Introduce PredicateMemoization cop and fix offenses

with StrongMemoize

2 files changed, 40 insertions, 0 deletions

diff --git a/rubocop/cop/gitlab/predicate_memoization.rb b/rubocop/cop/gitlab/predicate_memoization.rb
new file mode 100644
index 00000000000..3c25d61d087
--- /dev/null
+++ b/rubocop/cop/gitlab/predicate_memoization.rb
@@ -0,0 +1,39 @@
+module RuboCop
+  module Cop
+    module Gitlab
+      class PredicateMemoization < RuboCop::Cop::Cop
+        MSG = <<~EOL.freeze
+          Avoid using `@value ||= query` inside predicate methods in order to
+          properly memoize `false` or `nil` values.
+          https://docs.gitlab.com/ee/development/utilities.html#strongmemoize
+        EOL
+
+        def on_def(node)
+          return unless predicate_method?(node)
+
+          select_offenses(node).each do |offense|
+            add_offense(offense, location: :expression)
+          end
+        end
+
+        private
+
+        def predicate_method?(node)
+          node.method_name.to_s.end_with?('?')
+        end
+
+        def or_ivar_assignment?(or_assignment)
+          lhs = or_assignment.each_child_node.first
+
+          lhs.ivasgn_type?
+        end
+
+        def select_offenses(node)
+          node.each_descendant(:or_asgn).select do |or_assignment|
+            or_ivar_assignment?(or_assignment)
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 57af87f7fb9..9110237c538 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -1,4 +1,5 @@
 require_relative 'cop/gitlab/module_with_instance_variables'
+require_relative 'cop/gitlab/predicate_memoization'
 require_relative 'cop/include_sidekiq_worker'
 require_relative 'cop/line_break_around_conditional_block'
 require_relative 'cop/migration/add_column'