Rename <required> tag to <plugin>.

PiperOrigin-RevId: 509237786
Change-Id: I35e812830b479004ac5025cbd9477888f51fb43e

src/xml/xml_native_reader.cc

@@ -190,11 +190,11 @@ static const char* MJCF[nMJCF][mjXATTRNUM] = {
 
     {"extension", "*", "0"},
     {"<"},
-        {"required", "*", "1", "plugin"},
+        {"plugin", "*", "1", "plugin"},
         {"<"},
             {"instance", "*", "1", "name"},
             {"<"},
-              {"config", "*", "2", "key", "value"},
+                {"config", "*", "2", "key", "value"},
             {">"},
         {">"},
     {">"},
@@ -2196,7 +2196,7 @@ void mjXReader::Extension(XMLElement* section) {
     // get sub-element name
     std::string_view name = elem->Value();
 
-    if (name == "required") {
+    if (name == "plugin") {
       std::string plugin_name;
       int plugin_slot = -1;
       ReadAttrTxt(elem, "plugin", plugin_name, /* required = */ true);

src/xml/xml_native_writer.cc

@@ -1081,12 +1081,12 @@ void mjXWriter::Extension(XMLElement* root) {
   // create section
   XMLElement* section = InsertEnd(root, "extension");
 
-  // keep track of plugins whose <required> section have been created
+  // keep track of plugins whose <plugin> section have been created
   std::unordered_set<const mjpPlugin*> seen_plugins;
 
   // write all plugins
   const mjpPlugin* last_plugin = nullptr;
-  XMLElement* required_elem = nullptr;
+  XMLElement* plugin_elem = nullptr;
   for (int i = 0; i < model->plugins.size(); ++i) {
     mjCPlugin* pp = static_cast<mjCPlugin*>(model->GetObject(mjOBJ_PLUGIN, i));
 
@@ -1096,17 +1096,17 @@ void mjXWriter::Extension(XMLElement* root) {
       break;
     }
 
-    // check if we need to open a new <required> section
+    // check if we need to open a new <plugin> section
     const mjpPlugin* plugin = mjp_getPluginAtSlot(pp->plugin_slot);
     if (plugin != last_plugin) {
-      required_elem = InsertEnd(section, "required");
-      WriteAttrTxt(required_elem, "plugin", plugin->name);
+      plugin_elem = InsertEnd(section, "plugin");
+      WriteAttrTxt(plugin_elem, "plugin", plugin->name);
       seen_plugins.insert(plugin);
       last_plugin = plugin;
     }
 
     // write instance element
-    XMLElement* elem = InsertEnd(required_elem, "instance");
+    XMLElement* elem = InsertEnd(plugin_elem, "instance");
     WriteAttrTxt(elem, "name", pp->name);
 
     // write plugin config attributes
@@ -1123,11 +1123,11 @@ void mjXWriter::Extension(XMLElement* root) {
     }
   }
 
-  // write <required> elements for plugins without explicit instances
+  // write <plugin> elements for plugins without explicit instances
   for (const auto& [plugin, slot] : model->active_plugins) {
     if (seen_plugins.find(plugin) == seen_plugins.end()) {
-      required_elem = InsertEnd(section, "required");
-      WriteAttrTxt(required_elem, "plugin", plugin->name);
+      plugin_elem = InsertEnd(section, "plugin");
+      WriteAttrTxt(plugin_elem, "plugin", plugin->name);
     }
   }
 }