[PATCH v2] rust: sync: document `PhantomData` in `Arc`

[Tamir Duberstein <tamird@gmail.com>]

Add a comment explaining the relevant semantics of `PhantomData`. This
should help future readers who may, as I did, assume that this field is
redundant at first glance.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
Changes in v2:
- Reworked as a new comment (Boqun Feng, Miguel Ojeda).
- Link to v1: https://lore.kernel.org/r/20241104-simplify-arc-v1-1-a4ffc290f905@gmail.com
---
 rust/kernel/sync/arc.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index db9da352d588f65348aa7a5204abbb165b70197f..547d56766f507afe4bc733f7f1b465a55b64312a 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -127,6 +127,14 @@
 /// ```
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
+    // NB: this informs dropck that objects of type `ArcInner<T>` may be used in `<Arc<T> as
+    // Drop>::drop`. Note that dropck already assumes that objects of type `T` may be used in
+    // `<Arc<T> as Drop>::drop` and the distinction between `T` and `ArcInner<T>` is not presently
+    // meaningful with respect to dropck - but this may change in the future so this is left here
+    // out of an abundance of caution.
+    //
+    // See https://doc.rust-lang.org/nomicon/phantom-data.html#generic-parameters-and-drop-checking
+    // for more detail on the semantics of dropck in the presence of `PhantomData`.
     _p: PhantomData<ArcInner<T>>,
 }
 

---
base-commit: ae7851c29747fa3765ecb722fe722117a346f988
change-id: 20241104-simplify-arc-70c3574b5fac

Best regards,
-- 
Tamir Duberstein <tamird@gmail.com>