Use $root:default as a "default" rule

tools/gn/setup.cc

 // 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.
 
 #include "tools/gn/setup.h"
 
 #include <stdlib.h>
 #include <algorithm>
 #include <sstream>
 #include <utility>
@@ skipping 77 matching lines @@
       If unspecified, the ability to call exec_script is unrestricted.
 
       Example:
         exec_script_whitelist = [
           "//base/BUILD.gn",
           "//build/my_config.gni",
         ]
 
   root [optional]
       Label of the root build target. The GN build will start by loading the
-      build file containing this target name. This defaults to "//:" which will
+      build file containing this target name. This defaults to label which will

[brettw, 2017/04/19 19:49:25]

This new text isn't gramatical. I think the old text is probably fine. It's what
actually happens, and I don't think people are commonly reading this
documentation and setting the root such that they would get confused by it.

[Petr Hosek, 2017/04/19 20:20:26]

Done.

[Dirk Pranke, 2017/04/19 23:01:27]

FWIW, I still find it confusing.

       cause the file //BUILD.gn to be loaded.
 
   secondary_source [optional]
       Label of an alternate directory tree to find input files. When searching
       for a BUILD.gn file (or the build config file discussed above), the file
       will first be looked for in the source root. If it's not found, the
       secondary source root will be checked (which would contain a parallel
       directory hierarchy).
 
       This behavior is intended to be used when BUILD.gn files can't be checked
@@ skipping 582 matching lines @@
     err.PrintToStdout();
     return false;
   }
 
   return true;
 }
 
 bool Setup::FillOtherConfig(const base::CommandLine& cmdline) {
   Err err;
   SourceDir current_dir("//");
+  Label root_target_label(current_dir, "");
 
   // Secondary source path, read from the config file if present.
   // Read from the config file if present.
   const Value* secondary_value =
       dotfile_scope_.GetValue("secondary_source", true);
   if (secondary_value) {
     if (!secondary_value->VerifyTypeIs(Value::STRING, &err)) {
       err.PrintToStdout();
       return false;
     }
     build_settings_.SetSecondarySourcePath(
         SourceDir(secondary_value->string_value()));
   }
 
   // Root build file.
   const Value* root_value = dotfile_scope_.GetValue("root", true);
   if (root_value) {
     if (!root_value->VerifyTypeIs(Value::STRING, &err)) {
       err.PrintToStdout();
       return false;
     }
 
-    Label root_target_label =
-        Label::Resolve(current_dir, Label(), *root_value, &err);
+    root_target_label = Label::Resolve(current_dir, Label(), *root_value, &err);
     if (err.has_error()) {
       err.PrintToStdout();
       return false;
     }
 
     root_build_file_ = Loader::BuildFileForLabel(root_target_label);
   }
+  build_settings_.SetRootTargetLabel(root_target_label);
 
   // Build config file.
   const Value* build_config_value =
       dotfile_scope_.GetValue("buildconfig", true);
   if (!build_config_value) {
     Err(Location(), "No build config file.",
         "Your .gn file (\"" + FilePathToUTF8(dotfile_name_) + "\")\n"
         "didn't specify a \"buildconfig\" value.").PrintToStdout();
     return false;
   } else if (!build_config_value->VerifyTypeIs(Value::STRING, &err)) {
@@ skipping 58 matching lines @@
     if (!arg_file_template_value->VerifyTypeIs(Value::STRING, &err)) {
       err.PrintToStdout();
       return false;
     }
     SourceFile path(arg_file_template_value->string_value());
     build_settings_.set_arg_file_template_path(path);
   }
 
   return true;
 }