Avoid regression in derive(PartialOrd) for zero-variant enums

compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

@@ -57,8 +57,11 @@ pub(crate) fn expand_deriving_partial_ord(
     }));
     let (is_simple, substructure) = match item {
         Annotatable::Item(annitem) => match &annitem.kind {
-            // For unit structs, the default generated code is better.
+            // For unit structs/zero-variant enums, the default generated code is better.
             ItemKind::Struct(.., ast::VariantData::Unit(..)) => (false, default_substructure),
+            ItemKind::Enum(.., enum_def) if enum_def.variants.is_empty() => {
+                (false, default_substructure)
+            }
             ItemKind::Struct(_, ast::Generics { params, .. }, _)
             | ItemKind::Enum(_, ast::Generics { params, .. }, _)
                 if is_simple_candidate(params) =>

tests/ui/derives/deriving-all-codegen.stdout

@@ -1016,7 +1016,7 @@ impl ::core::cmp::PartialOrd for Enum0 {
     #[inline]
     fn partial_cmp(&self, other: &Enum0)
         -> ::core::option::Option<::core::cmp::Ordering> {
-        ::core::option::Option::Some(::core::cmp::Ord::cmp(self, other))
+        match *self {}
     }
 }
 #[automatically_derived]