Mojo C++ bindings: more readable error message for missing *Traits specialization.

BUG=None

Change-Id: I3cae8cee3d77006c659bb9784a7089478276016e
Reviewed-on: https://chromium-review.googlesource.com/574682Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487638}

ipc/ipc_param_traits.h

@@ -9,8 +9,19 @@
 // a data type is read, written and logged in the IPC system.
 
 namespace IPC {
+namespace internal {
+
+template <typename T>
+struct AlwaysFalse {
+  static const bool value = false;
+};
+
+}  // namespace internal
 
 template <class P> struct ParamTraits {
+  static_assert(internal::AlwaysFalse<P>::value,
+                "Cannot find the IPC::ParamTraits specialization. Did you "
+                "forget to include the corresponding header file?");
 };
 
 template <class P>

mojo/public/cpp/bindings/BUILD.gn

@@ -22,7 +22,6 @@ component("bindings") {
     "$interfaces_bindings_gen_dir/pipe_control_messages.mojom.cc",
     "$interfaces_bindings_gen_dir/pipe_control_messages.mojom.h",
     "array_data_view.h",
-    "array_traits.h",
     "array_traits_carray.h",
     "array_traits_stl.h",
     "associated_binding.h",
@@ -110,7 +109,6 @@ component("bindings") {
     "lib/sync_handle_watcher.cc",
     "lib/task_runner_helper.cc",
     "lib/task_runner_helper.h",
-    "lib/template_util.h",
     "lib/unserialized_message_context.cc",
     "lib/unserialized_message_context.h",
     "lib/validate_params.h",
@@ -122,7 +120,6 @@ component("bindings") {
     "lib/validation_util.h",
     "map.h",
     "map_data_view.h",
-    "map_traits.h",
     "map_traits_stl.h",
     "message.h",
     "message_header_validator.h",
@@ -135,7 +132,6 @@ component("bindings") {
     "raw_ptr_impl_ref_traits.h",
     "scoped_interface_endpoint_handle.h",
     "string_data_view.h",
-    "string_traits.h",
     "string_traits_stl.h",
     "string_traits_string16.h",
     "string_traits_string_piece.h",
@@ -149,7 +145,6 @@ component("bindings") {
     "sync_handle_watcher.h",
     "thread_safe_interface_ptr.h",
     "type_converter.h",
-    "union_traits.h",
     "unique_ptr_impl_ref_traits.h",
   ]
 
@@ -171,8 +166,13 @@ component("bindings") {
 
 source_set("struct_traits") {
   sources = [
+    "array_traits.h",
     "enum_traits.h",
+    "lib/template_util.h",
+    "map_traits.h",
+    "string_traits.h",
     "struct_traits.h",
+    "union_traits.h",
   ]
 }
 

mojo/public/cpp/bindings/array_traits.h

@@ -5,6 +5,8 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_H_
 
+#include "mojo/public/cpp/bindings/lib/template_util.h"
+
 namespace mojo {
 
 // This must be specialized for any type |T| to be serialized/deserialized as
@@ -64,7 +66,11 @@ namespace mojo {
 //   };
 //
 template <typename T>
-struct ArrayTraits;
+struct ArrayTraits {
+  static_assert(internal::AlwaysFalse<T>::value,
+                "Cannot find the mojo::ArrayTraits specialization. Did you "
+                "forget to include the corresponding header file?");
+};
 
 }  // namespace mojo
 

mojo/public/cpp/bindings/enum_traits.h

@@ -5,6 +5,8 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_ENUM_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_ENUM_TRAITS_H_
 
+#include "mojo/public/cpp/bindings/lib/template_util.h"
+
 namespace mojo {
 
 // This must be specialized for any type |T| to be serialized/deserialized as a
@@ -20,7 +22,11 @@ namespace mojo {
 //   };
 //
 template <typename MojomType, typename T>
-struct EnumTraits;
+struct EnumTraits {
+  static_assert(internal::AlwaysFalse<T>::value,
+                "Cannot find the mojo::EnumTraits specialization. Did you "
+                "forget to include the corresponding header file?");
+};
 
 }  // namespace mojo
 

mojo/public/cpp/bindings/lib/string_traits_wtf.cc

@@ -8,6 +8,7 @@
 
 #include "base/logging.h"
 #include "mojo/public/cpp/bindings/lib/array_internal.h"
+#include "mojo/public/cpp/bindings/string_data_view.h"
 #include "third_party/WebKit/Source/platform/wtf/text/StringUTF8Adaptor.h"
 
 namespace mojo {

mojo/public/cpp/bindings/lib/template_util.h

@@ -114,6 +114,11 @@ struct Conditional<false, T, F> {
   typedef F type;
 };
 
+template <typename T>
+struct AlwaysFalse {
+  static const bool value = false;
+};
+
 }  // namespace internal
 }  // namespace mojo
 

mojo/public/cpp/bindings/map_traits.h

@@ -5,6 +5,8 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_MAP_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_MAP_TRAITS_H_
 
+#include "mojo/public/cpp/bindings/lib/template_util.h"
+
 namespace mojo {
 
 // This must be specialized for any type |T| to be serialized/deserialized as
@@ -49,7 +51,11 @@ namespace mojo {
 //   };
 //
 template <typename T>
-struct MapTraits;
+struct MapTraits {
+  static_assert(internal::AlwaysFalse<T>::value,
+                "Cannot find the mojo::MapTraits specialization. Did you "
+                "forget to include the corresponding header file?");
+};
 
 }  // namespace mojo
 

mojo/public/cpp/bindings/string_traits.h

@@ -5,7 +5,7 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_H_
 
-#include "mojo/public/cpp/bindings/string_data_view.h"
+#include "mojo/public/cpp/bindings/lib/template_util.h"
 
 namespace mojo {
 
@@ -47,7 +47,11 @@ namespace mojo {
 // so that you can do any necessary cleanup.
 //
 template <typename T>
-struct StringTraits;
+struct StringTraits {
+  static_assert(internal::AlwaysFalse<T>::value,
+                "Cannot find the mojo::StringTraits specialization. Did you "
+                "forget to include the corresponding header file?");
+};
 
 }  // namespace mojo
 

mojo/public/cpp/bindings/string_traits_string16.h

@@ -7,6 +7,7 @@
 
 #include "base/strings/string16.h"
 #include "mojo/public/cpp/bindings/bindings_export.h"
+#include "mojo/public/cpp/bindings/string_data_view.h"
 #include "mojo/public/cpp/bindings/string_traits.h"
 
 namespace mojo {

mojo/public/cpp/bindings/struct_traits.h

@@ -5,6 +5,8 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_STRUCT_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRUCT_TRAITS_H_
 
+#include "mojo/public/cpp/bindings/lib/template_util.h"
+
 namespace mojo {
 
 // This must be specialized for any type |T| to be serialized/deserialized as
@@ -158,7 +160,11 @@ namespace mojo {
 //   };
 //
 template <typename DataViewType, typename T>
-struct StructTraits;
+struct StructTraits {
+  static_assert(internal::AlwaysFalse<T>::value,
+                "Cannot find the mojo::StructTraits specialization. Did you "
+                "forget to include the corresponding header file?");
+};
 
 }  // namespace mojo
 

mojo/public/cpp/bindings/union_traits.h

@@ -5,6 +5,8 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_UNION_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_UNION_TRAITS_H_
 
+#include "mojo/public/cpp/bindings/lib/template_util.h"
+
 namespace mojo {
 
 // This must be specialized for any type |T| to be serialized/deserialized as
@@ -32,7 +34,11 @@ namespace mojo {
 //      will be called.
 //
 template <typename DataViewType, typename T>
-struct UnionTraits;
+struct UnionTraits {
+  static_assert(internal::AlwaysFalse<T>::value,
+                "Cannot find the mojo::UnionTraits specialization. Did you "
+                "forget to include the corresponding header file?");
+};
 
 }  // namespace mojo