[PATCH 6.6.y] rust: init: fix `Zeroable` implementation for `Option<NonNull<T>>` and `Option<Box<T>>`

[Miguel Ojeda <ojeda@kernel.org>]

From: Benno Lossin <benno.lossin@proton.me>

commit df27cef153603b18a7d094b53cc3d5264ff32797 upstream.

According to [1], `NonNull<T>` and `#[repr(transparent)]` wrapper types
such as `Box<T>` have the null pointer optimization only if `T: Sized`.
Thus remove the `Zeroable` implementation for the unsized case.

Link: https://doc.rust-lang.org/stable/std/option/index.html#representation [1]
Reported-by: Alice Ryhl <aliceryhl@google.com>
Closes: https://lore.kernel.org/rust-for-linux/CAH5fLghL+qzrD8KiCF1V3vf2YcC6aWySzkmaE2Zzrnh1gKj-hw@mail.gmail.com/
Cc: stable@vger.kernel.org # v6.12+ (a custom patch will be needed for 6.6.y)
Fixes: 38cde0bd7b67 ("rust: init: add `Zeroable` trait and `init::zeroed` function")
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250305132836.2145476-1-benno.lossin@proton.me
[ Added Closes tag and moved up the Reported-by one. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/init.rs | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 2d4b19b86857..6ef9f6182018 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -1310,17 +1310,14 @@ macro_rules! impl_zeroable {
     // SAFETY: `T: Zeroable` and `UnsafeCell` is `repr(transparent)`.
     {<T: ?Sized + Zeroable>} UnsafeCell<T>,
 
-    // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
+    // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee:
+    // https://doc.rust-lang.org/stable/std/option/index.html#representation).
     Option<NonZeroU8>, Option<NonZeroU16>, Option<NonZeroU32>, Option<NonZeroU64>,
     Option<NonZeroU128>, Option<NonZeroUsize>,
     Option<NonZeroI8>, Option<NonZeroI16>, Option<NonZeroI32>, Option<NonZeroI64>,
     Option<NonZeroI128>, Option<NonZeroIsize>,
-
-    // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
-    //
-    // In this case we are allowed to use `T: ?Sized`, since all zeros is the `None` variant.
-    {<T: ?Sized>} Option<NonNull<T>>,
-    {<T: ?Sized>} Option<Box<T>>,
+    {<T>} Option<NonNull<T>>,
+    {<T>} Option<Box<T>>,
 
     // SAFETY: `null` pointer is valid.
     //
-- 
2.49.0