Suggested cop: time/date for const/prop defaults

[rvanmelle]

We were recently affected by a bug where had stale times/dates for struct defaults --- which is not surprising, just easy to overlook. We ended up writing the cop below which could be generally useful.

I'd be happy to work at creating a PR, but I thought it would be great to get general feedback first. My concerns:

• this cop currently isn't smart enough to know whether the prop or const declaration is within a T::Struct definition, so it is likely prone to false positives; it works in our code base fine but I imagine it might need to be a bit more contextual for general usage
• this is an easy cop for autocorrect, but I'm still learning rubocop 😅

  class StructDefaultTime < Cop
        MSG = "Struct property defaults are evaluated only once. Use a factory for time and date defaults"
        DATE_OR_TIME_METHODS = %i[since from_now after ago until before yesterday tomorrow now today].to_set.freeze
        STRUCT_DECLARATIONS = %i[const prop].to_set.freeze

        def on_send(node)
          struct_default(node) do |_, default_node|
            nested_date_or_time(default_node) do
              add_offense(node)
            end
          end
        end

        private

        def_node_matcher :struct_default, <<~PATTERN
          (send nil? $STRUCT_DECLARATIONS
            (sym _)
            (const nil? _)
            (hash
              (pair
                (sym :default) $(...))))
        PATTERN

        def_node_matcher :date_or_time, <<~PATTERN
          (send _ $DATE_OR_TIME_METHODS)
        PATTERN

        def nested_date_or_time(node, &callback)
          return if node.nil? || node.block_type?

          node.each_child_node do |child|
            nested_date_or_time(child, &callback)
          end

          date_or_time(node, &callback)
        end
      end

The rough test suite looks something like:

 class StructDefaultTimeTest < ActiveSupport::TestCase
        include RubocopTestHelper

        def setup
          @cop = RuboCop::Cop::Bourgeois::StructDefaultTime.new
        end

        test "ok for non date/time props and constants with or without defaults" do
          assert_no_offenses("const :foo, Integer, default: 3")
          assert_no_offenses("prop :bar, String, default: 'foo'")
          assert_no_offenses("const :baz, Time")
        end

        test "ok for factory date and time props and constants" do
          assert_no_offenses("const :baz, Time, factory: -> { Time.zone.now }")
        end

        test "offense when using current or relative date/time methods" do
          assert_offense("const :foo, Date, default: Time.zone.today")
          assert_offense("const :foo, Time, default: Time.zone.now")
          assert_offense("const :foo, Date, default: 3.days.from_now")
          assert_offense("const :foo, Date, default: Time.zone.now.yesterday")
          assert_offense("const :foo, Time, default: 10.minutes.ago")
          assert_offense("const :foo, Date, default: 5.days.before(Date.today)")

          assert_offense("prop :foo, Date, default: Time.zone.today")
          assert_offense("prop :foo, Time, default: Time.zone.now")
          assert_offense("prop :foo, Date, default: 3.days.from_now")
          assert_offense("prop :foo, Date, default: Time.zone.now.yesterday")
          assert_offense("prop :foo, Time, default: 10.minutes.ago")
          assert_offense("prop :foo, Date, default: 5.days.before(Date.today)")
        end
      end

[KaanOzkan]

Thank you 🙂 . This is my first time hearing about factory but the cop suggestion seems reasonable.

• Feel free to take a look at this cop to help alleviate your first concern and make the cop smarter:

  rubocop-sorbet/lib/rubocop/cop/sorbet/forbid_untyped_struct_props.rb

  Line 49 in 2fce9e3

  return unless subclass_of_t_struct?(node)

• It'd be great to have an autocorrect. We also have quite a few autocorrect examples in the repo. This is a simpler one you can take a look at:

  rubocop-sorbet/lib/rubocop/cop/sorbet/sigils/valid_sigil.rb

  Line 33 in ee5c723

  def autocorrect(_node)

  If autocorrecting is still an issue we can also do a pairing session.