Advise using `verify_doubled_constant_names`

[pirj]

In addition to verify_partial_doubles the above option may come handy to verify that you are stubbing an existing constant.

      # When this is set to true, an error will be raised when
      # `instance_double` or `class_double` is given the name of an undefined
      # constant. You probably only want to set this when running your entire
      # test suite, with all production code loaded. Setting this for an
      # isolated unit test will prevent you from being able to isolate it!

Example

With this option set to false the following example will, surprisingly, pass:

describe UserGreeter do
  it 'picks user name and prints greeting' do
    user = instance_double('Usor') # notice a typo
    allow(user).to receive(:name).and_return('John')
    expect { subject.process(user) }.to output('Hello, John')
end

When User renames its name method to full_name, this spec will still pass, but UserGreeter will blow up in production with NoMethodError.

Real-life Examples

Example violations (resulting with errors when the option is set to true):

# Typo: notice the leading space
let(:api) { instance_double(' ThirdParty::API') }

# `instance_double` expects a class name
let(:charge) { instance_double('payment', id: 1) } # "payment" is not a defined constant. Perhaps you misspelt it?

# Does not exist outside of the test code
let(:action) { instance_double('Action') } # "Action" is not a defined constant (though "Namespace::Action" is)

# Not loaded
let(:settings) { class_double('Settings') } # "Settings" is not a defined constant

# Double stubbing
before do
  stub_const('ActionClass', Class.new)
end
let(:action) { instance_double(ActionClass) } "ActionClass" is not a defined constant