Autoloading notes in the upgrading guide

Author: fxn

guides/source/upgrading_ruby_on_rails.md

@@ -78,6 +78,58 @@ To allow you to upgrade to new defaults one by one, the update task has created
 Upgrading from Rails 6.1 to Rails 7.0
 -------------------------------------
 
+### Applications need to run in `zeitwerk` mode
+
+Applications still running in `classic` mode have to switch to `zeitwerk` mode. Please check the [upgrading guide for Rails 6.0](https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#autoloading) for details.
+
+### The setter `config.autoloader=` has been deleted
+
+In Rails 7 there is no configuration point to set the autoloading mode, `config.autoloader=` has been deleted. If you had it set to `:zeitwerk` for whatever reason, just remove it.
+
+### `ActiveSupport::Dependencies` private API has been deleted
+
+The private API of `ActiveSupport::Dependencies` has been deleted. That includes methods like `hook!`, `unhook!`, `depend_on`, `require_or_load`, `mechanism`, and many others.
+
+A few of highlights:
+
+* If you used `ActiveSupport::Dependencies.constantize` or ``ActiveSupport::Dependencies.safe_constantize`, just change them to `String#constantize` or `String#safe_constantize`.
+
+  ```ruby
+  ActiveSupport::Dependencies.constantize("User") # NO LONGER POSSIBLE
+  "User".constantize # 👍
+  ```
+
+* Any usage of `ActiveSupport::Dependencies.mechanism`, reader or writer, has to be replaced by accessing `config.cache_classes` accordingly.
+
+* If you want to trace the activity of the autoloader, `ActiveSupport::Dependencies.verbose=` is no longer available, just throw `Rails.autoloaders.log!` in `config/application.rb`.
+
+Auxiliary internal classes or modules are also gone, like like `ActiveSupport::Dependencies::Reference`, `ActiveSupport::Dependencies::Blamable`, and others.
+
+### Autoloading during initialization
+
+Applications that autoloaded reloadable constants during initialization outside of `to_prepare` blocks got those constants unloaded and had this warning issued since Rails 6.0:
+
+```
+DEPRECATION WARNING: Initialization autoloaded the constant ....
+
+Being able to do this is deprecated. Autoloading during initialization is going
+to be an error condition in future versions of Rails.
+
+...
+```
+
+If you still get this warning in the logs, please check the section about autoloading when the application boots in the [autoloading guide](https://guides.rubyonrails.org/v7.0/autoloading_and_reloading_constants.html#autoloading-when-the-application-boots). You'd get a `NameError` in Rails 7 otherwise.
+
+### Ability to configure `config.autoload_once_paths`
+
+`config.autoload_once_paths` can be set in the body of the application class defined in `config/application.rb` or in the configuration for environments in `config/environments/*`.
+
+Similarly, engines can configure that collection in the class body of the engine class or in the configuration for environments.
+
+After that, the collection is frozen, and you can autoload from those paths. In particular, you can autoload from there during initialization. They are managed by the `Rails.autoloaders.once` autoloader, which does not reload, only autoloads/eager loads.
+
+If you configured this setting after the environments configuration has been processed and are getting `FrozenError`, please just move the code.
+
 ### `ActionDispatch::Request#content_type` now returned Content-Type header as it is.
 
 Previously, `ActionDispatch::Request#content_type` returned value does NOT contain charset part.
@@ -173,7 +225,7 @@ FFmpeg v3.4+.
 
 Image transformation will now use libvips instead of ImageMagick. This will reduce
 the time taken to generate variants as well as CPU and memory usage, improving response
-times in apps that rely on active storage to serve their images. 
+times in apps that rely on active storage to serve their images.
 
 The `:mini_magick` option is not being deprecated, so it is fine to keep using it.