如何在 Rust 中修复.. was mutably borrowed here in the previous iteration of the loop

我必须迭代键，在HashMap中按键查找值，可能会在找到的 struct 中作为值进行一些繁重的计算(lazy=&gt；mutate the struct)，然后缓存并以Rust的形式返回它.

我收到以下错误消息:

error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> src/main.rs:25:26
   |
23 |     fn it(&mut self) -> Option<&Box<Calculation>> {
   |           - let's call the lifetime of this reference `'1`
24 |         for key in vec!["1","2","3"] {
25 |             let result = self.find(&key.to_owned());
   |                          ^^^^ `*self` was mutably borrowed here in the previous iteration of the loop
...
28 |                 return result
   |                        ------ returning this value requires that `*self` is borrowed for `'1`

这是code in playground美元.

use std::collections::HashMap;

struct Calculation {
    value: Option<i32>
}

struct Struct {
    items: HashMap<String, Box<Calculation>> // cache
}

impl Struct {
    fn find(&mut self, key: &String) -> Option<&Box<Calculation>> {
        None // find, create, and/or calculate items
    }

    fn it(&mut self) -> Option<&Box<Calculation>> {
        for key in vec!["1","2","3"] {
            let result = self.find(&key.to_owned());
            if result.is_some() {
                return result
            }
        }
        None
    }
}

• 我无法避免循环，因为我必须判断多个键
• 我必须使其可变(self和 struct )，因为可能的计算会改变它

有没有关于如何改变设计的建议(因为Rust 迫使人们以一种有意义的不同方式思考)或解决它的建议？

另外，代码还有一些其他问题，但是让我们先把问题分开，解决这个问题.