rust: pin-init: 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
Link: https://github.com/Rust-for-Linux/pin-init/pull/56/commits/5ef1638c79e019d3dc0c62db5905601644c2e60a
Link: https://lore.kernel.org/all/20250523145125.523275-5-lossin@kernel.org
Signed-off-by: Benno Lossin <lossin@kernel.org>

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index a5bb3939b58b4caec2c5a1c848eaa07de08cc0d9..298a3e675b7f691b326c4533676d36e75d7beab5 100644 (file)
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/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`.