Re: [PATCH v5 1/5] rust: types: implement ForeignOwnable for ARef<T>

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Missing `#[inline]` annotations on small trait methods in `ForeignOwnable` for `ARef<T>`.
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commit 51b0cb5da3a412f66a80b1925a746644ac0ed5c1
Author: Danilo Krummrich <dakr@kernel.org>
Subject: rust: types: implement ForeignOwnable for ARef<T>

This commit implements the ForeignOwnable trait for ARef<T>. It allows C
code to take ownership of ARef instances and defines BorrowedMut as a
shared reference, matching the semantics of the underlying reference-counted
type.

> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> index b721b2e00b986..5407666136595 100644
> --- a/rust/kernel/sync/aref.rs
> +++ b/rust/kernel/sync/aref.rs

[ ... ]

> @@ -188,6 +193,41 @@ fn eq(&self, other: &ARef<U>) -> bool {
>  }
>  impl<T: AlwaysRefCounted + Eq> Eq for ARef<T> {}
>  
> +// SAFETY: `into_foreign` returns a pointer from `NonNull::as_ptr`, so it's non-null. The
> +// `ARef` invariant guarantees that `ptr` points to a valid `T`, so it's aligned to `T`.
> +unsafe impl<T: AlwaysRefCounted + 'static> ForeignOwnable for ARef<T> {
> +    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
> +
> +    type Borrowed<'a> = &'a T;
> +    type BorrowedMut<'a> = &'a T;
> +
> +    fn into_foreign(self) -> *mut c_void {

[Severity: Low]
This isn't a bug, but the Rust subsystem coding guidelines require functions
in abstractions that are small or just forwarding to binding calls to be
annotated with #[inline]. 

Would it be appropriate to add the #[inline] annotation to into_foreign(),
as well as from_foreign(), borrow(), and borrow_mut() below, to ensure
optimal cross-crate inlining?

> +        ARef::into_raw(self).as_ptr().cast()
> +    }
> +
> +    unsafe fn from_foreign(ptr: *mut c_void) -> Self {
> +        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
> +        // call to `Self::into_foreign`.
> +        let ptr = unsafe { NonNull::new_unchecked(ptr.cast()) };
> +
> +        // SAFETY: `ptr` came from `into_foreign`, which consumed an `ARef` without decrementing
> +        // the refcount, so we can transfer the ownership to the new `ARef`.
> +        unsafe { ARef::from_raw(ptr) }
> +    }
> +
> +    unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T {
> +        // SAFETY: The safety requirements of this method ensure that the object remains alive and
> +        // immutable for the duration of 'a.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a T {
> +        // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
> +        // requirements for `borrow`.
> +        unsafe { <Self as ForeignOwnable>::borrow(ptr) }
> +    }
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260703073141.3962604-2-phasta@kernel.org?part=1