Add new RSpec/ExpectInLet cop

Backport the addition of new cop RSpec/ExpectInLet added in #1885 from
the main branch to the 2-x-stable branch.

We want to get as much feedback as possible on new cops before
(eventually) enabling them when v3 is released.

.rubocop.yml

@@ -170,6 +170,8 @@ RSpec/Eq:
   Enabled: true
 RSpec/ExcessiveDocstringSpacing:
   Enabled: true
+RSpec/ExpectInLet:
+  Enabled: true
 RSpec/IdenticalEqualityAssertion:
   Enabled: true
 RSpec/IndexedLet:

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+- Add new `RSpec/ExpectInLet` cop. ([@yasu551])
+
 ## 2.29.2 (2024-05-02)
 
 - Fix beginless and endless range bug for RepeatedIncludeExample cop. ([@hasghari])
@@ -976,6 +978,7 @@ Compatibility release so users can upgrade RuboCop to 0.51.0. No new features.
 [@twalpole]: https://github.com/twalpole
 [@vzvu3k6k]: https://github.com/vzvu3k6k
 [@walf443]: https://github.com/walf443
+[@yasu551]: https://github.com/yasu551
 [@ybiquitous]: https://github.com/ybiquitous
 [@ydah]: https://github.com/ydah
 [@yevhene]: https://github.com/yevhene

config/default.yml

@@ -447,6 +447,12 @@ RSpec/ExpectInHook:
   VersionAdded: '1.16'
   Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExpectInHook
 
+RSpec/ExpectInLet:
+  Description: Do not use `expect` in let.
+  Enabled: pending
+  VersionAdded: "<<next>>"
+  Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExpectInLet
+
 RSpec/ExpectOutput:
   Description: Checks for opportunities to use `expect { ... }.to output`.
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -41,6 +41,7 @@
 * xref:cops_rspec.adoc#rspecexpectactual[RSpec/ExpectActual]
 * xref:cops_rspec.adoc#rspecexpectchange[RSpec/ExpectChange]
 * xref:cops_rspec.adoc#rspecexpectinhook[RSpec/ExpectInHook]
+* xref:cops_rspec.adoc#rspecexpectinlet[RSpec/ExpectInLet]
 * xref:cops_rspec.adoc#rspecexpectoutput[RSpec/ExpectOutput]
 * xref:cops_rspec.adoc#rspecfilepath[RSpec/FilePath]
 * xref:cops_rspec.adoc#rspecfocus[RSpec/Focus]

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -2042,6 +2042,39 @@ end
 
 * https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExpectInHook
 
+== RSpec/ExpectInLet
+
+|===
+| Enabled by default | Safe | Supports autocorrection | Version Added | Version Changed
+
+| Pending
+| Yes
+| No
+| <<next>>
+| -
+|===
+
+Do not use `expect` in let.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+let(:foo) do
+  expect(something).to eq 'foo'
+end
+
+# good
+it do
+  expect(something).to eq 'foo'
+end
+----
+
+=== References
+
+* https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExpectInLet
+
 == RSpec/ExpectOutput
 
 |===

lib/rubocop/cop/rspec/expect_in_let.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Do not use `expect` in let.
+      #
+      # @example
+      #   # bad
+      #   let(:foo) do
+      #     expect(something).to eq 'foo'
+      #   end
+      #
+      #   # good
+      #   it do
+      #     expect(something).to eq 'foo'
+      #   end
+      #
+      class ExpectInLet < Base
+        MSG = 'Do not use `%<expect>s` in let'
+
+        # @!method expectation(node)
+        def_node_search :expectation, '(send nil? #Expectations.all ...)'
+
+        def on_block(node)
+          return unless let?(node)
+          return if node.body.nil?
+
+          expectation(node.body) do |expect|
+            add_offense(expect.loc.selector, message: message(expect))
+          end
+        end
+
+        alias on_numblock on_block
+
+        private
+
+        def message(expect)
+          format(MSG, expect: expect.method_name)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -63,6 +63,7 @@
 require_relative 'rspec/expect_actual'
 require_relative 'rspec/expect_change'
 require_relative 'rspec/expect_in_hook'
+require_relative 'rspec/expect_in_let'
 require_relative 'rspec/expect_output'
 require_relative 'rspec/file_path'
 require_relative 'rspec/focus'

spec/rubocop/cop/rspec/expect_in_let_spec.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::ExpectInLet do
+  it 'adds an offense for `expect` in let' do
+    expect_offense(<<~RUBY)
+      let(:foo) do
+        expect(something).to eq 'foo'
+        ^^^^^^ Do not use `expect` in let
+      end
+    RUBY
+  end
+
+  it 'adds an offense for `is_expected` in let' do
+    expect_offense(<<~RUBY)
+      let(:foo) do
+        is_expected.to eq 'foo'
+        ^^^^^^^^^^^ Do not use `is_expected` in let
+      end
+    RUBY
+  end
+
+  it 'adds an offense for `expect_any_instance_of` in let' do
+    expect_offense(<<~RUBY)
+      let(:foo) do
+        expect_any_instance_of(Something).to receive :foo
+        ^^^^^^^^^^^^^^^^^^^^^^ Do not use `expect_any_instance_of` in let
+      end
+    RUBY
+  end
+
+  it 'adds offenses for multiple expectations in let' do
+    expect_offense(<<~RUBY)
+      let(:foo) do
+        expect(something).to eq 'foo'
+        ^^^^^^ Do not use `expect` in let
+        is_expected.to eq 'foo'
+        ^^^^^^^^^^^ Do not use `is_expected` in let
+      end
+    RUBY
+  end
+
+  it 'accepts an empty let' do
+    expect_no_offenses(<<~RUBY)
+      let(:foo) {}
+    RUBY
+  end
+
+  it 'accepts `expect` in `it`' do
+    expect_no_offenses(<<~RUBY)
+      it do
+        expect(something).to eq 'foo'
+        is_expected.to eq 'foo'
+        expect_any_instance_of(Something).to receive :foo
+      end
+    RUBY
+  end
+end