Add LetSetup cop

Closes rubocop#94

config/default.yml

@@ -51,6 +51,9 @@ RSpec/InstanceVariable:
   Description: 'Checks for the usage of instance variables.'
   Enabled: true
 
+RSpec/LetSetup:
+  Description: 'Checks for `let!` being used for test setup.'
+
 RSpec/FilePath:
   Description: 'Checks the file and folder naming of the spec file.'
   Enabled: true

lib/rubocop-rspec.rb

@@ -21,6 +21,7 @@
 require 'rubocop/cop/rspec/file_path'
 require 'rubocop/cop/rspec/focus'
 require 'rubocop/cop/rspec/instance_variable'
+require 'rubocop/cop/rspec/let_setup'
 require 'rubocop/cop/rspec/multiple_describes'
 require 'rubocop/cop/rspec/multiple_expectations'
 require 'rubocop/cop/rspec/named_subject'

lib/rubocop/cop/rspec/let_setup.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Detect unreferenced `let!` calls being used for test setup
+      #
+      # @example
+      #   # Bad
+      #   let!(:my_widget) { create(:widget) }
+      #
+      #   it 'counts widgets' do
+      #     expect(Widget.count).to eq(1)
+      #   end
+      #
+      #   # Good
+      #   it 'counts widgets' do
+      #     create(:widget)
+      #     expect(Widget.count).to eq(1)
+      #   end
+      #
+      #   # Good
+      #   before { create(:widget) }
+      #
+      #   it 'counts widegts' do
+      #     expect(Widget.count).to eq(1)
+      #   end
+      class LetSetup < Cop
+        include RuboCop::RSpec::TopLevelDescribe, RuboCop::RSpec::Language
+
+        MSG = 'Do not use `let!` for test setup.'.freeze
+
+        def_node_search :let_bang, '(block $(send nil :let! (sym $_)) args ...)'
+
+        def_node_search :method_called?, '(send nil %)'
+
+        def_node_matcher :example_group?, <<-PATTERN
+          (block (send _ {#{ExampleGroups::ALL.to_node_pattern}} ...) ...)
+        PATTERN
+
+        def on_block(node)
+          return unless example_group?(node)
+
+          unused_let_bang(node) do |let|
+            add_offense(let, :expression)
+          end
+        end
+
+        private
+
+        def unused_let_bang(node)
+          let_bang(node) do |method_send, method_name|
+            yield(method_send) unless method_called?(node, method_name)
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/rspec/let_setup_spec.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+describe RuboCop::Cop::RSpec::LetSetup do
+  subject(:cop) { described_class.new }
+
+  it 'complains when let! is used and not referenced' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        let!(:foo) { bar }
+        ^^^^^^^^^^ Do not use `let!` for test setup.
+
+        it 'does not use foo' do
+          expect(baz).to eq(qux)
+        end
+      end
+    RUBY
+  end
+
+  it 'ignores let! when used in `before`' do
+    expect_no_violations(<<-RUBY)
+      describe Foo do
+        let!(:foo) { bar }
+
+        before do
+          foo
+        end
+
+        it 'does not use foo' do
+          expect(baz).to eq(qux)
+        end
+      end
+    RUBY
+  end
+
+  it 'ignores let! when used in example' do
+    expect_no_violations(<<-RUBY)
+      describe Foo do
+        let!(:foo) { bar }
+
+        it 'uses foo' do
+          foo
+          expect(baz).to eq(qux)
+        end
+      end
+    RUBY
+  end
+
+  it 'complains when let! is used and not referenced within nested group' do
+    expect_violation(<<-RUBY)
+      describe Foo do
+        context 'when something special happens' do
+          let!(:foo) { bar }
+          ^^^^^^^^^^ Do not use `let!` for test setup.
+
+          it 'does not use foo' do
+            expect(baz).to eq(qux)
+          end
+        end
+
+        it 'references some other foo' do
+          foo
+        end
+      end
+    RUBY
+  end
+end