experiment.rb

require 'coverage'

Coverage.start
require './a_method.rb'  # => true
a_method
p Coverage.result        # => {"/Users/.../a_method.rb"=>[1, 1, 1, 10, nil, 1, nil]}

# Does not produce coverage because `Coverage.result` clears out the coverage
# p Coverage.result      # coverage measurement is not enabled (RuntimeError)

# Does not produce coverage because the file `a_method.rb` is not loaded
Coverage.start
a_method
p Coverage.result        # => {"/Users/.../a_method.rb"=>[]}

# Does not produce coverage because the file `a_method.rb` is not loaded
Coverage.start
require './a_method.rb'  # => false
a_method
p Coverage.result        # => {"/Users/.../a_method.rb"=>[]}

# Unload `a_method.rb`
$".delete_if { |s| s.include?('a_method.rb') }

# Does produce coverage because the file `a_method.rb` is loaded
Coverage.start
require './a_method.rb'  # => true
a_method
p Coverage.result        # => {"/Users/.../a_method.rb"=>[1, 1, 1, 10, nil, 1, nil]}