[base] Make USE a variadic template

This will allow for passing more than one variable. 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

Change-Id: I8f894d730bbcd195ed83705f98771994b4bc906f
Reviewed-on: https://chromium-review.googlesource.com/565561Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46527}

src/base/macros.h

@@ -190,9 +190,8 @@ V8_INLINE Dest bit_cast(Source const& source) {
 
 // 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) { }
-
+template <typename... Ts>
+inline void USE(Ts&&...) {}
 
 #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: {