implement ZeroableOption for &T and &mut T

`Option<&T>` and `Option<&mut T>` are documented [1] to have the `None`
variant be all zeroes.

Link: https://doc.rust-lang.org/stable/std/option/index.html#representation [1]
Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `impl<T, E> [Pin]Init<T, E> for Result<T, E>`, so results are now (pin-)initializers
 - add `Zeroable::init_zeroed()` delegating to `init_zeroed()`
 - add new `zeroed()`, a safe version of `mem::zeroed()` and also provide it via `Zeroable::zeroed()`
+- implement `Zeroable` for `Option<&T>` and `Option<&mut T>`
 
 ### Changed
 

src/lib.rs

@@ -1546,6 +1546,13 @@ pub unsafe trait ZeroableOption {}
 // SAFETY: by the safety requirement of `ZeroableOption`, this is valid.
 unsafe impl<T: ZeroableOption> Zeroable for Option<T> {}
 
+// SAFETY: `Option<&T>` is part of the option layout optimization guarantee:
+// <https://doc.rust-lang.org/stable/std/option/index.html#representation>.
+unsafe impl<T> ZeroableOption for &T {}
+// SAFETY: `Option<&mut T>` is part of the option layout optimization guarantee:
+// <https://doc.rust-lang.org/stable/std/option/index.html#representation>.
+unsafe impl<T> ZeroableOption for &mut T {}
+
 /// Create an initializer for a zeroed `T`.
 ///
 /// The returned initializer will write `0x00` to every byte of the given `slot`.