[PATCH v8 1/2] rust: types: add FOREIGN_ALIGN to ForeignOwnable

[Maíra Canal <mcanal@xxxxxxxxxx>]

There are cases where we need to check the alignment of the pointers
returned by `into_foreign`. Currently, this is not possible to be done
at build time. Therefore, add a property to the trait ForeignOwnable,
which specifies the alignment of the pointers returned by
`into_foreign`.

Suggested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Maíra Canal <mcanal@xxxxxxxxxx>
Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
 rust/kernel/sync/arc.rs | 2 ++
 rust/kernel/types.rs    | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 7d4c4bf58388..da5c8cc325b6 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -274,6 +274,8 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
 }

 impl<T: 'static> ForeignOwnable for Arc<T> {
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<ArcInner<T>>();
+
     type Borrowed<'a> = ArcBorrow<'a, T>;

     fn into_foreign(self) -> *const core::ffi::c_void {
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index aa77bad9bce4..76cd4226dd35 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -20,6 +20,9 @@
 /// This trait is meant to be used in cases when Rust objects are stored in C objects and
 /// eventually "freed" back to Rust.
 pub trait ForeignOwnable: Sized {
+    /// The alignment of pointers returned by `into_foreign`.
+    const FOREIGN_ALIGN: usize;
+
     /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
     /// [`ForeignOwnable::from_foreign`].
     type Borrowed<'a>;
@@ -68,6 +71,8 @@ unsafe fn try_from_foreign(ptr: *const core::ffi::c_void) -> Option<Self> {
 }

 impl<T: 'static> ForeignOwnable for Box<T> {
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
+
     type Borrowed<'a> = &'a T;

     fn into_foreign(self) -> *const core::ffi::c_void {
@@ -90,6 +95,8 @@ unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
 }

 impl ForeignOwnable for () {
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<()>();
+
     type Borrowed<'a> = ();

     fn into_foreign(self) -> *const core::ffi::c_void {
--
2.43.0