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

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
> 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>

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

One nit below:

> ---
>  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;

I think `ForeignOwnable` also implies a `T` maybe get dropped via a
pointer from `into_foreign()`. Not sure it's worth mentioning though.

Regards,
Boqun

> +
>      /// 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
>