test.rb

require 'contracts'

include Contracts

class ConvertableToHash
  def self.valid?(thing)
    thing.to_h
    true
  rescue TypeError
    false
  end
end

Contract ConvertableToHash => Any
def with_a_hashy_input(input)
  input = input.to_h
  input.delete(:abc)
end

class Thing
  def to_h
    { abc: 123 }
  end
end

puts with_a_hashy_input(Thing.new)