Deprecate SafeBuffer#clone_empty

This method is unused within the Rails codebase, the last caller was
removed by 479c7cac in 2014 (Rails 4.2.0).

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Deprecate `SafeBuffer#clone_empty`.
+
+    This method has not been used internally since Rails 4.2.0.
+
+    *Mike Dalessio*
+
 *   `MessageEncryptor`, `MessageVerifier`, and `config.active_support.message_serializer`
     now accept `:message_pack` and `:message_pack_allow_marshal` as serializers.
     These serializers require the [`msgpack` gem](https://rubygems.org/gems/msgpack)

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -77,7 +77,10 @@ def initialize_copy(other)
       @html_safe = other.html_safe?
     end
 
-    def clone_empty
+    def clone_empty # :nodoc:
+      ActiveSupport.deprecator.warn <<~EOM
+        ActiveSupport::SafeBuffer#clone_empty is deprecated and will be removed in Rails 7.2.
+      EOM
       self[0, 0]
     end
 

activesupport/test/safe_buffer_test.rb

@@ -196,12 +196,18 @@ def test_titleize
   end
 
   test "clone_empty returns an empty buffer" do
-    assert_equal "", ActiveSupport::SafeBuffer.new("foo").clone_empty
+    assert_deprecated(ActiveSupport.deprecator) do
+      assert_equal "", ActiveSupport::SafeBuffer.new("foo").clone_empty
+    end
   end
 
   test "clone_empty keeps the original dirtiness" do
-    assert_predicate @buffer.clone_empty, :html_safe?
-    assert_not_predicate @buffer.gsub!("", "").clone_empty, :html_safe?
+    assert_deprecated(ActiveSupport.deprecator) do
+      assert_predicate @buffer.clone_empty, :html_safe?
+    end
+    assert_deprecated(ActiveSupport.deprecator) do
+      assert_not_predicate @buffer.gsub!("", "").clone_empty, :html_safe?
+    end
   end
 
   test "Should be safe when sliced if original value was safe" do