[PATCH v4 10/13] rust: init: implement `Zeroable` for `UnsafeCell<T>` and `Opaque<T>`

[Benno Lossin <benno.lossin@proton.me>]

`UnsafeCell<T>` and `T` have the same layout so if `T` is `Zeroable`
then so should `UnsafeCell<T>` be. This allows using the derive macro
for `Zeroable` on types that contain an `UnsafeCell<T>`.
Since `Opaque<T>` contains a `MaybeUninit<T>`, all bytes zero is a valid
bit pattern for that type.

Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
---
v3 -> v4:
- added Reviewed-by's from Gary and Martin.

v2 -> v3:
- also implement Zeroable for `UnsafeCell<T>` when `T: Zeroable`,
- use `impl_zeroable!` instead of `derive(Zeroable)`.

 rust/kernel/init.rs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index b54f32017d00..f91ecd3b1edf 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -212,11 +212,12 @@
 use crate::{
     error::{self, Error},
     sync::UniqueArc,
-    types::ScopeGuard,
+    types::{Opaque, ScopeGuard},
 };
 use alloc::boxed::Box;
 use core::{
     alloc::AllocError,
+    cell::UnsafeCell,
     convert::Infallible,
     marker::PhantomData,
     mem::MaybeUninit,
@@ -1159,6 +1160,11 @@ macro_rules! impl_zeroable {
 
     // SAFETY: Type is allowed to take any value, including all zeros.
     {<T>} MaybeUninit<T>,
+    // SAFETY: Type is allowed to take any value, including all zeros.
+    {<T>} Opaque<T>,
+
+    // SAFETY: `T: Zeroable` and `UnsafeCell` is `repr(transparent)`.
+    {<T: ?Sized + Zeroable>} UnsafeCell<T>,
 
     // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
     Option<NonZeroU8>, Option<NonZeroU16>, Option<NonZeroU32>, Option<NonZeroU64>,
-- 
2.41.0