[PATCH 3/4] rust: list: document safety of List::remove

[Cian McGuire]

The call to `ListLinks::fields(T::view_links(item))` in `List::remove`
had its SAFETY comment left as "TODO.". This call chains two separate
safety requirements that need to be discharged together.

`view_links` requires its pointer argument to point at a valid value.
`item` is a `&T`, and Rust's reference guarantees mean it is always a
valid, non-dangling pointer to a live value of type `T`, so this is
satisfied without needing anything beyond ordinary reference validity
(`view_links`'s own doc comment notes it "need not be in an `Arc`", to
head off the assumption that anything more is required here).

`fields` requires its pointer argument to be dereferenceable. The
pointer returned by `view_links` is guaranteed dereferenceable by its
own documented guarantees: it is either the pointer from a preceding
`prepare_to_insert` call, which itself must point at a valid value, or
it points at a read-only `ListLinks` with null fields. Either way,
`fields`'s requirement is satisfied.

Suggested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Link: https://github.com/Rust-for-Linux/linux/issues/351
Signed-off-by: Cian McGuire <cianmcguire167@xxxxxxxxx>
---
 rust/kernel/list.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
index 406e3a028c55..22b497eb10c0 100644
--- a/rust/kernel/list.rs
+++ b/rust/kernel/list.rs
@@ -599,7 +599,12 @@ pub fn pop_front(&mut self) -> Option<ListArc<T, ID>> {
     ///
     /// `item` must not be in a different linked list (with the same id).
     pub unsafe fn remove(&mut self, item: &T) -> Option<ListArc<T, ID>> {
-        // SAFETY: TODO.
+        // SAFETY: `item` is a `&T`, and Rust's reference guarantees mean it is always a valid,
+        // non-dangling pointer to a live value of type `T`, satisfying `view_links`'s safety
+        // requirement. The pointer `view_links` returns is guaranteed dereferenceable by its own
+        // documented guarantees: it's either the pointer from a preceding `prepare_to_insert`
+        // call, which itself must point at a valid value, or it points at a read-only
+        // `ListLinks` with null fields. Either way, `fields`'s safety requirement is satisfied.
         let mut item = unsafe { ListLinks::fields(T::view_links(item)) };
         // SAFETY: The user provided a reference, and reference are never dangling.
         //
-- 
2.55.0