Presubmit: Warn about useless forward declarations

PRESUBMIT.py

 # Copyright (c) 2012 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
@@ skipping 1566 matching lines @@
         break
 
   if len(files):
     return [output_api.PresubmitError(
         'Do not introduce new mojom targets with use_new_wrapper_types set to '
         'false. The mode is deprecated and will be removed soon.',
         files)]
   return []
 
 
+def _CheckUselessForwardDeclarations(input_api, output_api):
+  """Checks that added or removed lines in affected header files
+     do not lead to new useless class or struct forward declaration.
+  """
+  results = []
+  class_pattern = input_api.re.compile(r'^class\s+(\w+);$',
+                                       input_api.re.MULTILINE)
+  struct_pattern = input_api.re.compile(r'^struct\s+(\w+);$',
+                                        input_api.re.MULTILINE)
+  for f in input_api.AffectedFiles(include_deletes=False):
+    if not f.LocalPath().endswith('.h'):
+      continue
+
+    contents = input_api.ReadFile(f)
+    fwd_decls = input_api.re.findall(class_pattern, contents)
+    fwd_decls.extend(input_api.re.findall(struct_pattern, contents))
+
+    useless_fwd_decls = []
+    for decl in fwd_decls:
+      count = sum(1 for _ in input_api.re.finditer(
+        r'\b%s\b' % input_api.re.escape(decl), contents))
+      if count == 1:
+        useless_fwd_decls.append(decl)
+
+    if not useless_fwd_decls:
+      continue
+
+    for line in f.GenerateScmDiff().splitlines():
+      if (line.startswith('-') and not line.startswith('--') or
+          line.startswith('+') and not line.startswith('++')):
+        for decl in useless_fwd_decls:
+          if input_api.re.search(r'\b%s\b' % decl, line[1:]):
+            results.append(output_api.PresubmitPromptWarning(
+              '%s: %s forward declaration is becoming useless' %
+              (f.LocalPath(), decl)))
+            useless_fwd_decls.remove(decl)
+
+  return results
+
+
 def _CheckAndroidToastUsage(input_api, output_api):
   """Checks that code uses org.chromium.ui.widget.Toast instead of
      android.widget.Toast (Chromium Toast doesn't force hardware
      acceleration on low-end devices, saving memory).
   """
   toast_import_pattern = input_api.re.compile(
       r'^import android\.widget\.Toast;$')
 
   errors = []
 
@@ skipping 429 matching lines @@
   results.extend(_CheckNoDeprecatedJS(input_api, output_api))
   results.extend(_CheckParseErrors(input_api, output_api))
   results.extend(_CheckForIPCRules(input_api, output_api))
   results.extend(_CheckForWindowsLineEndings(input_api, output_api))
   results.extend(_CheckSingletonInHeaders(input_api, output_api))
   results.extend(_CheckNoDeprecatedCompiledResourcesGYP(input_api, output_api))
   results.extend(_CheckPydepsNeedsUpdating(input_api, output_api))
   results.extend(_CheckJavaStyle(input_api, output_api))
   results.extend(_CheckIpcOwners(input_api, output_api))
   results.extend(_CheckMojoUsesNewWrapperTypes(input_api, output_api))
+  results.extend(_CheckUselessForwardDeclarations(input_api, output_api))
 
   if any('PRESUBMIT.py' == f.LocalPath() for f in input_api.AffectedFiles()):
     results.extend(input_api.canned_checks.RunUnitTestsInDirectory(
         input_api, output_api,
         input_api.PresubmitLocalPath(),
         whitelist=[r'^PRESUBMIT_test\.py$']))
   return results
 
 
 def _CheckAuthorizedAuthor(input_api, output_api):
@@ skipping 260 matching lines @@
   results.extend(input_api.canned_checks.CheckTreeIsOpen(
       input_api,
       output_api,
       json_url='http://chromium-status.appspot.com/current?format=json'))
 
   results.extend(input_api.canned_checks.CheckChangeHasBugField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   return results