handler_for_rescue

# File actionpack/lib/action_controller/rescue.rb, line 218
      def handler_for_rescue(exception)
        # We go from right to left because pairs are pushed onto rescue_handlers
        # as rescue_from declarations are found.
        _, handler = *rescue_handlers.reverse.detect do |klass_name, handler|
          # The purpose of allowing strings in rescue_from is to support the
          # declaration of handler associations for exception classes whose
          # definition is yet unknown.
          #
          # Since this loop needs the constants it would be inconsistent to
          # assume they should exist at this point. An early raised exception
          # could trigger some other handler and the array could include
          # precisely a string whose corresponding constant has not yet been
          # seen. This is why we are tolerant to unknown constants.
          #
          # Note that this tolerance only matters if the exception was given as
          # a string, otherwise a NameError will be raised by the interpreter
          # itself when rescue_from CONSTANT is executed.
          klass = self.class.const_get(klass_name) rescue nil
          klass ||= klass_name.constantize rescue nil
          exception.is_a?(klass) if klass
        end

        case handler
        when Symbol
          method(handler)
        when Proc
          handler.bind(self)
        end
      end