生命周期标注到底描述什么

  源码:
      1 | fn longest(x: &str, y: &str) -> &str { if x.len() > y.len() { x } else { y } }
      2 | fn main() { println!("{}", longest("abc", "de")); }
  编译:失败
    error[E0106]: missing lifetime specifier
     --> <实验目录>/src/u23a.rs:1:33
      |
    1 | fn longest(x: &str, y: &str) -> &str { if x.len() > y.len() { x } else { y } }
      |               ----     ----     ^ expected named lifetime parameter
      |
      = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `x` or `y`
    help: consider introducing a named lifetime parameter
      |
    1 | fn longest<'a>(x: &'a str, y: &'a str) -> &'a str { if x.len() > y.len() { x } else { y } }
      |           ++++     ++          ++          ++
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0106`.
  源码:
      1 | fn longest<'a>(x: &'a str, y: &'a str) -> &'a str { if x.len() > y.len() { x } else { y } }
      2 | fn main() { println!("{}", longest("abc", "de")); }
  编译:成功
  运行(退出码 0):
    abc

  源码:
      1 | fn longest<'a>(x: &'a str, y: &'a str) -> &'a str { if x.len() > y.len() { x } else { y } }
      2 | fn main() {
      3 |     let a = String::from("long string");
      4 |     let result;
      5 |     {
      6 |         let b = String::from("xyz");
      7 |         result = longest(a.as_str(), b.as_str());
      8 |     }
      9 |     println!("{}", result);
     10 | }
  编译:失败
    error[E0597]: `b` does not live long enough
     --> <实验目录>/src/u23c.rs:7:38
      |
    6 |         let b = String::from("xyz");
      |             - binding `b` declared here
    7 |         result = longest(a.as_str(), b.as_str());
      |                                      ^ borrowed value does not live long enough
    8 |     }
      |     - `b` dropped here while still borrowed
    9 |     println!("{}", result);
      |                    ------ borrow later used here
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0597`.
  源码:
      1 | fn longest<'a>(x: &'a str, y: &'a str) -> &'a str { if x.len() > y.len() { x } else { y } }
      2 | fn main() {
      3 |     let a = String::from("long string");
      4 |     {
      5 |         let b = String::from("xyz");
      6 |         let result = longest(a.as_str(), b.as_str());
      7 |         println!("在内层使用 result:{}", result);
      8 |     }
      9 | }
  编译:成功
  运行(退出码 0):
    在内层使用 result:long string

  源码:
      1 | fn first<'a, 'b>(x: &'a str, y: &'b str) -> &'a str { let _ = x; y }
      2 | fn main() { println!("{}", first("a", "b")); }
  编译:失败
    error: lifetime may not live long enough
     --> <实验目录>/src/u23e.rs:1:66
      |
    1 | fn first<'a, 'b>(x: &'a str, y: &'b str) -> &'a str { let _ = x; y }
      |          --  -- lifetime `'b` defined here                       ^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
      |          |
      |          lifetime `'a` defined here
      |
      = help: consider adding the following bound: `'b: 'a`
    
    error: aborting due to 1 previous error
  源码:
      1 | fn first<'a, 'b>(x: &'a str, _y: &'b str) -> &'a str { x }
      2 | fn main() {
      3 |     let a = String::from("long");
      4 |     let r;
      5 |     { let b = String::from("short"); r = first(&a, &b); }   // 返回值只与 a 相关,b 先死也没关系
      6 |     println!("{r}");
      7 | }
  编译:成功
  运行(退出码 0):
    long

  elided 与 explicit 共同的汇编: add	x0, x0, #4 ; ret
  别名行: `_explicit = _elided`(LLVM 合并了两个相同的函数)
  size_of::<&u32>() 在有无生命周期标注时都是 8 字节;生命周期只在编译期存在