Add HB_FALLTHROUGH

Borrowed from https://bugzilla.mozilla.org/show_bug.cgi?id=1215411

src/hb-ot-shape-complex-indic.cc

@@ -756,7 +756,7 @@ initial_reordering_consonant_syllable (const hb_ot_shape_plan_t *plan,
     {
       default:
         assert (false);
-	/* fallthrough */
+	HB_FALLTHROUGH;
 
       case BASE_POS_LAST:
       {

src/hb-ot-shape-complex-thai.cc

@@ -139,7 +139,7 @@ thai_pua_shape (hb_codepoint_t u, thai_action_t action, hb_font_t *font)
   };
 
   switch (action) {
-    default: assert (false); /* Fallthrough */
+    default: assert (false); HB_FALLTHROUGH;
     case NOP: return u;
     case SD:  pua_mappings = SD_mappings; break;
     case SDL: pua_mappings = SDL_mappings; break;

src/hb-ot-shape-fallback.cc

@@ -224,7 +224,7 @@ position_mark (const hb_ot_shape_plan_t *plan,
 	pos.x_offset += base_extents.x_bearing + base_extents.width - mark_extents.width / 2 - mark_extents.x_bearing;
         break;
       }
-      /* Fall through */
+      HB_FALLTHROUGH;
 
     default:
     case HB_UNICODE_COMBINING_CLASS_ATTACHED_BELOW:
@@ -259,6 +259,7 @@ position_mark (const hb_ot_shape_plan_t *plan,
     case HB_UNICODE_COMBINING_CLASS_BELOW_RIGHT:
       /* Add gap, fall-through. */
       base_extents.height -= y_gap;
+      HB_FALLTHROUGH;
 
     case HB_UNICODE_COMBINING_CLASS_ATTACHED_BELOW_LEFT:
     case HB_UNICODE_COMBINING_CLASS_ATTACHED_BELOW:
@@ -279,6 +280,7 @@ position_mark (const hb_ot_shape_plan_t *plan,
       /* Add gap, fall-through. */
       base_extents.y_bearing += y_gap;
       base_extents.height -= y_gap;
+      HB_FALLTHROUGH;
 
     case HB_UNICODE_COMBINING_CLASS_ATTACHED_ABOVE:
     case HB_UNICODE_COMBINING_CLASS_ATTACHED_ABOVE_RIGHT:

src/hb-private.hh

@@ -119,6 +119,36 @@ extern "C" void  hb_free_impl(void *ptr);
 #define HB_FUNC __func__
 #endif
 
+/*
+ * Borrowed from https://bugzilla.mozilla.org/show_bug.cgi?id=1215411
+ * HB_FALLTHROUGH is an annotation to suppress compiler warnings about switch
+ * cases that fall through without a break or return statement. HB_FALLTHROUGH
+ * is only needed on cases that have code:
+ *
+ * switch (foo) {
+ *   case 1: // These cases have no code. No fallthrough annotations are needed.
+ *   case 2:
+ *   case 3:
+ *     foo = 4; // This case has code, so a fallthrough annotation is needed:
+ *     HB_FALLTHROUGH;
+ *   default:
+ *     return foo;
+ * }
+ */
+#if defined(__clang__) && __cplusplus >= 201103L
+   /* clang's fallthrough annotations are only available starting in C++11. */
+#  define HB_FALLTHROUGH [[clang::fallthrough]]
+#elif defined(_MSC_VER)
+   /*
+    * MSVC's __fallthrough annotations are checked by /analyze (Code Analysis):
+    * https://msdn.microsoft.com/en-us/library/ms235402%28VS.80%29.aspx
+    */
+#  include <sal.h>
+#  define HB_FALLTHROUGH __fallthrough
+#else
+#  define HB_FALLTHROUGH /* FALLTHROUGH */
+#endif
+
 #if defined(_WIN32) || defined(__CYGWIN__)
    /* We need Windows Vista for both Uniscribe backend and for
     * MemoryBarrier.  We don't support compiling on Windows XP,