[base] Accept several values for USE

This CL changes the USE macro to accept more than one parameter.
This is particularly interesting for calling a method on each type in a
parameter pack, as in:

template<typename... T>
void foo(T&&... ts) {
  USE(do_something(ts)...);
}

Drive-by fix: Allow to pass arbitrary types to USE, including
references. This might prevent a copy for pass-by-value.

R=ishell@chromium.org, tebbi@chromium.org

Also-by: tebbi@chromium.org
Change-Id: I544e83bb996aaa638e7512295973dd3e742254bc
Reviewed-on: https://chromium-review.googlesource.com/567507Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46626}

src/base/macros.h

@@ -188,11 +188,18 @@ V8_INLINE Dest bit_cast(Source const& source) {
 #define IS_TRIVIALLY_COPYABLE(T) std::is_trivially_copyable<T>::value
 #endif
 
-// The USE(x) template is used to silence C++ compiler warnings
+// The USE(x, ...) template is used to silence C++ compiler warnings
 // issued for (yet) unused variables (typically parameters).
-template <typename T>
-inline void USE(T) { }
-
+// The arguments are guaranteed to be evaluated from left to right.
+struct Use {
+  template <typename T>
+  Use(T&&) {}  // NOLINT(runtime/explicit)
+};
+#define USE(...)                                         \
+  do {                                                   \
+    ::Use unused_tmp_array_for_use_macro[]{__VA_ARGS__}; \
+    (void)unused_tmp_array_for_use_macro;                \
+  } while (false)
 
 #define IS_POWER_OF_TWO(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
 

test/unittests/base/platform/platform-unittest.cc

@@ -154,7 +154,8 @@ class MemoryAllocationPermissionsTest : public ::testing::Test {
     if (!sigsetjmp(continuation_, save_sigs)) {
       switch (action) {
         case MemoryAction::kRead: {
-          USE(*buffer);
+          // static_cast to remove the reference and force a memory read.
+          USE(static_cast<int>(*buffer));
           break;
         }
         case MemoryAction::kWrite: {