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?

1 files changed, 40 insertions, 0 deletions

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