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: 0fc4424d ("rust: types: introduce `ForeignOwnable`")
Link: https://lore.kernel.org/r/20230706094615.3080784-1-aliceryhl@google.comSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/sync/arc.rs

@@ -243,8 +243,7 @@ unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, 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) }
     }
 

rust/kernel/types.rs

@@ -35,34 +35,16 @@ pub trait ForeignOwnable: Sized {
     ///
     /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
     /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
-    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow_mut`]
-    /// for this object must have been dropped.
     unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
 
-    /// Mutably borrows a foreign-owned object.
-    ///
-    /// # Safety
-    ///
-    /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
-    /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
-    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
-    /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
-    unsafe fn borrow_mut(ptr: *const core::ffi::c_void) -> ScopeGuard<Self, fn(Self)> {
-        // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
-        // `into_foreign`.
-        ScopeGuard::new_with_data(unsafe { Self::from_foreign(ptr) }, |d| {
-            d.into_foreign();
-        })
-    }
-
     /// Converts a foreign-owned object back to a Rust-owned one.
     ///
     /// # Safety
     ///
     /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
     /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
-    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
-    /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
+    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
+    /// this object must have been dropped.
     unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
 }