编译期断言

  源码:
      1 | #[repr(C)] struct Header { tag: u8, len: u32 }
      2 | const _: () = assert!(std::mem::size_of::<Header>() == 8, "Header 必须是 8 字节");
      3 | fn main() { println!("size = {}", std::mem::size_of::<Header>()); }
  编译:成功
  运行(退出码 0):
    size = 8
  源码:
      1 | #[repr(C)] struct Header { tag: u8, len: u32 }
      2 | const _: () = assert!(std::mem::size_of::<Header>() == 6, "Header 必须是 6 字节");
      3 | fn main() { println!("size = {}", std::mem::size_of::<Header>()); }
  编译:失败
    error[E0080]: evaluation panicked: Header 必须是 6 字节
     --> <实验目录>/src/u84b.rs:2:15
      |
    2 | const _: () = assert!(std::mem::size_of::<Header>() == 6, "Header 必须是 6 字节");
      |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `_` failed here
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0080`.

  有 const 断言的 f 汇编 2 行,没有断言的 2 行,逐行相同: True

  源码:
      1 | struct Buf<const N: usize>;
      2 | impl<const N: usize> Buf<N> {
      3 |     const OK: () = assert!(N.is_power_of_two(), "N 必须是 2 的幂");
      4 |     fn new() -> Self { let () = Self::OK; Buf }
      5 | }
      6 | fn main() { let _a = Buf::<8>::new(); println!("Buf::<8> 创建成功"); }
  编译:成功
  运行(退出码 0):
    Buf::<8> 创建成功
  源码:
      1 | struct Buf<const N: usize>;
      2 | impl<const N: usize> Buf<N> {
      3 |     const OK: () = assert!(N.is_power_of_two(), "N 必须是 2 的幂");
      4 |     fn new() -> Self { let () = Self::OK; Buf }
      5 | }
      6 | fn main() { let _a = Buf::<8>::new(); let _b = Buf::<6>::new(); }
  编译:失败
    error[E0080]: evaluation panicked: N 必须是 2 的幂
     --> <实验目录>/src/u84e.rs:3:20
      |
    3 |     const OK: () = assert!(N.is_power_of_two(), "N 必须是 2 的幂");
      |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `Buf::<6>::OK` failed here
    
    note: erroneous constant encountered
     --> <实验目录>/src/u84e.rs:4:33
      |
    4 |     fn new() -> Self { let () = Self::OK; Buf }
      |                                 ^^^^^^^^
    
    note: the above error was encountered while instantiating `fn Buf::<6>::new`
     --> <实验目录>/src/u84e.rs:6:48
      |
    6 | fn main() { let _a = Buf::<8>::new(); let _b = Buf::<6>::new(); }
      |                                                ^^^^^^^^^^^^^^^
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0080`.

  源码:
      1 | fn first_n<const N: usize>() -> [u8; N] { const { assert!(N > 0 && N <= 4, "N 必须在 1..=4") }; [0; N] }
      2 | fn main() { println!("{:?}", first_n::<3>()); }
  编译:成功
  运行(退出码 0):
    [0, 0, 0]
  源码:
      1 | fn first_n<const N: usize>() -> [u8; N] { const { assert!(N > 0 && N <= 4, "N 必须在 1..=4") }; [0; N] }
      2 | fn main() { println!("{:?}", first_n::<9>()); }
  编译:失败
    error[E0080]: evaluation panicked: N 必须在 1..=4
     --> <实验目录>/src/u84g.rs:1:51
      |
    1 | fn first_n<const N: usize>() -> [u8; N] { const { assert!(N > 0 && N <= 4, "N 必须在 1..=4") }; [0; N] }
      |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `first_n::<9>::{constant#1}` failed here
    
    note: erroneous constant encountered
     --> <实验目录>/src/u84g.rs:1:43
      |
    1 | fn first_n<const N: usize>() -> [u8; N] { const { assert!(N > 0 && N <= 4, "N 必须在 1..=4") }; [0; N] }
      |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    
    note: the above error was encountered while instantiating `fn first_n::<9>`
     --> <实验目录>/src/u84g.rs:2:30
      |
    2 | fn main() { println!("{:?}", first_n::<9>()); }
      |                              ^^^^^^^^^^^^^^
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0080`.

  源码:
      1 | const _: [(); 0 - (std::mem::size_of::<u32>() != 4) as usize] = [];
      2 | const _: [(); 0 - (std::mem::size_of::<u32>() != 5) as usize] = [];
      3 | fn main() {}
  编译:失败
    error[E0080]: attempt to compute `0_usize - 1_usize`, which would overflow
     --> <实验目录>/src/u84h.rs:2:15
      |
    2 | const _: [(); 0 - (std::mem::size_of::<u32>() != 5) as usize] = [];
      |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `_::{constant#0}` failed here
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0080`.