Object#constants_hash.

constants_hash.rb

class Object
  def constants_hash
    constants = self.constants
    constant_values = constants.map do |const|
      self.const_get const
    end
    constants.zip(constant_values).to_h
  end
end

require 'mathn'

Math.constants_hash
#=> {:DomainError=>Math::DomainError,
 :PI=>3.141592653589793,
 :E=>2.718281828459045}