foo.rb

module Foo
  module_function def foo
    :foo
  end
end

class Bar
  include Foo
end

Bar.new.foo
#!> NoMethodError: private method `foo' called for #<Bar:0x00007fe909ae8f40>

class Baz
  include Foo
  public :foo
end

Baz.new.foo
#=> :foo