Fix nested enum generation when enums are the sole nested types

PiperOrigin-RevId: 596066421

rust/test/shared/enum_test.rs

@@ -19,6 +19,13 @@ fn test_nested_enum_values() {
     assert_that!(i32::from(proto2_unittest::TestAllTypes_::NestedEnum::Neg), eq(-1));
 }
 
+#[test]
+fn test_isolated_nested_enum() {
+    // Ensure that the enum is generated even when it's the only nested type for the
+    // message.
+    assert_that!(i32::from(proto2_unittest::TestRequiredEnumNoMask_::NestedEnum::Foo), eq(2));
+}
+
 #[test]
 fn test_enum_value_name_same_as_enum() {
     assert_that!(i32::from(enums::TestEnumValueNameSameAsEnum::TestEnumValueNameSameAsEnum), eq(1));

src/google/protobuf/compiler/rust/message.cc

@@ -390,9 +390,9 @@ void GenerateRs(Context& ctx, const Descriptor& msg) {
              }},
             {"nested_in_msg",
              [&] {
-               // If we have no nested types or oneofs, bail out without
+               // If we have no nested types, enums, or oneofs, bail out without
                // emitting an empty mod SomeMsg_.
-               if (msg.nested_type_count() == 0 &&
+               if (msg.nested_type_count() == 0 && msg.enum_type_count() == 0 &&
                    msg.real_oneof_decl_count() == 0) {
                  return;
                }