Add initial prototype for the GN meta-buildsystem.

tools/gn/err.h

Index: tools/gn/err.h
diff --git a/tools/gn/err.h b/tools/gn/err.h
new file mode 100644
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+++ b/tools/gn/err.h
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+// 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_ERR_H_
+#define TOOLS_GN_ERR_H_
+
+#include <string>
+#include <vector>
+
+#include "tools/gn/location.h"
+#include "tools/gn/token.h"
+
+class ParseNode;
+class Value;
+
+// Result of doing some operation. Check has_error() to see if an error
+// occurred.
+//
+// An error has a location and a message. Below that, is some optional help
+// text to go with the annotation of the location.
+//
+// An error can also have sub-errors which are additionally printed out
+// below. They can provide additional context.
+class Err {
+ public:
+  typedef std::vector<LocationRange> RangeList;
+
+  // Indicates no error.
+  Err();
+
+  // Error at a single point.
+  Err(const Location& location,
+      const std::string& msg,
+      const std::string& help = std::string());
+
+  // Error at a given range.
+  Err(const LocationRange& range,
+      const std::string& msg,
+      const std::string& help = std::string());
+
+  // Error at a given token.
+  Err(const Token& token,
+      const std::string& msg,
+      const std::string& help_text = std::string());
+
+  // Error at a given node.
+  Err(const ParseNode* node,
+      const std::string& msg,
+      const std::string& help_text = std::string());
+
+  // Error at a given value.
+  Err(const Value& value,
+      const std::string msg,
+      const std::string& help_text = std::string());
+
+  ~Err();
+
+  bool has_error() const { return has_error_; }
+  const Location& location() const { return location_; }
+  const std::string& message() const { return message_; }
+  const std::string& help_text() const { return help_text_; }
+
+  void AppendRange(const LocationRange& range) { ranges_.push_back(range); }
+  const RangeList& ranges() const { return ranges_; }
+
+  void AppendSubErr(const Err& err);
+
+  void PrintToStdout() const;
+
+ private:
+  void InternalPrintToStdout(bool is_sub_err) const;
+
+  bool has_error_;
+  Location location_;
+
+  std::vector<LocationRange> ranges_;
+
+  std::string message_;
+  std::string help_text_;
+
+  std::vector<Err> sub_errs_;
+};
+
+#endif  // TOOLS_GN_ERR_H_