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::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 {
                self as *const Self as *const u8,

// Safe because u32 has a defined representation and no padding.
unsafe impl AsBytes for u32 {}
This slide should take about 5 minutes.

There should be a # Safety section on the Rustdoc for the trait explaining the requirements for the trait to be safely implemented.

The actual safety section for AsBytes is rather longer and more complicated.

The built-in Send and Sync traits are unsafe.