Reduce input dependencies in GN.

tools/gn/ninja_target_writer.h

 // 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.
 
 #ifndef TOOLS_GN_NINJA_TARGET_WRITER_H_
 #define TOOLS_GN_NINJA_TARGET_WRITER_H_
 
 #include <iosfwd>
 
 #include "base/basictypes.h"
 #include "tools/gn/path_output.h"
 #include "tools/gn/substitution_type.h"
 
 class FileTemplate;
+class OutputFile;
 class Settings;
 class Target;
 
 // Generates one target's ".ninja" file. The toplevel "build.ninja" file is
 // generated by the NinjaBuildWriter.
 class NinjaTargetWriter {
  public:
   NinjaTargetWriter(const Target* target, std::ostream& out);
   virtual ~NinjaTargetWriter();
 
   static void RunAndWriteFile(const Target* target);
 
   virtual void Run() = 0;
 
  protected:
   // Writes out the substitution values that are shared between the different
   // types of tools (target gen dir, target label, etc.). Only the substitutions
   // identified by the given bits will be written.
   void WriteSharedVars(const SubstitutionBits& bits);
 
   // Writes to the output stream a stamp rule for input dependencies, and
-  // returns the string to be appended to source rules that encodes the
-  // order-only dependencies for the current target. This will include the "|"
-  // character so can just be appended to the source rules. If there are no
-  // implicit dependencies and no extra target dependencies passed in, returns
-  // the empty string.
-  std::string WriteInputDepsStampAndGetDep(
+  // returns the file to be appended to source rules that encodes the
+  // order-only dependencies for the current target. The returned OutputFile
+  // will be empty if there are no implicit dependencies and no extra target
+  // dependencies passed in.
+  OutputFile WriteInputDepsStampAndGetDep(
       const std::vector<const Target*>& extra_hard_deps) const;
 
   // Writes to the output file a final stamp rule for the target that stamps
   // the given list of files. This function assumes the stamp is for the target
   // as a whole so the stamp file is set as the target's dependency output.
   void WriteStampForTarget(const std::vector<OutputFile>& deps,
                            const std::vector<OutputFile>& order_only_deps);
 
   const Settings* settings_;  // Non-owning.
   const Target* target_;  // Non-owning.
   std::ostream& out_;
   PathOutput path_output_;
 
  private:
   void WriteCopyRules();
 
   DISALLOW_COPY_AND_ASSIGN(NinjaTargetWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_TARGET_WRITER_H_