为什么不将 u8 指针转换为 [bool; 8] 指针逐位产生数字？

Question

我正在 Rust 中尝试原始指针。我有以下代码：

fn main() {
    let mut t: u8 = 0;

    let addr = &mut t as *mut u8 as usize;

    let x = addr as *mut [bool; 8];

    let y = addr as *mut u8;
    unsafe {
        *y = 0b10101010;
        println!("{:?} {:b}", *x, *y);
    }
}

它产生以下输出：[true, true, true, true, true, true, true, false] 10101010 虽然我希望它打印[true, false, true, false, true, false, true, false] 10101010. 到底是怎么回事？bool数组不是按位存储的吗？

Answer 1

该程序的行为未定义（因此输出毫无意义）。从美里出发：

error: Undefined Behavior: memory access failed: pointer must be in-bounds at offset 8, but is outside bounds of alloc1381 which has size 1
  --> src/main.rs:11:31
   |
11 |         println!("{:?} {:b}", *x, *y);
   |                               ^^ memory access failed: pointer must be in-bounds at offset 8, but is outside bounds of alloc1381 which has size 1
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

布尔数组是按字节存储的，而不是按位存储的。如果您想要逐位存储，请使用bitvec或crate。bitfield指针无法指向单个位：指针始终指向字节（基本上任何 ISA 都不支持指向位的指针）。bools 的长度为 1 个字节，不能安全地使用0_u8或 以外的任何值1_u8。