |     - immutable borrow ends here

这个来自 * Programming Rust * 的例子与我的例子非常相似，但它确实有效：

pub struct Queue {
    older: Vec<char>,   // older elements, eldest last.
    younger: Vec<char>, // younger elements, youngest last.
}

impl Queue {
    /// Push a character onto the back of a queue.
    pub fn push(&mut self, c: char) {
        self.younger.push(c);
    }

    /// Pop a character off the front of a queue. Return `Some(c)` if there /// was a character to pop, or `None` if the queue was empty.
    pub fn pop(&mut self) -> Option<char> {
        if self.older.is_empty() {
            if self.younger.is_empty() {
                return None;
            }

            // Bring the elements in younger over to older, and put them in // the promised order.
            use std::mem::swap;
            swap(&mut self.older, &mut self.younger);
            self.older.reverse();
        }

        // Now older is guaranteed to have something. Vec's pop method // already returns an Option, so we're set.
        self.older.pop()
    }

    pub fn split(self) -> (Vec<char>, Vec<char>) {
        (self.older, self.younger)
    }
}

pub fn main() {
    let mut q = Queue {
        older: Vec::new(),
        younger: Vec::new(),
    };

    q.push('P');
    q.push('D');

    assert_eq!(q.pop(), Some('P'));
    q.push('X');

    let (older, younger) = q.split(); // q is now uninitialized.
    assert_eq!(older, vec!['D']);
    assert_eq!(younger, vec!['X']);
}