Add NoLet cop

Fixes rubocop#862

CHANGELOG.md

@@ -53,6 +53,7 @@
 * Add `RSpec/RepeatedExampleGroupDescription` cop. ([@lazycoder9][])
 * Add block name and other lines to `RSpec/ScatteredSetup` message. ([@elebow][])
 * Fix `RSpec/RepeatedDescription` to take into account example metadata. ([@lazycoder9][])
+* Add `RSpec/NoLet` cop. ([@mockdeep][])
 
 ## 1.37.1 (2019-12-16)
 
@@ -539,3 +540,4 @@ Compatibility release so users can upgrade RuboCop to 0.51.0. No new features.
 [@harrylewis]: https://github.com/harrylewis
 [@elliterate]: https://github.com/elliterate
 [@mlarraz]: https://github.com/mlarraz
+[@mockdeep]: https://github.com/mockdeep

config/default.yml

@@ -415,6 +415,12 @@ RSpec/NestedGroups:
   VersionChanged: '1.10'
   StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NestedGroups
 
+RSpec/NoLet:
+  Description: Checks for usage of `let` blocks in specs.
+  Enabled: false
+  AllowSubject: false
+  StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NoLet
+
 RSpec/NotToNot:
   Description: Checks for consistent method usage for negating expectations.
   Enabled: true

lib/rubocop/cop/rspec/no_let.rb

@@ -0,0 +1,85 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Checks for usage of `let` blocks in specs.
+      #
+      # This cop can be configured with the option `AllowSubject` which
+      # will configure the cop to only register offenses on explicit calls to
+      # `let` and not calls to `subject`
+      #
+      # @example
+      #   # bad
+      #   describe MyClass do
+      #     let(:foo) { [] }
+      #     it { expect(foo).to be_empty }
+      #   end
+      #
+      #   describe MyClass do
+      #     subject(:foo) { [] }
+      #     it { expect(foo).to be_empty }
+      #   end
+      #
+      #   # good
+      #   describe MyClass do
+      #     it do
+      #       foo = []
+      #       expect(foo).to be_empty
+      #     end
+      #   end
+      #
+      # @example with AllowSubject configuration
+      #
+      #   # rubocop.yml
+      #   # RSpec/NoLet:
+      #   #   AllowSubject: true
+      #
+      #   # bad
+      #   describe MyClass do
+      #     let(:foo) { [] }
+      #     it { expect(foo).to be_empty }
+      #   end
+      #
+      #   # good
+      #   describe MyClass do
+      #     subject(:foo) { [] }
+      #     it { expect(foo).to be_empty }
+      #   end
+      #
+      #   describe MyClass do
+      #     it do
+      #       foo = []
+      #       expect(foo).to be_empty
+      #     end
+      #   end
+      #
+      class NoLet < Cop
+        MSG = 'Avoid using `%<method>s` ' \
+              '– use a method call or local variable instead.'
+
+        def_node_matcher :let?, <<~PATTERN
+          (send nil? :let ...)
+        PATTERN
+
+        def_node_matcher :subject?, <<~PATTERN
+          (send nil? :subject ...)
+        PATTERN
+
+        def on_send(node)
+          if subject?(node) && !allow_subject?
+            add_offense(node, message: format(MSG, method: 'subject'))
+          elsif let?(node)
+            add_offense(node, message: format(MSG, method: 'let'))
+          end
+        end
+
+        private
+
+        def allow_subject?
+          cop_config['AllowSubject']
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -68,6 +68,7 @@
 require_relative 'rspec/multiple_subjects'
 require_relative 'rspec/named_subject'
 require_relative 'rspec/nested_groups'
+require_relative 'rspec/no_let'
 require_relative 'rspec/not_to_not'
 require_relative 'rspec/overwriting_setup'
 require_relative 'rspec/pending'

manual/cops.md

@@ -67,6 +67,7 @@
 * [RSpec/MultipleSubjects](cops_rspec.md#rspecmultiplesubjects)
 * [RSpec/NamedSubject](cops_rspec.md#rspecnamedsubject)
 * [RSpec/NestedGroups](cops_rspec.md#rspecnestedgroups)
+* [RSpec/NoLet](cops_rspec.md#rspecnolet)
 * [RSpec/NotToNot](cops_rspec.md#rspecnottonot)
 * [RSpec/OverwritingSetup](cops_rspec.md#rspecoverwritingsetup)
 * [RSpec/Pending](cops_rspec.md#rspecpending)

manual/cops_rspec.md

@@ -2322,6 +2322,77 @@ Max | `3` | Integer
 
 * [https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NestedGroups](https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NestedGroups)
 
+## RSpec/NoLet
+
+Enabled by default | Supports autocorrection
+--- | ---
+Disabled | No
+
+Checks for usage of `let` blocks in specs.
+
+This cop can be configured with the option `AllowSubject` which
+will configure the cop to only register offenses on explicit calls to
+`let` and not calls to `subject`
+
+### Examples
+
+```ruby
+# bad
+describe MyClass do
+  let(:foo) { [] }
+  it { expect(foo).to be_empty }
+end
+
+describe MyClass do
+  subject(:foo) { [] }
+  it { expect(foo).to be_empty }
+end
+
+# good
+describe MyClass do
+  it do
+    foo = []
+    expect(foo).to be_empty
+  end
+end
+```
+#### with AllowSubject configuration
+
+```ruby
+# rubocop.yml
+# RSpec/NoLet:
+#   AllowSubject: true
+
+# bad
+describe MyClass do
+  let(:foo) { [] }
+  it { expect(foo).to be_empty }
+end
+
+# good
+describe MyClass do
+  subject(:foo) { [] }
+  it { expect(foo).to be_empty }
+end
+
+describe MyClass do
+  it do
+    foo = []
+    expect(foo).to be_empty
+  end
+end
+```
+
+### Configurable attributes
+
+Name | Default value | Configurable values
+--- | --- | ---
+AllowSubject | `false` | Boolean
+
+### References
+
+* [https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NoLet](https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NoLet)
+
 ## RSpec/NotToNot
 
 Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged

spec/rubocop/cop/rspec/no_let_spec.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::NoLet do
+  subject(:cop) { described_class.new(config) }
+
+  let(:config) { RuboCop::Config.new }
+
+  # TODO: Write test code
+  #
+  # For example
+  it 'flags an offense when using `#let`' do
+    expect_offense(<<~RUBY)
+      let(:foo) { Foo.new }
+      ^^^^^^^^^ Avoid using `let` – use a method call or local variable instead.
+    RUBY
+  end
+
+  it 'flags an offense when using `#subject` without name' do
+    expect_offense(<<~RUBY)
+      subject { Foo.new }
+      ^^^^^^^ Avoid using `subject` – use a method call or local variable instead.
+    RUBY
+  end
+
+  it 'flags an offense when using `#subject` with name' do
+    expect_offense(<<~RUBY)
+      subject(:foo) { Foo.new }
+      ^^^^^^^^^^^^^ Avoid using `subject` – use a method call or local variable instead.
+    RUBY
+  end
+
+  it 'does not flag an offense when using `#before`' do
+    expect_no_offenses(<<~RUBY)
+      before { foo }
+    RUBY
+  end
+
+  context 'when using AllowSubject configuration', :config do
+    subject(:cop) { described_class.new(config) }
+
+    let(:cop_config) do
+      { 'AllowSubject' => true }
+    end
+
+    it 'flags an offense when using `#let`' do
+      expect_offense(<<~RUBY)
+        let(:foo) { Foo.new }
+        ^^^^^^^^^ Avoid using `let` – use a method call or local variable instead.
+      RUBY
+    end
+
+    it 'does not flag an offense when using `#subject` without a name' do
+      expect_no_offenses(<<~RUBY)
+        subject { Foo.new }
+      RUBY
+    end
+
+    it 'does not flag an offense when using `#subject` with a name' do
+      expect_no_offenses(<<~RUBY)
+        subject(:foo) { Foo.new }
+      RUBY
+    end
+  end
+end