Implement `Borrow<T>` and `BorrowMut<T>` for `UniqueArc<T>`, and
`Borrow<T>` for `Arc<T>`. This allows these containers to be used in
generic APIs asking for types implementing those traits. `T` and `&mut
T` also implement those traits allowing users to use either owned,
shared or borrowed values.

`ForeignOwnable` makes a call to its own `borrow` method which must be
disambiguated.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/sync/arc.rs | 84 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 83 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index c7af0aa48a0a049bfeeba3a81080355f4d381738..c7832e18b6f129ea00f57fb8e38da68ebc209626 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -25,6 +25,7 @@
 };
 use core::{
     alloc::Layout,
+    borrow::{Borrow, BorrowMut},
     fmt,
     marker::PhantomData,
     mem::{ManuallyDrop, MaybeUninit},
@@ -406,7 +407,7 @@ unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
     unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
         // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
         // requirements for `borrow`.
-        unsafe { Self::borrow(ptr) }
+        unsafe { <Self as ForeignOwnable>::borrow(ptr) }
     }
 }
 
@@ -426,6 +427,33 @@ fn as_ref(&self) -> &T {
     }
 }
 
+/// Allows `Arc<T>` to be used as a `Borrow<T>`.
+///
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::sync::Arc;
+/// struct Foo<B: Borrow<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Shared instance.
+/// let arc = Arc::new(1, GFP_KERNEL)?;
+/// let foo_shared = Foo(arc.clone());
+///
+/// let i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T: ?Sized> Borrow<T> for Arc<T> {
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
@@ -834,6 +862,60 @@ fn deref_mut(&mut self) -> &mut Self::Target {
     }
 }
 
+/// Allows `UniqueArc<T>` to be used as a `Borrow<T>`.
+///
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::sync::UniqueArc;
+/// struct Foo<B: Borrow<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `UniqueArc`.
+/// let arc = UniqueArc::new(1, GFP_KERNEL)?;
+/// let foo_shared = Foo(arc);
+///
+/// let i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T: ?Sized> Borrow<T> for UniqueArc<T> {
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
+/// Allows `UniqueArc<T>` to be used as a `BorrowMut<T>`.
+///
+/// # Examples
+///
+/// ```
+/// # use core::borrow::BorrowMut;
+/// # use kernel::sync::UniqueArc;
+/// struct Foo<B: BorrowMut<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `UniqueArc`.
+/// let arc = UniqueArc::new(1, GFP_KERNEL)?;
+/// let foo_shared = Foo(arc);
+///
+/// let mut i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&mut i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T: ?Sized> BorrowMut<T> for UniqueArc<T> {
+    fn borrow_mut(&mut self) -> &mut T {
+        self.deref_mut()
+    }
+}
+
 impl<T: fmt::Display + ?Sized> fmt::Display for UniqueArc<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Display::fmt(self.deref(), f)

-- 
2.49.0

On Fri Jun 13, 2025 at 3:46 PM CEST, Alexandre Courbot wrote:
> Implement `Borrow<T>` and `BorrowMut<T>` for `UniqueArc<T>`, and
> `Borrow<T>` for `Arc<T>`. This allows these containers to be used in
> generic APIs asking for types implementing those traits. `T` and `&mut
> T` also implement those traits allowing users to use either owned,
> shared or borrowed values.
>
> `ForeignOwnable` makes a call to its own `borrow` method which must be
> disambiguated.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

With the doc comments fixed as detailed in the other thread:

Reviewed-by: Benno Lossin <lossin@kernel.org>

---
Cheers,
Benno

> ---
>  rust/kernel/sync/arc.rs | 84 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 83 insertions(+), 1 deletion(-)