nesting.rb

module A
  module B
    puts "module B inside module A nesting: #{Module.nesting}"

    module_function

    def show_nesting
      puts "show_nesting nesting: #{Module.nesting}"
    end

    def yielder
      yield
    end
  end
end

module A::B
  puts "module A::B nesting: #{Module.nesting}"
end

A::B.show_nesting

A::B.yielder do
  puts "A::B.yielder nesting: #{Module.nesting}"
end

include A::B

show_nesting

yielder do
  puts "toplevel.yielder nesting: #{Module.nesting}"
end

output

module B inside module A nesting: [A::B, A]
module A::B nesting: [A::B]
show_nesting nesting: [A::B, A]
A::B.yielder nesting: []
show_nesting nesting: [A::B, A]
toplevel.yielder nesting: []