bool QQmlImportsPrivate::populatePluginPairVector(...)
{
    static QVector<QStaticPlugin> plugins;
    if (plugins.isEmpty()) {
        // To avoid traversing all static plugins for all imports, we cut down
        // the list the first time called to only contain QML plugins:
        foreach (const QStaticPlugin &plugin, QPluginLoader::staticPlugins()) {
            if (qobject_cast<QQmlExtensionPlugin *>(plugin.instance()))
               plugins.append(plugin);
        }
    }
...