[PATCH v4 3/6] rust: arc: split unsafe block, add missing comment

[Tamir Duberstein <tamird@gmail.com>]

The new SAFETY comment style is taken from existing comments in `deref`
and `drop.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/sync/arc.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index bbb6ca8ff7d0e079c7f0efa7702a91622759cbd7..6f5fe24f0e1bad5ca141091e8945b8d2f94e16aa 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -377,10 +377,14 @@ fn as_ref(&self) -> &T {
 
 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
+        // safe to dereference it.
+        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
+
         // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
         // safe to increment the refcount.
-        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
+        unsafe { bindings::refcount_inc(refcount) };
 
         // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
         unsafe { Self::from_inner(self.ptr) }

-- 
2.47.0