# File lib/linecache.rb, line 402
  def trace_line_numbers(filename, reload_on_change=false)
    fullname = cache(filename, reload_on_change)
    return nil unless fullname
    e = @@file_cache[filename]
    unless e.line_numbers
      e.line_numbers =
        TraceLineNumbers.lnums_for_str_array(e.lines[:plain])
      e.line_numbers = false unless e.line_numbers
    end
    e.line_numbers
  end