Allow expect on impl for derive_ord_xor_partial_ord (#16303)

Closes rust-lang/rust-clippy#16298

This lint flags on the type defination, while also giving a warning on
the span of the impl block. It is counter-intuitive that you need to put
the `expect` on top of the type defination instead of the impl block to
suppress the warning, and this PR addresses that.

changelog: [`derive_ord_xor_partial_ord`] allow `expect` on `impl` block

clippy_lints/src/derive/derive_ord_xor_partial_ord.rs

@@ -1,15 +1,16 @@
 use clippy_utils::diagnostics::span_lint_hir_and_then;
+use clippy_utils::fulfill_or_allowed;
 use rustc_hir::{self as hir, HirId};
 use rustc_lint::LateContext;
 use rustc_middle::ty::Ty;
-use rustc_span::{Span, sym};
+use rustc_span::sym;
 
 use super::DERIVE_ORD_XOR_PARTIAL_ORD;
 
 /// Implementation of the `DERIVE_ORD_XOR_PARTIAL_ORD` lint.
 pub(super) fn check<'tcx>(
     cx: &LateContext<'tcx>,
-    span: Span,
+    item: &hir::Item<'_>,
     trait_ref: &hir::TraitRef<'_>,
     ty: Ty<'tcx>,
     adt_hir_id: HirId,
@@ -19,6 +20,8 @@ pub(super) fn check<'tcx>(
         && let Some(partial_ord_trait_def_id) = cx.tcx.lang_items().partial_ord_trait()
         && let Some(def_id) = &trait_ref.trait_def_id()
         && *def_id == ord_trait_def_id
+        && let item_hir_id = cx.tcx.local_def_id_to_hir_id(item.owner_id)
+        && !fulfill_or_allowed(cx, DERIVE_ORD_XOR_PARTIAL_ORD, [adt_hir_id])
     {
         // Look for the PartialOrd implementations for `ty`
         cx.tcx.for_each_relevant_impl(partial_ord_trait_def_id, ty, |impl_id| {
@@ -39,7 +42,7 @@ pub(super) fn check<'tcx>(
                     "you are deriving `Ord` but have implemented `PartialOrd` explicitly"
                 };
 
-                span_lint_hir_and_then(cx, DERIVE_ORD_XOR_PARTIAL_ORD, adt_hir_id, span, mess, |diag| {
+                span_lint_hir_and_then(cx, DERIVE_ORD_XOR_PARTIAL_ORD, item_hir_id, item.span, mess, |diag| {
                     if let Some(local_def_id) = impl_id.as_local() {
                         let hir_id = cx.tcx.local_def_id_to_hir_id(local_def_id);
                         diag.span_note(cx.tcx.hir_span(hir_id), "`PartialOrd` implemented here");

clippy_lints/src/derive/mod.rs

@@ -208,7 +208,7 @@ impl<'tcx> LateLintPass<'tcx> for Derive {
             let is_automatically_derived = cx.tcx.is_automatically_derived(item.owner_id.to_def_id());
 
             derived_hash_with_manual_eq::check(cx, item.span, trait_ref, ty, adt_hir_id, is_automatically_derived);
-            derive_ord_xor_partial_ord::check(cx, item.span, trait_ref, ty, adt_hir_id, is_automatically_derived);
+            derive_ord_xor_partial_ord::check(cx, item, trait_ref, ty, adt_hir_id, is_automatically_derived);
 
             if is_automatically_derived {
                 unsafe_derive_deserialize::check(cx, item, trait_ref, ty, adt_hir_id);

tests/ui/derive_ord_xor_partial_ord.rs

@@ -91,3 +91,17 @@ mod issue15708 {
         }
     }
 }
+
+mod issue16298 {
+    #[derive(Clone, Copy, Debug, Default, PartialEq, PartialOrd)]
+    struct Normalized<S>(S);
+
+    impl<S: Eq> Eq for Normalized<S> {}
+
+    #[expect(clippy::derive_ord_xor_partial_ord)]
+    impl<S: Eq + PartialOrd> Ord for Normalized<S> {
+        fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+            self.partial_cmp(other).unwrap()
+        }
+    }
+}