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>

Author: BennoLossin

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

rust/kernel/types.rs

@@ -2,7 +2,7 @@
 
 //! Kernel types.
 
-use crate::init::{self, PinInit};
+use crate::init::{self, PinInit, Zeroable};
 use core::{
     cell::UnsafeCell,
     marker::{PhantomData, PhantomPinned},
@@ -309,6 +309,9 @@ pub struct Opaque<T> {
     _pin: PhantomPinned,
 }
 
+// SAFETY: `Opaque<T>` allows the inner value to be any bit pattern, including all zeros.
+unsafe impl<T> Zeroable for Opaque<T> {}
+
 impl<T> Opaque<T> {
     /// Creates a new opaque value.
     pub const fn new(value: T) -> Self {

rust/pin-init/src/lib.rs

@@ -211,10 +211,7 @@
 //! [`pin_data`]: ::macros::pin_data
 //! [`pin_init!`]: crate::pin_init!
 
-use crate::{
-    alloc::KBox,
-    types::{Opaque, ScopeGuard},
-};
+use crate::{alloc::KBox, types::ScopeGuard};
 use core::{
     cell::UnsafeCell,
     convert::Infallible,
@@ -1342,8 +1339,6 @@ 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>,
@@ -1358,7 +1353,6 @@ impl_zeroable! {
     //
     // 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<KBox<T>>,
 
     // SAFETY: `null` pointer is valid.
     //