匹配陈述借来的价值不够长

Question

我想将数字转换为字符串。有时，该数字不存在，应该用空字符串表示。

我尝试编写以下代码：

struct Example {
    number: Option<usize>,
}

impl Example {
    fn example(&self) {
        let result = match self.number {
            Some(num) => num.to_string().as_str(),
            None => "",
        };
    }
}

fn main() {}

但是，借阅检查员不允许这样做：

error[E0597]: borrowed value does not live long enough
  --> src/main.rs:8:49
   |
8  |             Some(num) => num.to_string().as_str(),
   |                          ---------------        ^ temporary value dropped here while still borrowed
   |                          |
   |                          temporary value created here
...
11 |     }
   |     - temporary value needs to live until here

如何正确地/惯用地写这个？