Do not propose to auto-derive Clone in presence of unsafe fields (#14559)

`unsafe_fields` is an incomplete feature; comments have been put near
`#![expect(incomplete_features)]` to ensure that we revisit the
situation when the feature becomes complete.

changelog: [`expl_impl_clone_on_copy`]: do not lint in the presence of
`unsafe` fields

Fixes #14558

clippy_lints/src/derive.rs

@@ -349,6 +349,10 @@ fn check_copy_clone<'tcx>(cx: &LateContext<'tcx>, item: &Item<'_>, trait_ref: &h
     {
         return;
     }
+    // The presence of `unsafe` fields prevents deriving `Clone` automatically
+    if ty_adt.all_fields().any(|f| f.safety.is_unsafe()) {
+        return;
+    }
 
     span_lint_and_note(
         cx,