Warn if no Unicode functions implementation is found
diff --git a/src/hb-unicode-private.hh b/src/hb-unicode-private.hh
index c06dfe5..519fa0b 100644
--- a/src/hb-unicode-private.hh
+++ b/src/hb-unicode-private.hh
@@ -97,6 +97,7 @@
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
diff --git a/src/hb-warning.cc b/src/hb-warning.cc
index 6b5585f..c13731b 100644
--- a/src/hb-warning.cc
+++ b/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
