Make your Rails code notify you about those things you wanted to fix when you upgrade Rails.

EXAMPLE.rb

# This middleware is probably no longer needed in Rails 4.
RailsVersions.revisit_this_on_rails_4

rails_versions.rb

module RailsVersions
  def self.revisit_this_on_rails_4
    # Our unit tests may fake out Rails, so we guard against that.
    return unless defined?(Rails::VERSION)

    return if Rails::VERSION::MAJOR < 4
    return if Rails.env.production?

    raise "This code should be revisited on Rails 4!"
  end
end

rails_versions_spec.rb

require "spec_helper"
require "rails_versions"

describe RailsVersions, ".revisit_this_on_rails_4" do
  it "raises a helpful message on Rails >= 4" do
    stub_const "Rails::VERSION::MAJOR", 4

    expect {
      RailsVersions.revisit_this_on_rails_4
    }.to raise_error "This code should be revisited on Rails 4!"
  end

  it "does not raise on Rails < 4" do
    stub_const "Rails::VERSION::MAJOR", 3

    expect {
      RailsVersions.revisit_this_on_rails_4
    }.not_to raise_error
  end

  it "does not raise in production" do
    stub_const "Rails::VERSION::MAJOR", 4
    allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new("production"))

    expect {
      RailsVersions.revisit_this_on_rails_4
    }.not_to raise_error
  end

  it "does not explode if the Rails::VERSION constant is not defined" do
    hide_const "Rails::VERSION"

    expect {
      RailsVersions.revisit_this_on_rails_4
    }.not_to raise_error
  end
end