add module-visitation.h to include all visitation headers in a transitive manner

Summary:
`module-visitation.h` is a convenient header that includes all visitation APIs, along with dependent thrift visitation headers transitively.

This is useful for implementing general code since existing reflection is transitive. If user doesn't need transitivity, they can still include each individual header to reduce build-time.

Reviewed By: yfeldblum

Differential Revision: D23253246

fbshipit-source-id: ac4c07ffa71ccd849756dccaaf44b70d5d391b76

Author: Mizuchi

thrift/compiler/generate/t_mstch_cpp2_generator.cc

@@ -2038,6 +2038,9 @@ void t_mstch_cpp2_generator::generate_visitation(const t_program* program) {
         generators_->program_generator_->generate(program, generators_, cache_);
   }
 
+  render_to_file(
+      cache_->programs_[id], "module_visitation.h", name + "_visitation.h");
+
   render_to_file(
       cache_->programs_[id],
       "module_for_each_field.h",

thrift/compiler/generate/templates/cpp2/module_visitation.h.mustache

@@ -0,0 +1,23 @@
+<%!
+
+  Copyright (c) Facebook, Inc. and its affiliates.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+%><% > Autogen%>
+#pragma once
+<%#program:thrift_includes%>
+#include "<%program:include_prefix%><%program:name%>_visitation.h"
+<%/program:thrift_includes%>
+#include "<%program:include_prefix%><%program:name%>_for_each_field.h"
+#include "<%program:include_prefix%><%program:name%>_visit_union.h"

thrift/compiler/test/fixtures/visitation/gen-cpp2/module_visitation.h

@@ -0,0 +1,11 @@
+/**
+ * Autogenerated by Thrift
+ *
+ * DO NOT EDIT UNLESS YOU ARE SURE THAT YOU KNOW WHAT YOU ARE DOING
+ *  @generated
+ */
+#pragma once
+#include "thrift/compiler/test/fixtures/visitation/gen-cpp2/reflection_dep_B_visitation.h"
+#include "thrift/compiler/test/fixtures/visitation/gen-cpp2/reflection_dep_C_visitation.h"
+#include "thrift/compiler/test/fixtures/visitation/gen-cpp2/module_for_each_field.h"
+#include "thrift/compiler/test/fixtures/visitation/gen-cpp2/module_visit_union.h"

thrift/test/visitation_transitivity_test.cpp

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <thrift/test/gen-cpp2/reflection_visitation.h> // @manual=:reflection_if-cpp2-visitation
+
+#include <folly/Overload.h>
+#include <folly/portability/GTest.h>
+
+using namespace apache::thrift;
+using namespace std;
+
+namespace test_cpp2 {
+namespace cpp_reflection {
+TEST(dep_C_struct, test_transitivity) {
+  dep_C_struct s;
+  s.i_c_ref() = 10;
+  s.d_ref()->i_d_ref() = 20;
+  for_each_field(s, [](auto&&, auto&& ref) {
+    folly::overload(
+        [](int32_t i) { EXPECT_EQ(i, 10); },
+        [](dep_D_struct d) {
+          for_each_field(d, [](auto&&, auto&& ref) { EXPECT_EQ(*ref, 20); });
+        })(*ref);
+  });
+}
+TEST(union1, test_union) {
+  union1 s;
+  s.us_ref() = "foo";
+  visit_union(s, [](auto&&, auto&& value) {
+    folly::overload(
+        [](string& s) { EXPECT_EQ(s, "foo"); },
+        [](auto&&) { EXPECT_TRUE(false); })(value);
+  });
+}
+} // namespace cpp_reflection
+} // namespace test_cpp2