Implement unions as members of structs.

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

 {#  TODO(yzshen): Make these templates more readable. #}
 
 {#  Computes the serialized size for the specified struct.
     |struct| is the struct definition.
     |input_field_pattern| should be a pattern that contains one string
     placeholder, for example, "input->%s", "p_%s". The placeholder will be
     substituted with struct field names to refer to the input fields.
     This macro is expanded to compute seriailized size for both:
     - user-defined structs: the input is an instance of the corresponding struct
       wrapper class.
@@ skipping 28 matching lines @@
 {%-   set input_field = input_field_pattern|format(pf.field.name) %}
 {%-   set name = pf.field.name %}
 {%-   set kind = pf.field.kind %}
 {%-   if kind|is_object_kind %}
 {%-     if kind|is_array_kind %}
   mojo::SerializeArray_<{{kind|get_array_validate_params|indent(24)}}>(
       mojo::internal::Forward({{input_field}}), {{buffer}}, &{{output}}->{{name}}.ptr);
 {%-     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 %}
 {%-   elif kind|is_any_handle_kind %}
 {%-     if kind|is_interface_kind or kind|is_interface_request_kind %}
   {{output}}->{{name}} = {{input_field}}.PassMessagePipe().release();
 {%-     else %}
   {{output}}->{{name}} = {{input_field}}.release();
 {%-     endif %}
 {%-     if not kind|is_nullable_kind %}
@@ skipping 31 matching lines @@
 {%-   for pf in struct.packed.packed_fields_in_ordinal_order %}
 {%-     set output_field = output_field_pattern|format(pf.field.name) %}
 {%-     set name = pf.field.name %}
 {%-     set kind = pf.field.kind %}
 {%-     if pf.min_version > last_checked_version %}
 {%-       set last_checked_version = pf.min_version %}
     if ({{input}}->header_.version < {{pf.min_version}})
       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 %}
     {{output_field}}.reset(mojo::internal::FetchAndReset(&{{input}}->{{name}}));
 {%-     elif kind|is_enum_kind %}
     {{output_field}} = static_cast<{{kind|cpp_wrapper_type}}>({{input}}->{{name}});
 {%-     else %}
     {{output_field}} = {{input}}->{{name}};
 {%-     endif %}
 {%-   endfor %}
   } while (false);
 {%- endmacro %}