Add slide for unsafe traits.

src/SUMMARY.md

@@ -166,6 +166,7 @@
   - [Unions](unsafe/unions.md)
   - [Calling Unsafe Functions](unsafe/unsafe-functions.md)
     - [Extern Functions](unsafe/extern-functions.md)
+  - [Implementing Unsafe Traits](unsafe/unsafe-traits.md)
 - [Exercises](exercises/day-3/afternoon.md)
   - [Safe FFI Wrapper](exercises/day-3/safe-ffi-wrapper.md)
 

src/unsafe/unsafe-traits.md

@@ -0,0 +1,37 @@
+# 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](https://docs.rs/zerocopy/latest/zerocopy/trait.AsBytes.html):
+
+```rust,editable
+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 {}
+```
+
+<details>
+
+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.
+
+</details>