Modify ALLOW_UNUSED to allow enabling unused local warnings on MSVC.

base/compiler_specific.h

Index: base/compiler_specific.h
diff --git a/base/compiler_specific.h b/base/compiler_specific.h
index ba57cc38cdc2ef87dfe0697f25a950bcbec4dc0e..6210d1ace59ab6767e6f0fd1295955890f0feef5 100644
--- a/base/compiler_specific.h
+++ b/base/compiler_specific.h
@@ -94,11 +94,17 @@
 // (Typically used to silence a compiler warning when the assignment
 // is important for some other reason.)
 // Use like:
-//   int x ALLOW_UNUSED = ...;
+//   int x = ...;
+//   ALLOW_UNUSED_LOCAL(x);
+#define ALLOW_UNUSED_LOCAL(x) false ? (void)x : (void)0

[Nico, 2014/10/25 22:56:08]

How is this different from ignore_result()?

Why not just (void)x, why the `false?:`?

[Peter Kasting, 2014/10/27 17:58:44]

It doesn't implicitly take the address of |x| or in any other way evaluate it
(see below).

It would also be strange for code to write "ignore_result(x)" as a way of
marking |x| as "allowed to be unused" -- that could be very confusing to a
reader depending on the situation.
This references |x| without any possibility of actually evaluating it, just in
case evaluating the argument has some effect (e.g. x is declared volatile and so
we generate a load the compiler can't optimize away, or I suppose maybe this
gets used inside another macro and |x| turns out to be some expression with side
effects).

I think it's unlikely the distinction actually matters, but this form is at
least as safe as the other, so I don't see anything wrong with using it.

+
+// Annotate a typedef or function indicating it's ok if it's not used.
+// Use like:
+//   typedef Foo Bar ALLOW_UNUSED_TYPE;
 #if defined(COMPILER_GCC)
-#define ALLOW_UNUSED __attribute__((unused))
+#define ALLOW_UNUSED_TYPE __attribute__((unused))
 #else
-#define ALLOW_UNUSED
+#define ALLOW_UNUSED_TYPE
 #endif
 
 // Annotate a function indicating it should not be inlined.