Add an output_dir override to GN.

tools/gn/substitution_writer.cc

 // Copyright 2014 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 "tools/gn/substitution_writer.h"
 
 #include "tools/gn/build_settings.h"
 #include "tools/gn/escape.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/output_file.h"
@@ skipping 38 matching lines @@
     "\n"
     "  If no placeholders are found, the outputs or args list will be treated\n"
     "  as a static list of literal file names that do not depend on the\n"
     "  sources.\n"
     "\n"
     "  See \"gn help copy\" and \"gn help action_foreach\" for more on how\n"
     "  this is applied.\n"
     "\n"
     "Placeholders\n"
     "\n"
+    "  This section discusses only placeholders for actions. There are other\n"
+    "  placeholders used in the definition of tools. See \"gn help tool\" for\n"
+    "  those.\n"
+    "\n"
     "  {{source}}\n"
     "      The name of the source file including directory (*). This will\n"
     "      generally be used for specifying inputs to a script in the\n"
     "      \"args\" variable.\n"
     "        \"//foo/bar/baz.txt\" => \"../../foo/bar/baz.txt\"\n"
     "\n"
     "  {{source_file_part}}\n"
     "      The file part of the source including the extension.\n"
     "        \"//foo/bar/baz.txt\" => \"baz.txt\"\n"
     "\n"
@@ skipping 470 matching lines @@
     const Target* target,
     const Tool* tool,
     SubstitutionType type) {
   // First try the common tool ones.
   std::string result;
   if (GetTargetSubstitution(target, type, &result))
     return result;
 
   // Fall-through to the linker-specific ones.
   switch (type) {
+    case SUBSTITUTION_OUTPUT_DIR:
+      // Use the target's value if there is one (it will have no expansion
+      // patterns since it can directly use GN variables to compute whatever
+      // path it wants), or the tool's default (which will contain further
+      // expansions).
+      if (target->output_dir().is_null()) {
+        return ApplyPatternToLinkerAsOutputFile(
+            target, tool, tool->default_output_dir()).value();
+      } else {
+        SetDirOrDotWithNoSlash(RebasePath(
+                target->output_dir().value(),
+                target->settings()->build_settings()->build_dir()),
+            &result);
+        return result;
+      }
+      break;
+
     case SUBSTITUTION_OUTPUT_EXTENSION:
       // Use the extension provided on the target if specified, otherwise
       // fall back on the default. Note that the target's output extension
       // does not include the dot but the tool's does.
       if (!target->output_extension_set())
         return tool->default_output_extension();
       if (target->output_extension().empty())
         return std::string();  // Explicitly set to no extension.
       return std::string(".") + target->output_extension();
 
     default:
       NOTREACHED();
       return std::string();
   }
 }