#[repr(C)] 与内存布局

  编译:成功
  运行(退出码 0):
    repr(Rust): size 8 align 4  偏移 a=6 b=0 c=4
    repr(C):    size 12 align 4  偏移 a=0 b=4 c=8
    字段本身合计 7 字节;repr(C) 多出的 5 字节是填充(padding)

  编译:成功
  运行(退出码 0):
    repr(C) 声明顺序 u8,u64,u8,u32: 24 字节
    repr(C) 声明顺序 u64,u32,u8,u8: 16 字节(按大小降序,填充最少)
    repr(Rust) 同样四个字段(任意声明顺序): 16 字节(编译器自己重排)

  编译:成功
  运行(退出码 0):
    packed: size 7 align 1(没有填充,各字段紧挨着)
    align(16) 的 struct,只含一个 u32: size 16 align 16(大小被补到对齐的倍数)
    按值读 packed 字段是安全的: b = 2
  源码:
      1 | #[repr(C, packed)] struct P { a: u8, b: u32 }
      2 | fn main() { let p = P { a: 1, b: 2 }; let r = &p.b; println!("{}", r); }
  编译:失败
    error[E0793]: reference to field of packed struct is unaligned
     --> <实验目录>/src/u112d.rs:2:47
      |
    2 | fn main() { let p = P { a: 1, b: 2 }; let r = &p.b; println!("{}", r); }
      |                                               ^^^^
      |
      = note: this struct is 1-byte aligned, but the type of this field may require higher alignment
      = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
      = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0793`.

  编译:成功
  运行(退出码 0):
    默认 enum E0 {A,B,C}: 1  repr(u8): 1  repr(C): 4  repr(u32): 4
    带数据 enum {A(u8), B(u32), C}: 默认 8  repr(C): 8
    E1::C as u8 = 200(repr(u8) 的判别值可以读出来)

  编译:成功
  运行(退出码 0):
    repr(C) 结构体的 12 个字节(十六进制): aa 00 00 00 44 33 22 11 ef be 00 00
    tag 在字节 0,len 从字节 4 开始(小端:44 33 22 11),flags 从字节 8 开始(小端:ef be);其余是填充(内容未指定)