Bug 1324093 - Part 3.5: Add MOZ_MAYBE_UNUSED to mfbt/Attributes.h. r=froydnj

MozReview-Commit-ID: KXIaGGJnds3

--HG--
extra : rebase_source : 79738bfb55e562610f2c8501015a7c3fb80bdf9d

mfbt/Attributes.h

@@ -274,6 +274,27 @@
 #  define MOZ_MUST_USE
 #endif
 
+/**
+ * MOZ_MAYBE_UNUSED suppresses compiler warnings about functions that are
+ * never called (in this build configuration, at least).
+ *
+ * Place this attribute at the very beginning of a function declaration. For
+ * example, write
+ *
+ *   MOZ_MAYBE_UNUSED int foo();
+ *
+ * or
+ *
+ *   MOZ_MAYBE_UNUSED int foo() { return 42; }
+ */
+#if defined(__GNUC__) || defined(__clang__)
+#  define MOZ_MAYBE_UNUSED __attribute__ ((__unused__))
+#elif defined(_MSC_VER)
+#  define MOZ_MAYBE_UNUSED __pragma(warning(suppress:4505))
+#else
+#  define MOZ_MAYBE_UNUSED
+#endif
+
 /**
  * MOZ_FALLTHROUGH is an annotation to suppress compiler warnings about switch
  * cases that fall through without a break or return statement. MOZ_FALLTHROUGH