Mark build arguments implicitly used in GN.

tools/gn/args.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/args.h"
 
 #include "base/sys_info.h"
 #include "build/build_config.h"
 #include "tools/gn/variables.h"
 
@@ skipping 166 matching lines @@
         err->AppendSubErr(Err(previously_declared->second.origin(),
                               "Previous declaration.",
                               "See also \"gn help buildargs\" for more on how "
                               "build arguments work."));
         return false;
       }
     } else {
       declared_arguments.insert(arg);
     }
 
+    // In all the cases below, mark the variable used. If a variable is set
+    // that's only used in one toolchain, we don't want to report unused
+    // variable errors in other toolchains. Also, in some cases it's reasonable
+    // for the build file to overwrite the value with a different value based
+    // on some other condition without dereferencing the value first.
+
     // Check whether this argument has been overridden on the toolchain level
     // and use the override instead.
     Scope::KeyValueMap::const_iterator toolchain_override =
         toolchain_overrides.find(arg.first);
     if (toolchain_override != toolchain_overrides.end()) {
       scope_to_set->SetValue(toolchain_override->first,
                              toolchain_override->second,
                              toolchain_override->second.origin());
+      scope_to_set->MarkUsed(arg.first);
       continue;
     }
 
     // Check whether this argument has been overridden and use the override
     // instead.
     Scope::KeyValueMap::const_iterator override = overrides_.find(arg.first);
     if (override != overrides_.end()) {
       scope_to_set->SetValue(override->first, override->second,
                              override->second.origin());
+      scope_to_set->MarkUsed(override->first);
       continue;
     }
 
-    // Mark the variable used so the build script can override it in
-    // certain cases without getting unused value errors.
     scope_to_set->SetValue(arg.first, arg.second, arg.second.origin());
     scope_to_set->MarkUsed(arg.first);
   }
 
   return true;
 }
 
 bool Args::VerifyAllOverridesUsed(Err* err) const {
   base::AutoLock lock(lock_);
   Scope::KeyValueMap all_overrides(all_overrides_);
@@ skipping 121 matching lines @@
     Scope* scope) const {
   lock_.AssertAcquired();
   return declared_arguments_per_toolchain_[scope->settings()];
 }
 
 Scope::KeyValueMap& Args::OverridesForToolchainLocked(
     Scope* scope) const {
   lock_.AssertAcquired();
   return toolchain_overrides_[scope->settings()];
 }