Implement unions as members of structs.

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

Index: mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
index 03911b9b39b47e10ae9386c56624cb41e2c08d09..c585bb7c5f79010317c81e588726df536c9ea211 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
@@ -46,12 +46,19 @@
 {%-     elif kind|is_map_kind %}
   mojo::SerializeMap_<{{kind.value_kind|get_map_validate_params|indent(24)}}>(
       mojo::internal::Forward({{input_field}}), {{buffer}}, &{{output}}->{{name}}.ptr);
+{%-     elif kind|is_union_kind %}
+  internal::{{kind.name}}_Data* {{name}}_ptr = &{{output}}->{{name}};
+  SerializeUnion_(mojo::internal::Forward({{input_field}}), {{buffer}}, &{{name}}_ptr, true);
 {%-     else %}
   Serialize_(mojo::internal::Forward({{input_field}}), {{buffer}}, &{{output}}->{{name}}.ptr);
 {%-     endif %}
 {%-     if not kind|is_nullable_kind %}
   MOJO_INTERNAL_DLOG_SERIALIZATION_WARNING(
+{%-       if kind|is_union_kind %}
+      {{output}}->{{name}}.is_null(),
+{%-       else %}
       !{{output}}->{{name}}.ptr,
+{%-       endif %}
       mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER,
       "null {{name}} in {{struct_display_name}}");
 {%-     endif %}
@@ -103,7 +110,11 @@
       break;
 {%-     endif %}
 {%-     if kind|is_object_kind %}
+{%-       if kind|is_union_kind %}
+    Deserialize_(&{{input}}->{{name}}, &{{output_field}});
+{%-       else %}
     Deserialize_({{input}}->{{name}}.ptr, &{{output_field}});
+{%-       endif %}
 {%-     elif kind|is_interface_kind or kind|is_interface_request_kind %}
     {{output_field}}.Bind(mojo::MakeScopedHandle(mojo::internal::FetchAndReset(&{{input}}->{{name}})));
 {%-     elif kind|is_any_handle_kind %}