for tracepoint fallbacks, use c++-style mark-unused

Summary: Trying to mark macro params as unused using only the C preprocessor is painful. But we have an easier way in C++.

Reviewed By: nslingerland

Differential Revision: D60238723

fbshipit-source-id: c527bea47586e18dd40ecfa53cc60601771c8235

folly/tracing/StaticTracepoint.h

@@ -42,40 +42,13 @@
 
 #define FOLLY_HAVE_SDT 0
 
-// Mark variadic macro args as unused from https://stackoverflow.com/a/31470425
-#define FOLLY_UNUSED0()
-#define FOLLY_UNUSED1(a) (void)(a)
-#define FOLLY_UNUSED2(a, b) (void)(a), FOLLY_UNUSED1(b)
-#define FOLLY_UNUSED3(a, b, c) (void)(a), FOLLY_UNUSED2(b, c)
-#define FOLLY_UNUSED4(a, b, c, d) (void)(a), FOLLY_UNUSED3(b, c, d)
-#define FOLLY_UNUSED5(a, b, c, d, e) (void)(a), FOLLY_UNUSED4(b, c, d, e)
-#define FOLLY_UNUSED6(a, b, c, d, e, f) (void)(a), FOLLY_UNUSED5(b, c, d, e, f)
-#define FOLLY_UNUSED7(a, b, c, d, e, f, g) \
-  (void)(a), FOLLY_UNUSED6(b, c, d, e, f, g)
-#define FOLLY_UNUSED8(a, b, c, d, e, f, g, h) \
-  (void)(a), FOLLY_UNUSED7(b, c, d, e, f, g, h)
-
-#define FOLLY_VA_NUM_ARGS_IMPL(_0, _1, _2, _3, _4, _5, _6, _7, _8, N, ...) N
-#define FOLLY_VA_NUM_ARGS(...) \
-  FOLLY_VA_NUM_ARGS_IMPL(100, ##__VA_ARGS__, 8, 7, 6, 5, 4, 3, 2, 1, 0)
-
-#define FOLLY_ALL_UNUSED_IMPL_(nargs) FOLLY_UNUSED##nargs
-#define FOLLY_ALL_UNUSED_IMPL(nargs) FOLLY_ALL_UNUSED_IMPL_(nargs)
-
-#if defined(_MSC_VER)
-#define FOLLY_ALL_UNUSED(...)
-#else
-#define FOLLY_ALL_UNUSED(...) \
-  FOLLY_ALL_UNUSED_IMPL(FOLLY_VA_NUM_ARGS(__VA_ARGS__))(__VA_ARGS__)
-#endif
-
-#define FOLLY_SDT(provider, name, ...) \
-  do {                                 \
-    FOLLY_ALL_UNUSED(__VA_ARGS__);     \
+#define FOLLY_SDT(provider, name, ...)  \
+  do {                                  \
+    [](auto const&...) {}(__VA_ARGS__); \
   } while (0)
 #define FOLLY_SDT_WITH_SEMAPHORE(provider, name, ...) \
   do {                                                \
-    FOLLY_ALL_UNUSED(__VA_ARGS__);                    \
+    [](auto const&...) {}(__VA_ARGS__);               \
   } while (0)
 #define FOLLY_SDT_IS_ENABLED(provider, name) (false)
 #define FOLLY_SDT_SEMAPHORE(provider, name) \