Add cop to disallow delegating predicate methods

This cop looks for delegations to predicate methods with `allow_nil`
option. When delegating with this option enabled, we ends up with true,
false and nil and this does not preserve the boolean nature of predicate
methods.

The preferred implementation is to define a method to handle `nil` and
returns the correct Boolean value instead.

delegate :is_foo?, to: :bar, allow_nil: true

def is_foo?
  return false unless bar
  bar.is_foo?
end

def is_foo?
  !!bar&.is_foo?
end

.rubocop_manual_todo.yml

@@ -2590,3 +2590,19 @@ Performance/OpenStruct:
     - 'lib/gitlab/import_export/after_export_strategies/base_after_export_strategy.rb'
     - 'lib/gitlab/testing/request_inspector_middleware.rb'
     - 'lib/mattermost/session.rb'
+
+# WIP: https://gitlab.com/gitlab-org/gitlab/-/issues/324629
+Gitlab/DelegatePredicateMethods:
+  Exclude:
+    - 'app/models/clusters/cluster.rb'
+    - 'app/models/clusters/platforms/kubernetes.rb'
+    - 'app/models/concerns/ci/metadatable.rb'
+    - 'app/models/concerns/diff_positionable_note.rb'
+    - 'app/models/concerns/resolvable_discussion.rb'
+    - 'app/models/concerns/services/data_fields.rb'
+    - 'app/models/project.rb'
+    - 'ee/app/models/concerns/ee/ci/metadatable.rb'
+    - 'ee/app/models/ee/group.rb'
+    - 'ee/app/models/ee/namespace.rb'
+    - 'ee/app/models/license.rb'
+    - 'lib/gitlab/ci/trace/stream.rb'

rubocop/cop/gitlab/delegate_predicate_methods.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Gitlab
+      # This cop looks for delegations to predicate methods with `allow_nil: true` option.
+      # This construct results in three possible results: true, false and nil.
+      # In other words, it does not preserve the strict Boolean nature of predicate method return value.
+      # This cop suggests creating a method to handle `nil` delegator and ensure only Boolean type is returned.
+      #
+      # @example
+      #   # bad
+      #   delegate :is_foo?, to: :bar, allow_nil: true
+      #
+      #   # good
+      #   def is_foo?
+      #     return false unless bar
+      #     bar.is_foo?
+      #   end
+      #
+      #   def is_foo?
+      #     !!bar&.is_foo?
+      #   end
+      class DelegatePredicateMethods < RuboCop::Cop::Cop
+        MSG = "Using `delegate` with `allow_nil` on the following predicate methods is discouraged: %s."
+        RESTRICT_ON_SEND = %i[delegate].freeze
+        def_node_matcher :predicate_allow_nil_option, <<~PATTERN
+          (send nil? :delegate
+            (sym $_)*
+            (hash <$(pair (sym :allow_nil) true) ...>)
+          )
+        PATTERN
+
+        def on_send(node)
+          predicate_allow_nil_option(node) do |delegated_methods, _options|
+            offensive_methods = delegated_methods.select { |method| method.end_with?('?') }
+            next if offensive_methods.empty?
+
+            add_offense(node, message: format(MSG, offensive_methods.join(', ')))
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/gitlab/delegate_predicate_methods_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require_relative '../../../../rubocop/cop/gitlab/delegate_predicate_methods'
+
+RSpec.describe RuboCop::Cop::Gitlab::DelegatePredicateMethods do
+  subject(:cop) { described_class.new }
+
+  it 'registers offense for single predicate method with allow_nil:true' do
+    expect_offense(<<~SOURCE)
+      delegate :is_foo?, :do_foo, to: :bar, allow_nil: true
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Using `delegate` with `allow_nil` on the following predicate methods is discouraged: is_foo?.
+    SOURCE
+  end
+
+  it 'registers offense for multiple predicate methods with allow_nil:true' do
+    expect_offense(<<~SOURCE)
+      delegate :is_foo?, :is_bar?, to: :bar, allow_nil: true
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Using `delegate` with `allow_nil` on the following predicate methods is discouraged: is_foo?, is_bar?.
+    SOURCE
+  end
+
+  it 'registers no offense for non-predicate method with allow_nil:true' do
+    expect_no_offenses(<<~SOURCE)
+      delegate :do_foo, to: :bar, allow_nil: true
+    SOURCE
+  end
+
+  it 'registers no offense with predicate method with allow_nil:false' do
+    expect_no_offenses(<<~SOURCE)
+      delegate :is_foo?, to: :bar, allow_nil: false
+    SOURCE
+  end
+
+  it 'registers no offense with predicate method without allow_nil' do
+    expect_no_offenses(<<~SOURCE)
+      delegate :is_foo?, to: :bar
+    SOURCE
+  end
+end