Avoid regression in derive(PartialOrd) for single fieldless variant enums

Author: makai410

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

@@ -59,9 +59,16 @@ pub(crate) fn expand_deriving_partial_ord(
         Annotatable::Item(annitem) => match &annitem.kind {
             // For unit structs/zero-variant enums, the default generated code is better.
             ItemKind::Struct(.., ast::VariantData::Unit(..)) => (false, default_substructure),
+            // Also for single fieldless variant enum
             ItemKind::Enum(.., enum_def) if enum_def.variants.is_empty() => {
                 (false, default_substructure)
             }
+            ItemKind::Enum(.., enum_def)
+                if enum_def.variants.len() == 1
+                    && matches!(enum_def.variants[0].data, ast::VariantData::Unit(..)) =>
+            {
+                (false, default_substructure)
+            }
             ItemKind::Struct(_, ast::Generics { params, .. }, _)
             | ItemKind::Enum(_, ast::Generics { params, .. }, _)
                 if is_simple_candidate(params) =>

tests/mir-opt/pre-codegen/derived_ord_debug.{impl#0}-partial_cmp.runtime-optimized.after.panic-abort.mir

@@ -4,49 +4,14 @@ fn <impl at $DIR/derived_ord_debug.rs:6:10: 6:20>::partial_cmp(_1: &MultiField,
     debug self => _1;
     debug other => _2;
     let mut _0: std::option::Option<std::cmp::Ordering>;
-    let _3: &char;
-    let _4: &char;
-    let mut _5: std::option::Option<std::cmp::Ordering>;
-    let mut _6: isize;
-    let mut _7: i8;
-    let _8: &i16;
-    let _9: &i16;
-    scope 1 {
-        debug cmp => _5;
-    }
+    let mut _3: std::cmp::Ordering;
 
     bb0: {
-        _3 = &((*_1).0: char);
-        _4 = &((*_2).0: char);
-        _5 = <char as PartialOrd>::partial_cmp(copy _3, copy _4) -> [return: bb1, unwind unreachable];
+        _3 = <MultiField as Ord>::cmp(copy _1, copy _2) -> [return: bb1, unwind unreachable];
     }
 
     bb1: {
-        _6 = discriminant(_5);
-        switchInt(move _6) -> [1: bb2, 0: bb4, otherwise: bb6];
-    }
-
-    bb2: {
-        _7 = discriminant(((_5 as Some).0: std::cmp::Ordering));
-        switchInt(move _7) -> [0: bb3, otherwise: bb4];
-    }
-
-    bb3: {
-        _8 = &((*_1).1: i16);
-        _9 = &((*_2).1: i16);
-        _0 = <i16 as PartialOrd>::partial_cmp(copy _8, copy _9) -> [return: bb5, unwind unreachable];
-    }
-
-    bb4: {
-        _0 = copy _5;
-        goto -> bb5;
-    }
-
-    bb5: {
+        _0 = Option::<std::cmp::Ordering>::Some(move _3);
         return;
     }
-
-    bb6: {
-        unreachable;
-    }
 }

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

@@ -1149,7 +1149,7 @@ impl ::core::cmp::PartialOrd for Fieldless1 {
     #[inline]
     fn partial_cmp(&self, other: &Fieldless1)
         -> ::core::option::Option<::core::cmp::Ordering> {
-        ::core::option::Option::Some(::core::cmp::Ord::cmp(self, other))
+        ::core::option::Option::Some(::core::cmp::Ordering::Equal)
     }
 }
 #[automatically_derived]