hash_filter

# File actionpack/lib/action_controller/metal/strong_parameters.rb, line 764
      def hash_filter(params, filter)
        filter = filter.with_indifferent_access

        # Slicing filters out non-declared keys.
        slice(*filter.keys).each do |key, value|
          next unless value
          next unless has_key? key

          if filter[key] == EMPTY_ARRAY
            # Declaration { comment_ids: [] }.
            array_of_permitted_scalars?(self[key]) do |val|
              params[key] = val
            end
          elsif non_scalar?(value)
            # Declaration { user: :name } or { user: [:name, :age, { address: ... }] }.
            params[key] = each_element(value) do |element|
              element.permit(*Array.wrap(filter[key]))
            end
          end
        end
      end