GN flag to fail for unused build args.

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 176 matching lines @@
 
 bool Args::VerifyAllOverridesUsed(Err* err) const {
   base::AutoLock lock(lock_);
   Scope::KeyValueMap all_overrides(all_overrides_);
   for (const auto& map_pair : declared_arguments_per_toolchain_)
     RemoveDeclaredOverrides(map_pair.second, &all_overrides);
 
   if (all_overrides.empty())
     return true;
 
-  // Get a list of all possible overrides for help with error finding.
-  //
-  // It might be nice to do edit distance checks to see if we can find one close
-  // to what you typed.
-  std::string all_declared_str;
-  for (const auto& map_pair : declared_arguments_per_toolchain_) {
-    for (const auto& cur_str : map_pair.second) {
-      if (!all_declared_str.empty())
-        all_declared_str += ", ";
-      all_declared_str += cur_str.first.as_string();
-    }
-  }
-
   *err = Err(
       all_overrides.begin()->second.origin(), "Build argument has no effect.",
       "The variable \"" + all_overrides.begin()->first.as_string() +
           "\" was set as a build argument\nbut never appeared in a " +
-          "declare_args() block in any buildfile.\n\nPossible arguments: " +
-          all_declared_str);
+          "declare_args() block in any buildfile.\n\n"
+          "To view possible args, run \"gn args --list <builddir>\"");
   return false;
 }
 
 void Args::MergeDeclaredArguments(Scope::KeyValueMap* dest) const {
   base::AutoLock lock(lock_);
   for (const auto& map_pair : declared_arguments_per_toolchain_) {
     for (const auto& arg : map_pair.second)
       (*dest)[arg.first] = arg.second;
   }
 }
@@ skipping 81 matching lines @@
   lock_.AssertAcquired();
   for (const auto& val : values)
     all_overrides_[val.first] = val.second;
 }
 
 Scope::KeyValueMap& Args::DeclaredArgumentsForToolchainLocked(
     Scope* scope) const {
   lock_.AssertAcquired();
   return declared_arguments_per_toolchain_[scope->settings()];
 }