display_coverage.rb

require 'coverage'
Coverage.start
STDOUT.reopen("/dev/null")
require_relative 'fizzbuzz.rb'
Coverage.result.each do |file_name, counts|
  File.readlines(file_name).each_with_index do |code_line, line_number|
    count = counts[line_number] || "--"
    STDERR.printf "%3s: %s", count, code_line
  end
end
  
__END__

produces

  1: 1.upto(100).with_object('') do |i, x|
100:   if i % 3 == 0
 33:     x += 'Fizz' 
 --:   end
100:   if i % 5 == 0
 20:     x += 'Buzz' 
 --:   end
100:   if x.empty?
 53:     puts i
 --:   else
 47:     puts x
 --:   end
 --: end