example.rb

puts 1
puts %w(hello world) * ' '
puts %w(h l t).join('igh')

exprint.rb

#!/usr/bin/env ruby
require 'stringio'

path = ARGV.first or abort('')

class OutputRecorder < StringIO
  def self.record
    original_stdout = $stdout
    $stdout = output_recorder = new
    yield
    output_recorder.outputs.extend(OutputsHelper)
  ensure
    $stdout = original_stdout
  end

  def initialize
    super
    close_read
    @outputs = {}
  end

  attr_reader :outputs

  def write(string)
    file, line_number = caller.first.split(':')
    line_number = line_number.to_i
    @outputs[file] ||= {}
    @outputs[file][line_number] ||= ''
    @outputs[file][line_number] << string
    super
  end

  module OutputsHelper
    def record?(file, line_number)
      !!(self[file] && self[file][line_number])
    end

    def [](*args)
      return super unless args.size == 2
      file, line_number = args
      self[file] && self[file][line_number]
    end
  end
end

outputs = OutputRecorder.record { load path }

File.open(path, 'r') do |file|
  file.each_line.with_index do |line, index|
    print line.chomp
    print " # => #{outputs[path, index + 1].chomp}" if outputs.record?(path, index + 1)
    puts
  end
end

gistfile1.txt

$ ./exprint.rb example.rb
puts 1 # => 1
puts %w(hello world) * ' ' # => hello world
puts %w(h l t).join('igh') # => highlight