Remove ~MaybeHandle and statically assert that handles are trivially copyable

https://codereview.chromium.org/2632713003 with workaround for old GCC.

Drive-by: fix unused variable in src/wasm/wasm-js.cc

Bug:chromium:457078

Change-Id: I6c1b65076bae783c31869552bc87d05c28550e26
Reviewed-on: https://chromium-review.googlesource.com/538463
Commit-Queue: Loo Rong Jie <loorongjie@gmail.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46049}

src/base/macros.h

@@ -181,6 +181,12 @@ V8_INLINE Dest bit_cast(Source const& source) {
 // TODO(all) Replace all uses of this macro with static_assert, remove macro.
 #define STATIC_ASSERT(test) static_assert(test, #test)
 
+// TODO(rongjie) Remove this workaround once we require gcc >= 5.0
+#if __GNUG__ && __GNUC__ < 5
+#define IS_TRIVIALLY_COPYABLE(T) __has_trivial_copy(T)
+#else
+#define IS_TRIVIALLY_COPYABLE(T) std::is_trivially_copyable<T>::value
+#endif
 
 // The USE(x) template is used to silence C++ compiler warnings
 // issued for (yet) unused variables (typically parameters).

src/handles.cc

@@ -12,6 +12,16 @@
 namespace v8 {
 namespace internal {
 
+// Handles should be trivially copyable so that they can be efficiently passed
+// by value. If they are not trivially copyable, they cannot be passed in
+// registers.
+static_assert(IS_TRIVIALLY_COPYABLE(HandleBase),
+              "HandleBase should be trivially copyable");
+static_assert(IS_TRIVIALLY_COPYABLE(Handle<Object>),
+              "Handle<Object> should be trivially copyable");
+static_assert(IS_TRIVIALLY_COPYABLE(MaybeHandle<Object>),
+              "MaybeHandle<Object> should be trivially copyable");
+
 #ifdef DEBUG
 bool HandleBase::IsDereferenceAllowed(DereferenceCheckMode mode) const {
   DCHECK_NOT_NULL(location_);

src/handles.h

@@ -185,7 +185,6 @@ template <typename T>
 class MaybeHandle final {
  public:
   V8_INLINE MaybeHandle() {}
-  V8_INLINE ~MaybeHandle() {}
 
   // Constructor for handling automatic up casting from Handle.
   // Ex. Handle<JSArray> can be passed when MaybeHandle<Object> is expected.

src/wasm/wasm-js.cc

@@ -357,7 +357,7 @@ void WebAssemblyInstance(const v8::FunctionCallbackInfo<v8::Value>& args) {
 
   ErrorThrower thrower(i_isolate, "WebAssembly.Instance()");
 
-  auto maybe_module = GetFirstArgumentAsModule(args, &thrower);
+  GetFirstArgumentAsModule(args, &thrower);
   if (thrower.error()) return;
 
   // If args.Length < 2, this will be undefined - see FunctionCallbackInfo.