Allow GN configs to have sub-configs.

tools/gn/config.h

 // 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.
 
 #ifndef TOOLS_GN_CONFIG_H_
 #define TOOLS_GN_CONFIG_H_
 
+#include "base/logging.h"
 #include "base/macros.h"
 #include "tools/gn/config_values.h"
 #include "tools/gn/item.h"
+#include "tools/gn/label_ptr.h"
+#include "tools/gn/unique_vector.h"
 
 // Represents a named config in the dependency graph.
+//
+// A config can list other configs. We track both the data assigned directly
+// on the config, this list of sub-configs, and (when the config is resolved)
+// the resulting values of everything merged together. The flatten step
+// means we can avoid doing a recursive config walk for every target to compute
+// flags.
 class Config : public Item {
  public:
   Config(const Settings* settings, const Label& label);
   ~Config() override;
 
+  // Item implementation.
   Config* AsConfig() override;
   const Config* AsConfig() const override;
+  bool OnResolved(Err* err) override;
 
-  ConfigValues& config_values() { return config_values_; }
-  const ConfigValues& config_values() const { return config_values_; }
+  // The values set directly on this config. This will not contain data from
+  // sub-configs.
+  ConfigValues& own_values() { return own_values_; }
+  const ConfigValues& own_values() const { return own_values_; }
+
+  // The values that represent this config and all sub-configs combined into
+  // one. This is only valid after the config is resolved (when we know the
+  // contents of the sub-configs).
+  const ConfigValues& resolved_values() const {
+    DCHECK(resolved_);
+    if (configs_.empty())  // No sub configs, just use the regular values.
+      return own_values_;
+    return composite_values_;
+  }
+
+  // List of sub-configs.
+  const UniqueVector<LabelConfigPair>& configs() const { return configs_; }
+  UniqueVector<LabelConfigPair>& configs() { return configs_; }
 
  private:
-  ConfigValues config_values_;
+  ConfigValues own_values_;
+
+  // Contains the own_values combined with sub-configs. Most configs don't have
+  // sub-configs. So as an optimization, this is not populated if there are no
+  // items in configs_. The resolved_values() getter handles this.
+  bool resolved_;
+  ConfigValues composite_values_;
+
+  UniqueVector<LabelConfigPair> configs_;
 
   DISALLOW_COPY_AND_ASSIGN(Config);
 };
 
 #endif  // TOOLS_GN_CONFIG_H_