Avoid unused variable warning on Windows about cpp11::preserved

Author: DavisVaughan

cpp11test/src/test-protect.cpp

@@ -1,3 +1,16 @@
+// Avoid unused variable warnings regarding `cpp11::preserved`
+// clang-format off
+#ifdef __clang__
+# pragma clang diagnostic push
+# pragma clang diagnostic ignored "-Wunused-variable"
+#endif
+
+#ifdef __GNUC__
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wunused-variable"
+#endif
+// clang-format on
+
 #define CPP11_USE_FMT
 #include "cpp11/protect.hpp"
 #include "testthat.h"