實作不安全的特徵

與函式類似，如果實作程序必須保證符合特定條件才能避免未定義的行為，您可以將特徵標示為 unsafe。

舉例來說，zerocopy crate 就具有不安全的特徵，如這個頁面所示：

use std::mem::size_of_val;
use std::slice;

/// ...
/// # Safety
/// The type must have a defined representation and no padding.
pub unsafe trait AsBytes {
    fn as_bytes(&self) -> &[u8] {
        unsafe {
            slice::from_raw_parts(
                self as *const Self as *const u8,
                size_of_val(self),
            )
        }
    }
}

// Safe because u32 has a defined representation and no padding.
unsafe impl AsBytes for u32 {}