将格式！（）从String转换为＆amp;＆＃39; static str

Question

我很难理解如何插入数字并以正确的格式返回它。我开始使用返回String的函数，但编译器需要&'static str。我已经阅读了to_owned，但它还没有点击我的脑袋。什么是＆＃34;权利＆＃34;做我尝试的方法？

fn ordinal_name(num: i32) -> &'static str {
    if num == 1 {
        "1st"
    } else if num == 2 {
        "2nd"
    } else if num == 3 {
        "3rd"
    } else {
        format!("{}th", num)
    }
}

fn main() {
    println!("{}", ordinal_name(5));
}

error[E0308]: mismatched types
 --> src/main.rs:9:9
  |
1 | fn ordinal_name(num: i32) -> &'static str {
  |                              ------------ expected `&'static str` because of return type
...
9 |         format!("{}th", num)
  |         ^^^^^^^^^^^^^^^^^^^^ expected reference, found struct `std::string::String`
  |
  = note: expected type `&'static str`
             found type `std::string::String`
  = note: this error originates in a macro outside of the current crate

这不是String to &'static str的重复 - 我试图计算如何用字符串插入整数然后返回&static str。