[PATCH v4 4/5] rust: types: ForeignOwnable: Add type Target

[Danilo Krummrich <dakr@kernel.org>]

ForeignOwnable::Target defines the payload data of a ForeignOwnable. For
Arc<T> for instance, ForeignOwnable::Target would just be T.

This is useful for cases where a trait bound is required on the target
type of the ForeignOwnable. For instance:

	fn example<P>(data: P)
	   where
	      P: ForeignOwnable,
	      P::Target: MyTrait,
	{}

Suggested-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/alloc/kbox.rs | 2 ++
 rust/kernel/sync/arc.rs   | 1 +
 rust/kernel/types.rs      | 4 ++++
 3 files changed, 7 insertions(+)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c386ff771d50..66fad9777567 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -403,6 +403,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
 where
     A: Allocator,
 {
+    type Target = T;
     type PointedTo = T;
     type Borrowed<'a> = &'a T;
     type BorrowedMut<'a> = &'a mut T;
@@ -435,6 +436,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 where
     A: Allocator,
 {
+    type Target = T;
     type PointedTo = T;
     type Borrowed<'a> = Pin<&'a T>;
     type BorrowedMut<'a> = Pin<&'a mut T>;
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index c7af0aa48a0a..24fb63597d35 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -374,6 +374,7 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
 
 // SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
 unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
+    type Target = T;
     type PointedTo = ArcInner<T>;
     type Borrowed<'a> = ArcBorrow<'a, T>;
     type BorrowedMut<'a> = Self::Borrowed<'a>;
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 3958a5f44d56..74c787b352a9 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -27,6 +27,9 @@
 /// [`into_foreign`]: Self::into_foreign
 /// [`PointedTo`]: Self::PointedTo
 pub unsafe trait ForeignOwnable: Sized {
+    /// The payload type of the foreign-owned value.
+    type Target;
+
     /// Type used when the value is foreign-owned. In practical terms only defines the alignment of
     /// the pointer.
     type PointedTo;
@@ -128,6 +131,7 @@ unsafe fn try_from_foreign(ptr: *mut Self::PointedTo) -> Option<Self> {
 
 // SAFETY: The `into_foreign` function returns a pointer that is dangling, but well-aligned.
 unsafe impl ForeignOwnable for () {
+    type Target = ();
     type PointedTo = ();
     type Borrowed<'a> = ();
     type BorrowedMut<'a> = ();
-- 
2.49.0