Rust的生命周期：返回指针

Question

当我编写这样的函数时，

fn must_get_env<'a>(key: &'a str) -> &'a str {
    match std::env::var(key) {
        Ok(val) => {
            return &val;
        }
        Err(_) => panic!("Missing required environment variable: {}", key),
    }
}

我收到此错误：

error[E0597]: `val` does not live long enough
  --> src/main.rs:19:21
   |
19 |             return &val;
   |                     ^^^ borrowed value does not live long enough
...
22 |     }
   |     - borrowed value only lives until here

我以为我对锈的寿命有了很好的了解，但显然没有。除了返回String之外，我还有什么选择？