避免悬垂引用

  1. {
  2.     let r;
  4.     {
  5.         let x = 5;
  6.         r = &x;
  7.     }
  9.     println!("r: {}", r);
  10. }
  1. fn longest(x: &str, y: &str) -> &str {
  2.     if x.len() > y.len() {
  3.         x
  4.     } else {
  5.         y
  6.     }
  7. }
  1.  --> src\main.rs:6:33
  2.   |
  3. 6 | fn longest(x: &str, y: &str) -> &str {
  4.   |                                 ^ expected lifetime parameter
  5.   |
  6.   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `x` or `y`
  1. fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
  2.     if x.len() > y.len() {
  3.         x
  4.     } else {
  5.         y
  6.     }
  7. }