不能将 `*` 作为不可变借用，因为 `*self` 也作为可变借用 [E0502]

Question

这是我所有的代码：

use std::collections::HashMap;

pub struct Book<'a> {
    page: Vec<&'a str>,
    histories: HashMap<&'a str, &'a str>,
}

impl<'a> Book<'a> {
    pub fn new(page: Vec<&'a str>) -> Book<'a> {
        let histories = HashMap::new();
        Book {
            page: page,
            histories: histories
        }
    }

    pub fn process(&mut self, line: &str, book_id: &'a str) {

        let page_c = self.get_page(book_id);
        println!("page_c {}", page_c);

        for history in self.histories.iter() {
            println!("histories...");
        }
    }

    fn get_page(&mut self, book_id: &'a str) -> &str {
        if !self.histories.contains_key(book_id) {
            println!("not history found for book {}", book_id);
            self.histories.insert(book_id, "history A");
        }
        self.histories.get(book_id).unwrap()
    }
}

fn main() {
    println!("Hello, world!");
    let mut pages = Vec::new();
    let st = "page1";
    pages.push(st);

    let mut biblio = Book::new(pages);

    let sentence = "this is a line of page";
    let book_id = "onebook";
    biblio.process(sentence, book_id);
}

这不能编译：

src/main.rs:22:24: 22:38 error: cannot borrow `self.histories` as immutable because `*self` is also borrowed as mutable [E0502]
src/main.rs:22         for history in self.histories.iter() {
                                      ^~~~~~~~~~~~~~
src/main.rs:19:22: 19:26 note: previous borrow of `*self` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `*self` until the borrow ends
src/main.rs:19         let page_c = self.get_page(book_id);
                                    ^~~~
src/main.rs:25:6: 25:6 note: previous borrow ends here
src/main.rs:17     pub fn process(&mut self, line: &str, book_id: &'a str) {
...
src/main.rs:25     }
                   ^
error: aborting due to previous error
Could not compile `tempo`.

我理解错误消息，但在研究和阅读similar questions 之后，我不明白如何修复我的代码。

Answer 1

修复代码的最简单方法是不要为 page_c 引入额外的变量，而是直接使用来自 get_page 的结果：

pub fn process(&mut self, line: &str, book_id: &'a str) {
    println!("page_c {}", self.get_page(book_id));

    for history in self.histories.iter() {
        println!("histories...");
    }
}

这样，当您进入for 循环时，self 不会被借用，因为它仅在调用println 时被借用。如果您确实想为 page_c 提供一个变量，您可以在额外的范围内引入它，因此借用将在范围的末尾（因此在循环之前）：

pub fn process(&mut self, line: &str, book_id: &'a str) {
    {
        let page_c = self.get_page(book_id);
        println!("page_c {}", page_c);
    } // page_c ceases to exist here, so the borrow of self ends
    for history in self.histories.iter() {
        println!("histories...");
    }
}