error_matcher.rb

module Rescuer
  class << self
    attr_accessor :error_class

    def error_matcher
      error_class = self.error_class

      Module.new do
        define_singleton_method :=== do |e|
          error_class === e
        end
      end
    end
  end
end

class ErrorSetBeforeDefinition < StandardError; end
class ErrorSetAfterDefinition < StandardError; end

Rescuer.error_class = ErrorSetBeforeDefinition

def try_rescue(error_class)
  raise error_class
rescue Rescuer.error_matcher => e
end

try_rescue ErrorSetBeforeDefinition #=> Nothing was raised

Rescuer.error_class = ErrorSetAfterDefinition

try_rescue ErrorSetAfterDefinition #=> Nothing was raised