rust: delete `ForeignOwnable::borrow_mut`

We discovered that the current design of `borrow_mut` is problematic.
This patch removes it until a better solution can be found.

Specifically, the current design gives you access to a `&mut T`, which
lets you change where the `ForeignOwnable` points (e.g., with
`core::mem::swap`). No upcoming user of this API intended to make that
possible, making all of them unsound.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Fixes: 0fc4424d24a2 ("rust: types: introduce `ForeignOwnable`")
Link: https://lore.kernel.org/r/20230706094615.3080784-1-aliceryhl@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index a89843cacaad..172f563976a9 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -243,8 +243,7 @@ impl<T: 'static> ForeignOwnable for Arc<T> {
         let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
 
         // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
-        // for the lifetime of the returned value. Additionally, the safety requirements of
-        // `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
+        // for the lifetime of the returned value.
         unsafe { ArcBorrow::new(inner) }
     }