example.rb

# Vanilla

Thing1.transaction do
  Thing2.transaction do
    Governor.allowing_malarkey do
      Logger.with_context(some: :context) do
        Auditor.with_context(some: [:more, :context]) do
          do_a_thing
        end
      end
    end
  end
end

# Unnested

Nestor
  .(Thing1.method(:transaction))
  .(Thing2.method(:transaction))
  .(Governor.method(:allowing_malarkey))
  .(Logger.method(:with_context), some: :context)
  .(Auditor.method(:with_context), some: [:more, :context]) do
    do_a_thing
  end

unnestify.rb

# Almost definitely doesn't work… Recursion and all that… Haven't tested this one bit. Shame, shame…
class Nestor
  def initialize(nestor, nestee, params)
    @parent = nestor
    @nestee = nestee
    @params = params
  end

  def call(nestee, *params, &block)
    Nestor.new(self, nestee, params).tap do |nestor|
      nestor.run(&block) if block
    end
  end
  
  def run(&block)
    @parent.run do
      @nestee.call(*params) do
        block.call
      end
    end
  end
  
  def self.call(nestee, *params)
    Nestor.new(nil, nestee, params)
  end
end