[PATCH v21 5/9] rust: Add missing SAFETY documentation for `ARef` example

[Andreas Hindborg <a.hindborg@kernel.org>]

From: Oliver Mangold <oliver.mangold@pm.me>

SAFETY comment in rustdoc example was just 'TODO'. Fixed.

Link: https://github.com/Rust-for-Linux/linux/issues/351
Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Assisted-by: LLM
Co-developed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/sync/aref.rs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index 215e7eb139fa..ae76bd9b6c1c 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -147,7 +147,10 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
     ///
     /// struct Empty {}
     ///
-    /// # // SAFETY: TODO.
+    /// // SAFETY: The `RefCounted` implementation for `Empty` does not count references, and
+    /// // `dec_ref` never frees the underlying object, so a decrement cannot invalidate it. The
+    /// // object instead lives as long as the `Empty` value itself, so creators of `ARef<Empty>`
+    /// // must guarantee that the value outlives every `ARef` derived from it (as done below).
     /// unsafe impl RefCounted for Empty {
     ///     fn inc_ref(&self) {}
     ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
@@ -155,7 +158,9 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
     ///
     /// let mut data = Empty {};
     /// let ptr = NonNull::<Empty>::new(&mut data).unwrap();
-    /// # // SAFETY: TODO.
+    /// // SAFETY: As the refcount operations of `Empty` are no-ops, we can treat `ptr` as owning
+    /// // an increment on the refcount. `data` outlives the created `ARef`, upholding the
+    /// // liveness guarantee required by the `RefCounted` implementation above.
     /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
     /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
     ///

-- 
2.51.2