Borrow Checker 如何阻止悬垂引用

  源码:
      1 | fn dangle() -> &String {
      2 |     let s = String::from("hello");
      3 |     &s
      4 | }
      5 | fn main() { println!("{}", dangle()); }
  编译:失败
    error[E0106]: missing lifetime specifier
     --> <实验目录>/src/u18a.rs:1:16
      |
    1 | fn dangle() -> &String {
      |                ^ expected named lifetime parameter
      |
      = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
    help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
      |
    1 | fn dangle() -> &'static String {
      |                 +++++++
    help: instead, you are more likely to want to return an owned value
      |
    1 - fn dangle() -> &String {
    1 + fn dangle() -> String {
      |
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0106`.
  源码:
      1 | fn dangle<'a>() -> &'a String {
      2 |     let s = String::from("hello");
      3 |     &s
      4 | }
      5 | fn main() { println!("{}", dangle()); }
  编译:失败
    error[E0515]: cannot return reference to local variable `s`
     --> <实验目录>/src/u18b.rs:3:5
      |
    3 |     &s
      |     ^^ returns a reference to data owned by the current function
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0515`.

  源码:
      1 | fn main() {
      2 |     let r;
      3 |     {
      4 |         let x = 5;
      5 |         r = &x;
      6 |     }
      7 |     println!("r = {}", r);
      8 | }
  编译:失败
    error[E0597]: `x` does not live long enough
     --> <实验目录>/src/u18c.rs:5:13
      |
    4 |         let x = 5;
      |             - binding `x` declared here
    5 |         r = &x;
      |             ^^ borrowed value does not live long enough
    6 |     }
      |     - `x` dropped here while still borrowed
    7 |     println!("r = {}", r);
      |                        - 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 main() {
      2 |     let mut v = vec![1, 2, 3];
      3 |     let first = &v[0];
      4 |     v.push(4);
      5 |     println!("first = {}", first);
      6 | }
  编译:失败
    error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
     --> <实验目录>/src/u18d.rs:4:5
      |
    3 |     let first = &v[0];
      |                  - immutable borrow occurs here
    4 |     v.push(4);
      |     ^^^^^^^^^ mutable borrow occurs here
    5 |     println!("first = {}", first);
      |                            ----- immutable borrow later used here
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0502`.
  为什么要禁止?用 unsafe 的裸指针亲眼看看 push 之后元素搬家了(只是为了观察,真实代码不要这么写):
  编译:成功
  运行(退出码 0):
    push 之前 cap = 3,v[0] 的地址 = 0x100b65300
    push 之后 cap = 6,v[0] 的地址 = 0x100b65170
    地址变了:true(旧地址上的内存已经被释放)

  源码:
      1 | fn longest_owned() -> String { let s = String::from("hello"); s }       // 返回所有权
      2 | fn main() {
      3 |     println!("{}", longest_owned());
      4 |     let r; let x = 5; r = &x;                                           // x 活得比 r 久
      5 |     println!("r = {r}");
      6 |     let mut v = vec![1, 2, 3];
      7 |     let first = v[0];                                                   // 拷贝出值,不再持有引用
      8 |     v.push(4);
      9 |     println!("first = {first},v = {:?}", v);
     10 | }
  编译:成功
  运行(退出码 0):
    hello
    r = 5
    first = 1,v = [1, 2, 3, 4]