MaybeUninit

  源码:
      1 | fn main() { let x: u32; println!("{}", x); }
  编译:失败
    error[E0381]: used binding `x` isn't initialized
     --> <实验目录>/src/u118a.rs:1:40
      |
    1 | fn main() { let x: u32; println!("{}", x); }
      |                 -                      ^ `x` used here but it isn't initialized
      |                 |
      |                 binding declared here but left uninitialized
      |
    help: consider assigning a value
      |
    1 | fn main() { let x: u32 = 42; println!("{}", x); }
      |                        ++++
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0381`.
  编译:成功
  运行(退出码 0):
      size_of::<u64>() = 8  size_of::<MaybeUninit<u64>>() = 8  align 同样 8 / 8
      size_of::<Option<u64>>() = 16(Option 要标签)而 MaybeUninit<u64> 没有额外字节:它不记录是否已初始化
      write 之后 assume_init = 42

  编译:成功
  运行(退出码 0):
      逐个 write 之后整体转成 [String; 4]:["item-0", "item-1", "item-4", "item-9"]
      对照:std::array::from_fn 的安全写法得到相同结果: true

  编译:成功
  运行(退出码 0):
      1) MaybeUninit<Noisy> 离开作用域:
         (没有看到 Noisy(1) drop —— MaybeUninit 不负责析构,值泄漏了)
      2) 手动 assume_init_drop:
      Noisy(2) drop
      3) 数组只初始化了前 2 个,只 drop 这 2 个:
      Noisy(10) drop
      Noisy(11) drop

  编译:成功
  运行(退出码 0):
      vec![0u8; 1 MiB](清零)          每次    11.07 µs
      with_capacity + set_len(不清零) 每次     0.93 µs(只写了 1/4096 的字节,其余保持未初始化)