Implementing Unsafe Traits

Like with functions, you can mark a trait as unsafe if the implementation must guarantee particular conditions to avoid undefined behaviour.

For example, the zerocopy crate has an unsafe trait that looks something like this:

use std::{mem, slice};

/// ...
/// # Safety
/// The type must have a defined representation and no padding.
pub unsafe trait IntoBytes {
    fn as_bytes(&self) -> &[u8] {
        let len = mem::size_of_val(self);
        unsafe { slice::from_raw_parts((&raw const self).cast::<u8>(), len) }
    }
}

// SAFETY: `u32` has a defined representation and no padding.
unsafe impl IntoBytes for u32 {}