Update Message_index to account for oneof's

This is mostly patterned off of `Message_method_missing`, but that
function does a bunch of extra stuff (to handle, e.g., `has_` and
`clear_` and setter methods), while `Message_index` only needs to handle
getter methods. So instead of copying the `Message_method_missing`
implementation verbatim, we can "constant fold" away the parts of the
method that we know cannot (or should not) happen for `msg[...]` calls.

Author: jez

ruby/ext/google/protobuf_c/message.c

@@ -895,16 +895,23 @@ VALUE Message_freeze(VALUE _self) {
  */
 static VALUE Message_index(VALUE _self, VALUE field_name) {
   Message* self = ruby_to_Message(_self);
+  const upb_OneofDef* o;
   const upb_FieldDef* f;
 
   Check_Type(field_name, T_STRING);
-  f = upb_MessageDef_FindFieldByName(self->msgdef, RSTRING_PTR(field_name));
+  const char* name = RSTRING_PTR(field_name);
+  size_t sn = strlen(name);
 
-  if (f == NULL) {
+  if (!upb_MessageDef_FindByNameWithSize(self->msgdef, name, sn, &f, &o)) {
     return Qnil;
   }
 
-  return Message_getfield(_self, f);
+  // Dispatch accessor.
+  if (o != NULL) {
+    return Message_oneof_accessor(_self, o, METHOD_GETTER);
+  } else {
+    return Message_getfield(_self, f);
+  }
 }
 
 /*

ruby/tests/basic.rb

@@ -800,7 +800,7 @@ def test_oneof_fields_index
 
       assert_equal :a, msg.my_oneof
       assert_equal :a, msg.method_missing(:my_oneof)
-      assert_equal nil, msg["my_oneof"]
+      assert_equal :a, msg["my_oneof"]
     end
 
     def test_string_subclass