Did you know? Rails is written by people no smarter than you (I know, low bar! 😘). They show up, Google a few things for the common good and submit patches. They even get their name on this here fancy page!
Rails 5's thread_mattr_accessor wouldn't expose a variable set at the class level to instances and vice versa, because it generated a key that wasn't uniform both at the class and instance level. Now it does 😁
Rails' logger and formatter — ActiveSupport::TaggedLogging — had a habit of saying, "tag, you're it!", when figuring out how to glue the pieces together. With new documentation, this rigamarole is easier to suss out.
Spinning assigns and assert_template out into its own gem exposed a circular require in Action View and Action Pack. Good news is, adding gem 'rails-controller-testing' to your Gemfile should be hassle-free after this (though if you're using RSpec it needs a teensy bit more setup).
Rails 5 adds new application wide classes to inherit from, one being ApplicationMailer. New mailers automatically inherit from said class. However, the file for it wasn't being generated if missing. Rest assured, it now is.