mojo::Serialize*_() calls now propogate/return validation errors.

mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl

+{% import 'struct_macros.tmpl' as struct_macros without context %} 
 size_t GetSerializedSize_(const {{union.name}}Ptr& input, bool inlined) {
   size_t size = 0U;
   if (!inlined) {
     size += sizeof(internal::{{union.name}}_Data);
   }
 
   if (!input)
     return size;
 
   mojo::internal::UnionAccessor<{{union.name}}> input_acc(input.get());
@@ skipping 10 matching lines @@
 {%      endif %}
       break;
 {%-   endif %}
 {%- endfor %}
     default:
       break;
   }
   return size;
 }
 
-void SerializeUnion_({{union.name}}* input, mojo::internal::Buffer* buf,
-                     internal::{{union.name}}_Data** output, bool inlined) {
+mojo::internal::ValidationError SerializeUnion_(
+    {{union.name}}* input,
+    mojo::internal::Buffer* buf,
+    internal::{{union.name}}_Data** output,
+    bool inlined) {
   internal::{{union.name}}_Data* result = *output;
   if (input) {
     if (!inlined) {
       result = internal::{{union.name}}_Data::New(buf);
     }
     mojo::internal::UnionAccessor<{{union.name}}> input_acc(input);
     // TODO(azani): Handle unknown and objects.
     // Set the not-null flag.
     result->size = 16;
     result->tag = input->which();
     switch (input->which()) {
 {%  for field in union.fields %}
       case {{union.name}}::Tag::{{field.name|upper}}: {
 {%    if field.kind|is_object_kind %}
 {%      if field.kind|is_string_kind %}
         SerializeString_(
             *input_acc.data()->{{field.name}},
             buf, &result->data.f_{{field.name}}.ptr);
 {%      elif field.kind|is_struct_kind %}
-        Serialize_(
-            input_acc.data()->{{field.name}}->get(),
-            buf, &result->data.f_{{field.name}}.ptr);
+          {{struct_macros.call_serialize_struct(
+              input = "input_acc.data()->%s->get()"|format(field.name),
+              buffer = "buf",
+              output = "&result->data.f_%s.ptr"|format(field.name),
+              should_return_errors = true)|indent(6)}}
 {%      elif field.kind|is_union_kind %}
-        SerializeUnion_(
-            input_acc.data()->{{field.name}}->get(),
-            buf, &result->data.f_{{field.name}}.ptr, false);
+          {{struct_macros.call_serialize_union(
+              input = "input_acc.data()->%s->get()"|format(field.name),
+              buffer = "buf",
+              output = "&result->data.f_%s.ptr"|format(field.name),
+              inlined = "false",
+              should_return_errors = true)|indent(6)}}
 {%      elif field.kind|is_array_kind %}
-        const mojo::internal::ArrayValidateParams {{field.name}}_validate_params(
-            {{field.kind|get_array_validate_params_ctor_args|indent(16)}});
-        SerializeArray_(
-            input_acc.data()->{{field.name}},
-            buf, &result->data.f_{{field.name}}.ptr, &{{field.name}}_validate_params);
+          {{struct_macros.call_serialize_array(
+              name = field.name,
+              kind = field.kind,
+              input = "input_acc.data()->%s"|format(field.name),
+              buffer = "buf",
+              output = "&result->data.f_%s.ptr"|format(field.name),
+              should_return_errors = true,
+              indent_size = 16)|indent(6)}}
 {%      elif field.kind|is_map_kind %}
-        const mojo::internal::ArrayValidateParams {{field.name}}_validate_params(
-            {{field.kind.value_kind|get_map_validate_params_ctor_args|indent(16)}});
-        SerializeMap_(
-            input_acc.data()->{{field.name}},
-            buf, &result->data.f_{{field.name}}.ptr, &{{field.name}}_validate_params);
+          {{struct_macros.call_serialize_map(
+              name = field.name,
+              kind = field.kind,
+              input = "input_acc.data()->%s"|format(field.name),
+              buffer = "buf",
+              output = "&result->data.f_%s.ptr"|format(field.name),
+              should_return_errors = true,
+              indent_size = 16)|indent(6)}}
 {%-     endif %}
 {%    elif field.kind|is_any_handle_kind %}
         result->data.f_{{field.name}} =
             input_acc.data()->{{field.name}}->release().value();
 {%    elif field.kind|is_interface_kind %}
         mojo::internal::Interface_Data* {{field.name}} =
             reinterpret_cast<mojo::internal::Interface_Data*>(
                 &result->data.f_{{field.name}});
         mojo::internal::InterfacePointerToData(
             input_acc.data()->{{field.name}}->Pass(), {{field.name}});
 {%    elif field.kind|is_enum_kind %}
         result->data.f_{{field.name}} = 
           static_cast<int32_t>(input_acc.data()->{{field.name}});
 {%    else %}
         result->data.f_{{field.name}} = input_acc.data()->{{field.name}};
 {%-   endif %}
         break;
       }
 {%- endfor %}
       default:
+        // TODO(vardhan): Should this return an error code instead?
         MOJO_CHECK(false) << "No sane way to serialize a union with an unknown tag.";
         break;
     }
   } else if (inlined) {
     result->set_null();
   } else {
     result = nullptr;
   }
   *output = result;
+  return mojo::internal::ValidationError::VALIDATION_ERROR_NONE;
 }
 
 void Deserialize_(internal::{{union.name}}_Data* input,
                   {{union.name}}* output) {
   if (input && !input->is_null()) {
     mojo::internal::UnionAccessor<{{union.name}}> result_acc(output);
     switch (input->tag) {
 {#- TODO(vardhan):  There is a lot of overlap with struct_macros.deserialize()
     here, is it possible to merge? (currently looks very hairy) #}
 {%  for field in union.fields %}
@@ skipping 28 matching lines @@
         break;
       }
 {%- endfor %}
       default:
         MOJO_LOG(WARNING) << "Deserializing {{union.name}} with unknown tag!";
         // No way to deserialize the data when we encounter an unknown tag.
         break;
     }
   }
 }