[PATCH 10/22] rust: add `ZeroableOption` and implement it instead of `Zeroable` for `Option<Box<T, A>>`

[Benno Lossin]

When making pin-init its own crate, `Zeroable` will no longer be defined
by the kernel crate and thus implementing it for `Option<Box<T, A>>` is
no longer possible due to the orphan rule.
For this reason introduce a new `ZeroableOption` trait that circumvents
this problem.

Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>
---
 rust/kernel/alloc/kbox.rs |  4 ++--
 rust/pin-init/src/lib.rs  | 11 +++++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 9861433559dc..07150c038e3f 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -15,7 +15,7 @@
 use core::ptr::NonNull;
 use core::result::Result;
 
-use crate::init::{InPlaceWrite, Init, PinInit, Zeroable};
+use crate::init::{InPlaceWrite, Init, PinInit, ZeroableOption};
 use crate::init_ext::InPlaceInit;
 use crate::types::ForeignOwnable;
 
@@ -104,7 +104,7 @@
 //
 // In this case we are allowed to use `T: ?Sized`, since all zeros is the `None` variant and there
 // is no problem with a VTABLE pointer being null.
-unsafe impl<T: ?Sized, A: Allocator> Zeroable for Option<Box<T, A>> {}
+unsafe impl<T: ?Sized, A: Allocator> ZeroableOption for Box<T, A> {}
 
 // SAFETY: `Box` is `Send` if `T` is `Send` because the `Box` owns a `T`.
 unsafe impl<T, A> Send for Box<T, A>
diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 5fd8fa7c2ce9..fd2c4fdd0ca4 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -1297,6 +1297,17 @@ pub unsafe trait PinnedDrop: __internal::HasPinData {
 /// ```
 pub unsafe trait Zeroable {}
 
+/// Marker trait for types that allow `Option<Self>` to be set to all zeroes in order to write
+/// `None` to that location.
+///
+/// # Safety
+///
+/// The implementer needs to ensure that `unsafe impl Zeroable for Option<Self> {}` is sound.
+pub unsafe trait ZeroableOption {}
+
+// SAFETY: by the safety requirement of `ZeroableOption`, this is valid.
+unsafe impl<T: ZeroableOption> Zeroable for Option<T> {}
+
 /// Create a new zeroed T.
 ///
 /// The returned initializer will write `0x00` to every byte of the given `slot`.
-- 
2.47.2