[PATCH v15 8/9] rust: implement `ForeignOwnable` for `Owned`

[Andreas Hindborg <a.hindborg@kernel.org>]

Implement `ForeignOwnable` for `Owned<T>`. This allows use of `Owned<T>` in
places such as the `XArray`.

Note that `T` does not need to implement `ForeignOwnable` for `Owned<T>` to
implement `ForeignOwnable`.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/owned.rs | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
index a9bc871e07ce1..b115b4f3db6d0 100644
--- a/rust/kernel/owned.rs
+++ b/rust/kernel/owned.rs
@@ -16,7 +16,10 @@
 };
 use kernel::{
     sync::aref::ARef,
-    types::RefCounted, //
+    types::{
+        ForeignOwnable, //
+        RefCounted,
+    }, //
 };
 
 /// Types that specify their own way of performing allocation and destruction. Typically, this trait
@@ -120,6 +123,7 @@ pub unsafe trait Ownable {
 ///
 /// - The [`Owned<T>`] has exclusive access to the instance of `T`.
 /// - The instance of `T` will stay alive at least as long as the [`Owned<T>`] is alive.
+#[repr(transparent)]
 pub struct Owned<T: Ownable> {
     ptr: NonNull<T>,
 }
@@ -201,6 +205,45 @@ fn drop(&mut self) {
     }
 }
 
+// SAFETY: We derive the pointer to `T` from a valid `T`, so the returned
+// pointer satisfy alignment requirements of `T`.
+unsafe impl<T: Ownable + 'static> ForeignOwnable for Owned<T> {
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<Owned<T>>();
+
+    type Borrowed<'a> = &'a T;
+    type BorrowedMut<'a> = Pin<&'a mut T>;
+
+    fn into_foreign(self) -> *mut kernel::ffi::c_void {
+        let ptr = self.ptr.as_ptr().cast();
+        core::mem::forget(self);
+        ptr
+    }
+
+    unsafe fn from_foreign(ptr: *mut kernel::ffi::c_void) -> Self {
+        Self {
+            // SAFETY: By function safety contract, `ptr` came from
+            // `into_foreign` and cannot be null.
+            ptr: unsafe { NonNull::new_unchecked(ptr.cast()) },
+        }
+    }
+
+    unsafe fn borrow<'a>(ptr: *mut kernel::ffi::c_void) -> Self::Borrowed<'a> {
+        // SAFETY: By function safety requirements, `ptr` is valid for use as a
+        // reference for `'a`.
+        unsafe { &*ptr.cast() }
+    }
+
+    unsafe fn borrow_mut<'a>(ptr: *mut kernel::ffi::c_void) -> Self::BorrowedMut<'a> {
+        // SAFETY: By function safety requirements, `ptr` is valid for use as a
+        // unique reference for `'a`.
+        let inner = unsafe { &mut *ptr.cast() };
+
+        // SAFETY: We never move out of inner, and we do not hand out mutable
+        // references when `T: !Unpin`.
+        unsafe { Pin::new_unchecked(inner) }
+    }
+}
+
 /// A trait for objects that can be wrapped in either one of the reference types [`Owned`] and
 /// [`ARef`].
 ///

-- 
2.51.2