GN: Generate error if multiple rules generate same file

tools/gn/command_gen.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 "base/atomicops.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/timer/elapsed_timer.h"
 #include "tools/gn/build_settings.h"
@@ skipping 76 matching lines @@
   // the writing to maintain a counter.
   base::subtle::Atomic32 write_counter = 0;
   setup->builder()->set_resolved_callback(
       base::Bind(&ItemResolvedCallback, &write_counter,
                  scoped_refptr<Builder>(setup->builder())));
 
   // Do the actual load. This will also write out the target ninja files.
   if (!setup->Run())
     return 1;
 
+  Err err;
   // Write the root ninja files.
   if (!NinjaWriter::RunAndWriteFiles(&setup->build_settings(),
-                                     setup->builder()))
+                                     setup->builder(),
+                                     &err)) {
+    err.PrintToStdout();
     return 1;
+  }
 
   base::TimeDelta elapsed_time = timer.Elapsed();
 
   if (!CommandLine::ForCurrentProcess()->HasSwitch(kSwitchQuiet)) {
     OutputString("Done. ", DECORATION_GREEN);
 
     std::string stats = "Wrote " +
         base::IntToString(static_cast<int>(write_counter)) +
         " targets from " +
         base::IntToString(
             setup->scheduler().input_file_manager()->GetInputFileCount()) +
         " files in " +
         base::IntToString(elapsed_time.InMilliseconds()) + "ms\n";
     OutputString(stats);
   }
 
   return 0;
 }
 
 }  // namespace commands