Use either out/Release or out/Debug for YouCompleteMe, depending on mtime.

tools/vim/chromium.ycm_extra_conf.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.
 
 # Autocompletion config for YouCompleteMe in Chromium.
 #
 # USAGE:
 #
 #   1. Install YCM [https://github.com/Valloric/YouCompleteMe]
 #          (Googlers should check out [go/ycm])
@@ skipping 58 matching lines @@
   while not (PathExists(curdir, 'src')
              and (PathExists(curdir, '.gclient')
                   or PathExists(curdir, 'src', '.git'))):
     nextdir = os.path.normpath(os.path.join(curdir, '..'))
     if nextdir == curdir:
       return None
     curdir = nextdir
   return os.path.join(curdir, 'src')
 
 
+# Largely copied from ninja-build.vim (guess_configuration)
+def GetNinjaOutputDirectory(chrome_root):
+  """Returns either <chrome_root>/out/Release or <chrome_root>/out/Debug.
+
+  The configuration chosen is the one most recently generated/built."""
+  root = os.path.join(chrome_root, 'out')
+  debug_path = os.path.join(root, 'Debug')
+  release_path = os.path.join(root, 'Release')
+
+  def is_release_15s_newer(test_path):
+    try:
+      debug_mtime = os.path.getmtime(os.path.join(debug_path, test_path))
+    except os.error:
+      debug_mtime = 0
+    try:
+      rel_mtime = os.path.getmtime(os.path.join(release_path, test_path))
+    except os.error:
+      rel_mtime = 0
+    return rel_mtime - debug_mtime >= 15
+
+  if is_release_15s_newer('build.ninja') or is_release_15s_newer('protoc'):
+    return release_path
+  return debug_path
+
+
 def GetClangCommandFromNinjaForFilename(chrome_root, filename):
   """Returns the command line to build |filename|.
 
   Asks ninja how it would build the source file. If the specified file is a
   header, tries to find its companion source file first.
 
   Args:
     chrome_root: (String) Path to src/.
     filename: (String) Path to source file being edited.
 
@@ skipping 19 matching lines @@
     else:
       # If this is a standalone .h file with no source, the best we can do is
       # try to use the default flags.
       return chrome_flags
 
   # Ninja needs the path to the source file from the output build directory.
   # Cut off the common part and /.
   subdir_filename = filename[len(chrome_root)+1:]
   rel_filename = os.path.join('..', '..', subdir_filename)
 
+  out_dir = GetNinjaOutputDirectory(chrome_root)
+
   # Ask ninja how it would build our source file.
-  p = subprocess.Popen(['ninja', '-v', '-C', chrome_root + '/out/Release', '-t',
+  p = subprocess.Popen(['ninja', '-v', '-C', out_dir, '-t',
                         'commands', rel_filename + '^'],
                        stdout=subprocess.PIPE)
   stdout, stderr = p.communicate()
   if p.returncode:
     return chrome_flags
 
   # Ninja might execute several commands to build something. We want the last
   # clang command.
   clang_line = None
   for line in reversed(stdout.split('\n')):
     if 'clang' in line:
       clang_line = line
       break
   else:
     return chrome_flags
 
   # Parse out the -I and -D flags. These seem to be the only ones that are
   # important for YCM's purposes.
   for flag in clang_line.split(' '):
     if flag.startswith('-I'):
       # Relative paths need to be resolved, because they're relative to the
       # output dir, not the source.
       if flag[2] == '/':
         chrome_flags.append(flag)
       else:
-        abs_path = os.path.normpath(os.path.join(
-            chrome_root, 'out', 'Release', flag[2:]))
+        abs_path = os.path.normpath(os.path.join(out_dir, flag[2:]))
         chrome_flags.append('-I' + abs_path)
     elif flag.startswith('-') and flag[1] in 'DWFfmO':
       chrome_flags.append(flag)
 
   return chrome_flags
 
 
 def FlagsForFile(filename):
   """This is the main entry point for YCM. Its interface is fixed.
 
   Args:
     filename: (String) Path to source file being edited.
 
   Returns:
     (Dictionary)
       'flags': (List of Strings) Command line flags.
       'do_cache': (Boolean) True if the result should be cached.
   """
   chrome_root = FindChromeSrcFromFilename(filename)
   chrome_flags = GetClangCommandFromNinjaForFilename(chrome_root,
                                                      filename)
   final_flags = flags + chrome_flags
 
   return {
     'flags': final_flags,
     'do_cache': True
   }