Custom Rspec matcher for checking class for constant, with optional class type.

example.rb

# frozen_string_literal: true

class ExampleClass
  PI = 3.14159265359
end

example_spec.rb

# frozen_string_literal: true

require 'spec_helper'

RSpec.describe ExampleClass do
  it { expect(described_class).to have_constant(:PI, Float) }
  it { expect(described_class).to define_constant(:PI, Float) }
end

have_constant.rb

# frozen_string_literal: true
#
# File: spec/support/matchers/have_constant.rb

RSpec::Matchers.define :have_constant do |const, klass|
  match do |owner|
    if klass.nil?
      owner.const_defined?(const)
    else
      owner.const_defined?(const) && owner.const_get(const).class == klass
    end
  end

  failure_message do |actual|
    msg = +"constant #{expected[0]} not defined in #{actual}"
    msg += " as a #{expected[1]}" unless expected[1].nil?
    msg
  end

  failure_message_when_negated do |actual|
    msg = +"constant #{expected[0]} is defined in #{actual}"
    msg += " as a #{expected[1]}" unless expected[1].nil?
    msg
  end

  description do
    msg = +"have constant #{const}"
    msg += " defined with class #{klass}" unless klass.nil?
    msg
  end
end

RSpec::Matchers.alias_matcher :define_constant, :have_constant

spec_helper.rb

# frozen_string_literal: true
#
# File: spec/spec_helper.rb

Dir[File.join(__dir__, "/support/**/*.rb")].each { |f| require f }