Warn if no Unicode functions implementation is found

src/hb-unicode-private.hh

@@ -97,6 +97,7 @@ extern HB_INTERNAL hb_unicode_funcs_t _hb_glib_unicode_funcs;
 extern HB_INTERNAL hb_unicode_funcs_t _hb_icu_unicode_funcs;
 #define _hb_unicode_funcs_default _hb_icu_unicode_funcs
 #else
+#define HB_UNICODE_FUNCS_NIL 1
 extern HB_INTERNAL hb_unicode_funcs_t _hb_unicode_funcs_nil;
 #define _hb_unicode_funcs_default _hb_unicode_funcs_nil
 #endif

src/hb-warning.cc

@@ -51,3 +51,16 @@
 #warning "To suppress these warnings, define HB_NO_MT"
 #endif
 #endif
+
+
+#include "hb-unicode-private.hh"
+
+#if !defined(HB_NO_UNICODE_FUNCS) && defined(HB_UNICODE_FUNCS_NIL)
+#ifdef _MSC_VER
+#pragma message("Could not find any Unicode functions implementation, you have to provide your own")
+#pragma message("To suppress this warnings, define HB_NO_UNICODE_FUNCS")
+#else
+#warning "Could not find any Unicode functions implementation, you have to provide your own"
+#warning "To suppress this warning, define HB_NO_UNICODE_FUNCS"
+#endif
+#endif