Add error raise for deep_symbolize_keys!

lib/bson/document.rb

@@ -321,6 +321,14 @@ def symbolize_keys!
       raise ArgumentError, 'symbolize_keys! is not supported on BSON::Document instances. Please convert the document to hash first (using #to_h), then call #symbolize_keys! on the Hash instance'
     end
 
+    def deep_symbolize_keys!
+      warn <<~WARN
+        [DEPRECATION] `deep_symbolize_keys!` is not supported on BSON::Document instances.
+        Please convert the document to a Hash first (using `#to_h`), then call `#deep_symbolize_keys!` on the Hash.
+        This will raise an error starting with the v6.0.0 release.
+      WARN
+    end
+
     # Override the Hash implementation of to_bson_normalized_value.
     #
     # BSON::Document is already of the correct type and already provides

spec/bson/document_as_spec.rb

@@ -44,4 +44,18 @@
       end
     end
   end
+
+  describe '#deep_symbolize_keys!' do
+    context 'string keys' do
+      let(:doc) do
+        described_class.new('foo' => 'bar')
+      end
+
+      it 'raises ArgumentError' do
+        expect do
+          doc.deep_symbolize_keys!
+        end.to output(/\[DEPRECATION\] `deep_symbolize_keys!` is not supported on BSON::Document instances./).to_stderr
+      end
+    end
+  end
 end