知乎上一篇2018年初关于nll的文章，当时nll还未进stable。大致意思是在nll情况下这个代码可以通过编译，以前看的时候没有测试过，今天试了一下是编译不过的，应该如何更正一下？

use std::collections::HashMap;

fn main() {
    let map = &mut HashMap::new();
    map.insert(22, format!("Hello, world"));
    map.insert(44, format!("Goodbye, world"));
    assert_eq!(&*get_default(map, 22), "Hello, world");
    assert_eq!(&*get_default(map, 66), "");

}

fn get_default(map: &mut HashMap<usize, String>, key: usize) -> &mut String {
    match map.get_mut(&key) {
        Some(value) => value,
        None => {
            map.insert(key, "".to_string());
            map.get_mut(&key).unwrap()
        }
    }
}

报错如下

error[E0499]: cannot borrow `*map` as mutable more than once at a time
  --> examples\nll_test.rs:38:13
   |
34 |   fn get_default(map: &mut HashMap<usize, String>, key: usize) -> &mut String {
   |                       - let's call the lifetime of this reference `'1`
35 |       match map.get_mut(&key) {
   |       -     --- first mutable borrow occurs here
   |  _____|
   | |
36 | |         Some(value) => value,
37 | |         None => {
38 | |             map.insert(key, "".to_string());
   | |             ^^^ second mutable borrow occurs here
39 | |             map.get_mut(&key).unwrap()
40 | |         }
41 | |     }
   | |_____- returning this value requires that `*map` is borrowed for `'1`