RefCell<T> 运行时借用检查

  源码:
      1 | use std::cell::RefCell;
      2 | fn main() {
      3 |     let c = RefCell::new(vec![1, 2, 3]);       // 注意:c 不是 mut
      4 |     c.borrow_mut().push(4);                     // 通过共享引用修改
      5 |     println!("{:?},长度 {}", c.borrow(), c.borrow().len());
      6 |     {
      7 |         let r1 = c.borrow(); let r2 = c.borrow();      // 多个不可变借用:可以
      8 |         println!("同时两个 borrow():{} {}", r1.len(), r2.len());
      9 |     }
     10 | }
  编译:成功
  运行(退出码 0):
    [1, 2, 3, 4],长度 4
    同时两个 borrow():4 4

  编译:成功
  运行(退出码 101):
    持有一个 borrow() 时再 borrow_mut() ...
    
    thread 'main' (6994266) panicked at <实验目录>/src/u32b.rs:6:15:
    RefCell already borrowed
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  源码:
      1 | use std::cell::RefCell;
      2 | fn main() {
      3 |     let c = RefCell::new(5);
      4 |     let _w1 = c.borrow_mut();
      5 |     println!("try_borrow_mut() 在已有可变借用时:{:?}", c.try_borrow_mut().is_err());
      6 |     println!("try_borrow()     在已有可变借用时:{:?}", c.try_borrow().is_err());
      7 | }
  编译:成功
  运行(退出码 0):
    try_borrow_mut() 在已有可变借用时:true
    try_borrow()     在已有可变借用时:true

  源码:
      1 | fn main() {
      2 |     let mut x = 5;
      3 |     let r = &x;
      4 |     let w = &mut x;
      5 |     println!("{} {}", r, w);
      6 | }
  编译:失败
    error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable
     --> <实验目录>/src/u32d.rs:4:13
      |
    3 |     let r = &x;
      |             -- immutable borrow occurs here
    4 |     let w = &mut x;
      |             ^^^^^^ mutable borrow occurs here
    5 |     println!("{} {}", r, w);
      |                       - immutable borrow later used here
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0502`.

  源码:
      1 | use std::cell::RefCell;
      2 | use std::rc::Rc;
      3 | fn main() {
      4 |     let shared = Rc::new(RefCell::new(Vec::<i32>::new()));
      5 |     let a = Rc::clone(&shared); let b = Rc::clone(&shared);
      6 |     a.borrow_mut().push(1);
      7 |     b.borrow_mut().push(2);
      8 |     println!("两个所有者各自 push 一次,共享的 Vec = {:?}", shared.borrow());
      9 | }
  编译:成功
  运行(退出码 0):
    两个所有者各自 push 一次,共享的 Vec = [1, 2]
  编译:成功
  运行(退出码 0):
    size_of::<u64>() = 8,size_of::<RefCell<u64>>() = 16(借用标志是一个 isize,占 8 字节)
    size_of::<u8>() = 1,size_of::<RefCell<u8>>() = 16