不能把"自我"借给不可变因为"self.chars"也被借用为可变的

Question

我需要对一个struct字段的不可变访问和对另一个struct字段的可变访问,但它们必须相互堆叠.我多次遇到这个问题而且我不知道如何解决这个问题.

use std::collections::HashMap;

struct Bar;
impl Bar {
    fn get(&self) -> i32 {
        100
    }
}
struct Foo {
    chars: HashMap<char, i32>,
    b: Bar,
}

impl Foo {
    fn run(&mut self) {
        self.chars.entry('c').or_insert_with(|| self.b.get() * 100);
    }
}

fn main() {
    let mut m = Foo {
        chars: HashMap::new(),
        b: Bar,
    };
    m.run();
}

error[E0502]: cannot borrow `self` as immutable because `self.chars` is also borrowed as mutable
  --> src/main.rs:16:46
   |
16 |         self.chars.entry('c').or_insert_with(|| self.b.get() * 100);
   |         ----------                           ^^ ----              - mutable borrow ends here
   |         |                                    |  |
   |         |                                    |  borrow occurs due to use of `self` in closure
   |         |                                    immutable borrow occurs here
   |         mutable borrow occurs here

Answer 1

正如编译器所说的那样,问题在于你正在尝试self可变地和不可变地借用.正如Stefan所指出的,借用检查器无法区分跨越闭包边界的字段访问,因此我们需要通过更明确地了解我们想要借用的内容并传递给闭包来帮助它.

一种方法是在以下内容中取出引用self.b并使用它or_insert_with():

use std::collections::HashMap;

struct Bar;
impl Bar {
    fn get(&self) -> i32 {
        100
    }
}
struct Foo {
    chars: HashMap<char, i32>,
    b: Bar,
}

impl Foo {
    fn run(&mut self) {
        let b = &self.b;
        self.chars.entry('c').or_insert_with(|| b.get() * 100);
    }
}

fn main() {
    let mut m = Foo {
        chars: HashMap::new(),
        b: Bar,
    };
    m.run();
}