ninja windows: Make pch work on VS2012, and simplify implementation

pylib/gyp/msvs_emulation.py

Index: pylib/gyp/msvs_emulation.py
===================================================================
--- pylib/gyp/msvs_emulation.py	(revision 1564)
+++ pylib/gyp/msvs_emulation.py	(working copy)
@@ -610,29 +610,25 @@
   return ((source_ext in c_exts and pch_source_ext in c_exts) or
           (source_ext in cc_exts and pch_source_ext in cc_exts))
 
+
 class PrecompiledHeader(object):
   """Helper to generate dependencies and build rules to handle generation of
   precompiled headers. Interface matches the GCH handler in xcode_emulation.py.
   """
-  def __init__(self, settings, config, gyp_to_build_path):
+  def __init__(
+      self, settings, config, gyp_to_build_path, gyp_to_unique_output, obj_ext):
     self.settings = settings
     self.config = config
-    self.gyp_to_build_path = gyp_to_build_path
+    pch_source = self.settings.msvs_precompiled_source[self.config]
+    self.pch_source = gyp_to_build_path(pch_source)
+    filename, _ = os.path.splitext(pch_source)
+    self.output_obj = gyp_to_unique_output(filename + obj_ext).lower()
 
   def _PchHeader(self):
     """Get the header that will appear in an #include line for all source
     files."""
     return os.path.split(self.settings.msvs_precompiled_header[self.config])[1]
 
-  def _PchSource(self):
-    """Get the source file that is built once to compile the pch data."""
-    return self.gyp_to_build_path(
-        self.settings.msvs_precompiled_source[self.config])
-
-  def _PchOutput(self):
-    """Get the name of the output of the compiled pch data."""
-    return '${pchprefix}.' + self._PchHeader() + '.pch'
-
   def GetObjDependencies(self, sources, objs):
     """Given a list of sources files and the corresponding object files,
     returns a list of the pch files that should be depended upon. The
@@ -640,26 +636,32 @@
     with make.py on Mac, and xcode_emulation.py."""
     if not self._PchHeader():
       return []
-    source = self._PchSource()
-    assert source
-    pch_ext = os.path.splitext(self._PchSource())[1]
+    pch_ext = os.path.splitext(self.pch_source)[1]
     for source in sources:
       if _LanguageMatchesForPch(os.path.splitext(source)[1], pch_ext):
-        return [(None, None, self._PchOutput())]
+        return [(None, None, self.output_obj)]
     return []
 
   def GetPchBuildCommands(self):
-    """Returns [(path_to_pch, language_flag, language, header)].
-    |path_to_gch| and |header| are relative to the build directory."""
-    header = self._PchHeader()
-    source = self._PchSource()
-    if not source or not header:
-      return []
-    ext = os.path.splitext(source)[1]
-    lang = 'c' if ext == '.c' else 'cc'
-    return [(self._PchOutput(), '/Yc' + header, lang, source)]
+    """Not used on Windows as there are no additional build steps required
+    (instead, existing steps are modified in GetFlagsModifications below)."""
+    return []
 
+  def GetFlagsModifications(self, input, output, implicit, command,
+                            cflags_c, cflags_cc, expand_special):
+    """Get the modified cflags and implicit dependencies that should be used
+    for the pch compilation step."""
+    if input == self.pch_source:
+      pch_output = ['/Yc' + self._PchHeader()]
+      if command == 'cxx':
+        return ([('cflags_cc', map(expand_special, cflags_cc + pch_output))],
+                self.output_obj, [])
+      elif command == 'cc':
+        return ([('cflags_c', map(expand_special, cflags_c + pch_output))],
+                self.output_obj, [])
+    return [], output, implicit
 
+
 vs_version = None
 def GetVSVersion(generator_flags):
   global vs_version