GH-30923: [C++] Clarify error when serializing custom FunctionOptions

cpp/examples/arrow/acero_register_example.cc

@@ -137,7 +137,8 @@ arrow::Status RunComputeRegister(int argc, char** argv) {
   // FunctionOptions for which serialization is not supported.
   auto expr = cp::call(name, {}, options);
   auto maybe_serialized = cp::Serialize(expr);
-  std::cerr << maybe_serialized.status().ToString() << std::endl;
+  std::cout << "Serializing an expression with ExampleFunctionOptions fails as expected: "
+            << maybe_serialized.status().ToString() << std::endl;
 
   auto exec_registry = ac::default_exec_factory_registry();
   ARROW_RETURN_NOT_OK(

cpp/src/arrow/compute/expression_test.cc

@@ -2247,6 +2247,15 @@ TEST(Expression, Filter) {
   ])");
 }
 
+TEST(Expression, SerializeCustomOptions) {
+  // GH-30923: custom FunctionOptionsType cannot be serialized
+  auto expr = call("widgetify", {}, std::make_shared<WidgetifyOptions>());
+  EXPECT_RAISES_WITH_MESSAGE_THAT(
+      NotImplemented,
+      HasSubstr("FunctionOptions type 'widgetify' is not supported: only built-in"),
+      Serialize(expr));
+}
+
 TEST(Expression, SerializationRoundTrips) {
   auto ExpectRoundTrips = [](const Expression& expr) {
     ASSERT_OK_AND_ASSIGN(auto serialized, Serialize(expr));

cpp/src/arrow/compute/function_internal.cc

@@ -45,8 +45,10 @@ Result<std::shared_ptr<StructScalar>> FunctionOptionsToStructScalar(
   const auto* options_type =
       dynamic_cast<const GenericOptionsType*>(options.options_type());
   if (!options_type) {
-    return Status::NotImplemented("serializing ", options.type_name(),
-                                  " to StructScalar");
+    return Status::NotImplemented("Serialization of FunctionOptions type '",
+                                  options.type_name(),
+                                  "' is not supported: only built-in options types "
+                                  "can be serialized");
   }
   RETURN_NOT_OK(options_type->ToStructScalar(options, &field_names, &values));
   field_names.push_back(kTypeNameField);