Add custom RuboCop for `assert_not` over `refute`

Since at least cf4afc4 we have preferred `assert_not` methods over
`refute` methods. I have seen plenty of comments in PRs about this,
and we have tried to fix it a few times (5294ad8, e45f176, 8910f12,
41f50be, d4cfd54, 48a183e, and 211adb4), but the `refute` methods
keep sneaking back in.

This custom RuboCop will take care of enforcing this preference, so we
don't have to think about it again. I suspect there are other similar
stylistic preferences that could be solved with some custom RuboCops, so
I will definitely keep my eyes open. `assert_not` over `assert !` might
be a good candidate, for example.

I wasn't totally sure if `ci/custom_cops` was the best place to put
this, but nothing else seemed quite right. At one point I had it set up
as a gem, but I think custom cops like this would have limited value
in another context.

I want to see how code climate handles the new cops before
autocorrecting the existing violations. If things go as expected, I will
push another commit with those corrections.

5 files changed, 189 insertions, 0 deletions

diff --git a/ci/custom_cops/bin/test b/ci/custom_cops/bin/test
new file mode 100755
index 0000000000..495ffec83a
--- /dev/null
+++ b/ci/custom_cops/bin/test
@@ -0,0 +1,5 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+COMPONENT_ROOT = File.expand_path("..", __dir__)
+require_relative "../../../tools/test"
diff --git a/ci/custom_cops/lib/custom_cops.rb b/ci/custom_cops/lib/custom_cops.rb
new file mode 100644
index 0000000000..d5d17f8856
--- /dev/null
+++ b/ci/custom_cops/lib/custom_cops.rb
@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+require_relative "custom_cops/refute_not"
diff --git a/ci/custom_cops/lib/custom_cops/refute_not.rb b/ci/custom_cops/lib/custom_cops/refute_not.rb
new file mode 100644
index 0000000000..3e89e0fd32
--- /dev/null
+++ b/ci/custom_cops/lib/custom_cops/refute_not.rb
@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+module CustomCops
+  # Enforces the use of `#assert_not` methods over `#refute` methods.
+  #
+  # @example
+  #   # bad
+  #   refute false
+  #   refute_empty [1, 2, 3]
+  #   refute_equal true, false
+  #
+  #   # good
+  #   assert_not false
+  #   assert_not_empty [1, 2, 3]
+  #   assert_not_equal true, false
+  #
+  class RefuteNot < RuboCop::Cop::Cop
+    MSG = "Prefer `%<assert_method>s` over `%<refute_method>s`"
+
+    CORRECTIONS = {
+      refute:             "assert_not",
+      refute_empty:       "assert_not_empty",
+      refute_equal:       "assert_not_equal",
+      refute_in_delta:    "assert_not_in_delta",
+      refute_in_epsilon:  "assert_not_in_epsilon",
+      refute_includes:    "assert_not_includes",
+      refute_instance_of: "assert_not_instance_of",
+      refute_kind_of:     "assert_not_kind_of",
+      refute_nil:         "assert_not_nil",
+      refute_operator:    "assert_not_operator",
+      refute_predicate:   "assert_not_predicate",
+      refute_respond_to:  "assert_not_respond_to",
+      refute_same:        "assert_not_same",
+      refute_match:       "assert_no_match"
+    }.freeze
+
+    OFFENSIVE_METHODS = CORRECTIONS.keys.freeze
+
+    def_node_matcher :offensive?, "(send nil? #offensive_method? ...)"
+
+    def on_send(node)
+      return unless offensive?(node)
+
+      message = offense_message(node.method_name)
+      add_offense(node, location: :selector, message: message)
+    end
+
+    def autocorrect(node)
+      ->(corrector) do
+        corrector.replace(
+          node.loc.selector,
+          CORRECTIONS[node.method_name]
+        )
+      end
+    end
+
+    private
+
+      def offensive_method?(method_name)
+        OFFENSIVE_METHODS.include?(method_name)
+      end
+
+      def offense_message(method_name)
+        format(
+          MSG,
+          refute_method: method_name,
+          assert_method: CORRECTIONS[method_name]
+        )
+      end
+  end
+end
diff --git a/ci/custom_cops/test/custom_cops/refute_not_test.rb b/ci/custom_cops/test/custom_cops/refute_not_test.rb
new file mode 100644
index 0000000000..5dbd8bf32a
--- /dev/null
+++ b/ci/custom_cops/test/custom_cops/refute_not_test.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require "support/cop_helper"
+require "./lib/custom_cops/refute_not"
+
+class RefuteNotTest < ActiveSupport::TestCase
+  include CopHelper
+
+  setup do
+    @cop = CustomCops::RefuteNot.new
+  end
+
+  {
+    refute:             :assert_not,
+    refute_empty:       :assert_not_empty,
+    refute_equal:       :assert_not_equal,
+    refute_in_delta:    :assert_not_in_delta,
+    refute_in_epsilon:  :assert_not_in_epsilon,
+    refute_includes:    :assert_not_includes,
+    refute_instance_of: :assert_not_instance_of,
+    refute_kind_of:     :assert_not_kind_of,
+    refute_nil:         :assert_not_nil,
+    refute_operator:    :assert_not_operator,
+    refute_predicate:   :assert_not_predicate,
+    refute_respond_to:  :assert_not_respond_to,
+    refute_same:        :assert_not_same,
+    refute_match:       :assert_no_match
+  }.each do |refute_method, assert_method|
+    test "rejects `#{refute_method}` with a single argument" do
+      inspect_source(@cop, "#{refute_method} a")
+      assert_offense @cop, offense_message(refute_method, assert_method)
+    end
+
+    test "rejects `#{refute_method}` with multiple arguments" do
+      inspect_source(@cop, "#{refute_method} a, b, c")
+      assert_offense @cop, offense_message(refute_method, assert_method)
+    end
+
+    test "autocorrects `#{refute_method}` with a single argument" do
+      corrected = autocorrect_source(@cop, "#{refute_method} a")
+      assert_equal "#{assert_method} a", corrected
+    end
+
+    test "autocorrects `#{refute_method}` with multiple arguments" do
+      corrected = autocorrect_source(@cop, "#{refute_method} a, b, c")
+      assert_equal "#{assert_method} a, b, c", corrected
+    end
+
+    test "accepts `#{assert_method}` with a single argument" do
+      inspect_source(@cop, "#{assert_method} a")
+      assert_empty @cop.offenses
+    end
+
+    test "accepts `#{assert_method}` with multiple arguments" do
+      inspect_source(@cop, "#{assert_method} a, b, c")
+      assert_empty @cop.offenses
+    end
+  end
+
+  private
+
+    def assert_offense(cop, expected_message)
+      assert_not_empty cop.offenses
+
+      offense = cop.offenses.first
+      carets = "^" * offense.column_length
+
+      assert_equal expected_message, "#{carets} #{offense.message}"
+    end
+
+    def offense_message(refute_method, assert_method)
+      carets = "^" * refute_method.to_s.length
+      "#{carets} Prefer `#{assert_method}` over `#{refute_method}`"
+    end
+end
diff --git a/ci/custom_cops/test/support/cop_helper.rb b/ci/custom_cops/test/support/cop_helper.rb
new file mode 100644
index 0000000000..d259154df5
--- /dev/null
+++ b/ci/custom_cops/test/support/cop_helper.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "rubocop"
+
+module CopHelper
+  def inspect_source(cop, source)
+    processed_source = parse_source(source)
+    raise "Error parsing example code" unless processed_source.valid_syntax?
+    investigate(cop, processed_source)
+    processed_source
+  end
+
+  def autocorrect_source(cop, source)
+    cop.instance_variable_get(:@options)[:auto_correct] = true
+    processed_source = inspect_source(cop, source)
+    rewrite(cop, processed_source)
+  end
+
+  private
+    TARGET_RUBY_VERSION = 2.4
+
+    def parse_source(source)
+      RuboCop::ProcessedSource.new(source, TARGET_RUBY_VERSION)
+    end
+
+    def rewrite(cop, processed_source)
+      RuboCop::Cop::Corrector.new(processed_source.buffer, cop.corrections)
+        .rewrite
+    end
+
+    def investigate(cop, processed_source)
+      RuboCop::Cop::Commissioner.new([cop], [], raise_error: true)
+        .investigate(processed_source)
+    end
+end