rust: pin-init: move impl `Zeroable` for `Opaque` and `Option<KBox<T>>` into the kernel crate

In order to make pin-init a standalone crate, move kernel-specific code
directly into the kernel crate. Since `Opaque<T>` and `KBox<T>` are part
of the kernel, move their `Zeroable` implementation into the kernel
crate.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/20250308110339.2997091-10-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 39a3ea7542da..9861433559dc 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -15,7 +15,7 @@ use core::pin::Pin;
 use core::ptr::NonNull;
 use core::result::Result;
 
-use crate::init::{InPlaceWrite, Init, PinInit};
+use crate::init::{InPlaceWrite, Init, PinInit, Zeroable};
 use crate::init_ext::InPlaceInit;
 use crate::types::ForeignOwnable;
 
@@ -100,6 +100,12 @@ pub type VBox<T> = Box<T, super::allocator::Vmalloc>;
 /// ```
 pub type KVBox<T> = Box<T, super::allocator::KVmalloc>;
 
+// 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 and there
+// is no problem with a VTABLE pointer being null.
+unsafe impl<T: ?Sized, A: Allocator> Zeroable for Option<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>
 where