...

# Gems used only for assets and not required
# in production environments by default.
group :assets do
  gem 'sass-rails'
  gem 'coffee-rails'
  gem 'uglifier'

  # See https://github.com/sstephenson/execjs#readme for more supported runtimes
  # gem 'therubyracer',:platforms => :ruby
end

现在，根据(仍在进行中)upgrade documentation：

Rails 4.0 removed the assets group from Gemfile. You’d need to remove that line from your Gemfile when upgrading.

果然，使用RC1创建一个新项目会生成一个Gemfile，它包含默认情况下与任何组之外的资产相关的gem：

source 'https://rubygems.org'

# Bundle edge Rails instead: gem 'rails',github: 'rails/rails'
gem 'rails','4.0.0.rc1'

# Use sqlite3 as the database for Active Record
gem 'sqlite3'

# Use SCSS for stylesheets
gem 'sass-rails','~> 4.0.0.rc1'

# Use Uglifier as compressor for JavaScript assets
gem 'uglifier','>= 1.3.0'

# Use CoffeeScript for .js.coffee assets and views
gem 'coffee-rails','~> 4.0.0'

# See https://github.com/sstephenson/execjs#readme for more supported runtimes
# gem 'therubyracer',platforms: :ruby

...

这是否意味着这些gem将默认捆绑在生产构建中？如果是，为什么心的变化？ Rails 4是否朝着动态生成资产的方向发展？