🐣 GN: Print the import trail when parse errors occur.

tools/gn/location.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_LOCATION_H_
 #define TOOLS_GN_LOCATION_H_
 
 #include <string>
 
 class InputFile;
 
 // Represents a place in a source file. Used for error reporting.
 class Location {
  public:
   Location();
   Location(const InputFile* file, int line_number, int column_number, int byte);
 
   const InputFile* file() const { return file_; }
   int line_number() const { return line_number_; }
   int column_number() const { return column_number_; }
   int byte() const { return byte_; }
+  bool is_null() const { return *this == Location(); }
 
   bool operator==(const Location& other) const;
   bool operator!=(const Location& other) const;
   bool operator<(const Location& other) const;
 
   // Returns a string with the file, line, and (optionally) the character
   // offset for this location. If this location is null, returns an empty
   // string.
   std::string Describe(bool include_column_number) const;
 
  private:
   const InputFile* file_;  // Null when unset.
   int line_number_;        // -1 when unset. 1-based.
   int column_number_;      // -1 when unset. 1-based.
   int byte_;               // Index into the buffer, 0-based.
 };
 
 // Represents a range in a source file. Used for error reporting.
 // The end is exclusive i.e. [begin, end)
 class LocationRange {
  public:
   LocationRange();
   LocationRange(const Location& begin, const Location& end);
 
   const Location& begin() const { return begin_; }
   const Location& end() const { return end_; }
+  bool is_null() const {
+    return begin_.is_null();  // No need to check both for the null case.
+  }
+
 
   LocationRange Union(const LocationRange& other) const;
 
  private:
   Location begin_;
   Location end_;
 };
 
 #endif  // TOOLS_GN_LOCATION_H_