Add RuboCop for `assert_not` over `assert !`

We added `assert_not` in f75addd "to replace warty 'assert !foo'".
fa8d35b agrees that it is warty, and so do I. This custom Rubocop rule
turns the wart into a violation.

As with my last custom cop, https://github.com/rails/rails/pull/32441,
I want to make sure this looks right on code climate before pushing
another commit to autocorrect everything.

@toshimaru I just noticed
https://github.com/toshimaru/rubocop-rails/pull/26
Is there a better way to add these custom cops, or were you saying we
shouldn't have custom cops at all?

5 files changed, 96 insertions, 10 deletions

diff --git a/ci/custom_cops/lib/custom_cops.rb b/ci/custom_cops/lib/custom_cops.rb
index d5d17f8856..157b8247e4 100644
--- a/ci/custom_cops/lib/custom_cops.rb
+++ b/ci/custom_cops/lib/custom_cops.rb
@@ -1,3 +1,4 @@
 # frozen_string_literal: true
 
 require_relative "custom_cops/refute_not"
+require_relative "custom_cops/assert_not"
diff --git a/ci/custom_cops/lib/custom_cops/assert_not.rb b/ci/custom_cops/lib/custom_cops/assert_not.rb
new file mode 100644
index 0000000000..e722448e21
--- /dev/null
+++ b/ci/custom_cops/lib/custom_cops/assert_not.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+module CustomCops
+  # Enforces the use of `assert_not` over `assert !`.
+  #
+  # @example
+  #   # bad
+  #   assert !x
+  #   assert ! x
+  #
+  #   # good
+  #   assert_not x
+  #
+  class AssertNot < RuboCop::Cop::Cop
+    MSG = "Prefer `assert_not` over `assert !`"
+
+    def_node_matcher :offensive?, "(send nil? :assert (send ... :!))"
+
+    def on_send(node)
+      add_offense(node) if offensive?(node)
+    end
+
+    def autocorrect(node)
+      expression = node.loc.expression
+
+      ->(corrector) do
+        corrector.replace(
+          expression,
+          corrected_source(expression.source)
+        )
+      end
+    end
+
+    private
+
+      def corrected_source(source)
+        source.gsub(/^assert(\(| ) *! */, "assert_not\\1")
+      end
+  end
+end
diff --git a/ci/custom_cops/test/custom_cops/assert_not_test.rb b/ci/custom_cops/test/custom_cops/assert_not_test.rb
new file mode 100644
index 0000000000..abb151aeb4
--- /dev/null
+++ b/ci/custom_cops/test/custom_cops/assert_not_test.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require "support/cop_helper"
+require_relative "../../lib/custom_cops/assert_not"
+
+class AssertNotTest < ActiveSupport::TestCase
+  include CopHelper
+
+  setup do
+    @cop = CustomCops::AssertNot.new
+  end
+
+  test "rejects 'assert !'" do
+    inspect_source @cop, "assert !x"
+    assert_offense @cop, "^^^^^^^^^ Prefer `assert_not` over `assert !`"
+  end
+
+  test "rejects 'assert !' with a complex value" do
+    inspect_source @cop, "assert !a.b(c)"
+    assert_offense @cop, "^^^^^^^^^^^^^^ Prefer `assert_not` over `assert !`"
+  end
+
+  test "autocorrects `assert !`" do
+    corrected = autocorrect_source(@cop, "assert !false")
+    assert_equal "assert_not false", corrected
+  end
+
+  test "autocorrects `assert !` with extra spaces" do
+    corrected = autocorrect_source(@cop, "assert   !  false")
+    assert_equal "assert_not false", corrected
+  end
+
+  test "autocorrects `assert !` with parentheses" do
+    corrected = autocorrect_source(@cop, "assert(!false)")
+    assert_equal "assert_not(false)", corrected
+  end
+
+  test "accepts `assert_not`" do
+    inspect_source @cop, "assert_not x"
+    assert_empty @cop.offenses
+  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
index 5dbd8bf32a..f0f6eaeda0 100644
--- a/ci/custom_cops/test/custom_cops/refute_not_test.rb
+++ b/ci/custom_cops/test/custom_cops/refute_not_test.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require "support/cop_helper"
-require "./lib/custom_cops/refute_not"
+require_relative "../../lib/custom_cops/refute_not"
 
 class RefuteNotTest < ActiveSupport::TestCase
   include CopHelper
@@ -59,15 +59,6 @@ class RefuteNotTest < ActiveSupport::TestCase
 
   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}`"
diff --git a/ci/custom_cops/test/support/cop_helper.rb b/ci/custom_cops/test/support/cop_helper.rb
index d259154df5..c2c6b969dd 100644
--- a/ci/custom_cops/test/support/cop_helper.rb
+++ b/ci/custom_cops/test/support/cop_helper.rb
@@ -16,6 +16,18 @@ module CopHelper
     rewrite(cop, processed_source)
   end
 
+  def assert_offense(cop, expected_message)
+    assert_not_empty(
+      cop.offenses,
+      "Expected offense with message \"#{expected_message}\", but got no offense"
+    )
+
+    offense = cop.offenses.first
+    carets = "^" * offense.column_length
+
+    assert_equal expected_message, "#{carets} #{offense.message}"
+  end
+
   private
     TARGET_RUBY_VERSION = 2.4