Bug 1215411 - Define MOZ_FALLTHROUGH annotation to suppress clang's -Wimplicit-fallthrough warnings. r=botond

mfbt/Attributes.h

@@ -336,6 +336,35 @@
 #  define MOZ_WARN_UNUSED_RESULT
 #endif
 
+/**
+ * MOZ_FALLTHROUGH is an annotation to suppress compiler warnings about switch
+ * cases that fall through without a break or return statement. MOZ_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:
+ *     MOZ_FALLTHROUGH;
+ *   default:
+ *     return foo;
+ * }
+ */
+#if defined(__clang__) && __cplusplus >= 201103L
+   /* clang's fallthrough annotations are only available starting in C++11. */
+#  define MOZ_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 MOZ_FALLTHROUGH __fallthrough
+#else
+#  define MOZ_FALLTHROUGH /* FALLTHROUGH */
+#endif
+
 #ifdef __cplusplus
 
 /*