5 files changed, 221 insertions, 4 deletions
diff --git a/guides/source/6_0_release_notes.md b/guides/source/6_0_release_notes.md
index 4eb5296c41..cb3ea7737c 100644
--- a/guides/source/6_0_release_notes.md
+++ b/guides/source/6_0_release_notes.md
@@ -467,7 +467,7 @@ Please refer to the [Changelog][active-record] for detailed changes.
     ([Pull Request](https://github.com/rails/rails/pull/35631))
 
 *   Add `rails db:seed:replant` that truncates tables of each database
-    for ther current environment and loads the seeds.
+    for the current environment and loads the seeds.
     ([Pull Request](https://github.com/rails/rails/pull/34779))
 
 *   Add `reselect` method, which is a short-hand for `unscope(:select).select(fields)`.
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 3863323bd2..3adc956ef9 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -145,7 +145,7 @@ defaults to `:debug` for all environments. The available log levels are: `:debug
 
 * `secret_key_base` is used for specifying a key which allows sessions for the application to be verified against a known secure key to prevent tampering. Applications get a random generated key in test and development environments, other environments should set one in `config/credentials.yml.enc`.
 
-* `config.public_file_server.enabled` configures Rails to serve static files from the public directory. This option defaults to `true`, but in the production environment it is set to `false` because the server software (e.g. NGINX or Apache) used to run the application should serve static files instead. If you are running or testing your app in production mode using WEBrick (it is not recommended to use WEBrick in production) set the option to `true.` Otherwise, you won't be able to use page caching and request for files that exist under the public directory.
+* `config.public_file_server.enabled` configures Rails to serve static files from the public directory. This option defaults to `true`, but in the production environment it is set to `false` because the server software (e.g. NGINX or Apache) used to run the application should serve static files instead. If you are running or testing your app in production mode using WEBrick (it is not recommended to use WEBrick in production) set the option to `true`. Otherwise, you won't be able to use page caching and request for files that exist under the public directory.
 
 * `config.session_store` specifies what class to use to store the session. Possible values are `:cookie_store` which is the default, `:mem_cache_store`, and `:disabled`. The last one tells Rails not to deal with sessions. Defaults to a cookie store with application name as the session key. Custom session stores can also be specified:
 
diff --git a/guides/source/contributing_to_ruby_on_rails.md b/guides/source/contributing_to_ruby_on_rails.md
index f86589bdf1..a6eb9907a0 100644
--- a/guides/source/contributing_to_ruby_on_rails.md
+++ b/guides/source/contributing_to_ruby_on_rails.md
@@ -340,7 +340,7 @@ $ TEST_DIR=generators bundle exec rake test
 You can run the tests for a particular file by using:
 
 ```bash
-$ cd actionpack
+$ cd actionview
 $ bundle exec ruby -w -Itest test/template/form_helper_test.rb
 ```
 
diff --git a/guides/source/testing.md b/guides/source/testing.md
index 18eecf49fa..9540bb2af5 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -781,7 +781,7 @@ This can be helpful for viewing the browser at the point a test failed, or
 to view screenshots later for debugging.
 
 Two methods are provided: `take_screenshot` and `take_failed_screenshot`.
-`take_failed_screenshot` is automatically included in `after_teardown` inside
+`take_failed_screenshot` is automatically included in `before_teardown` inside
 Rails.
 
 The `take_screenshot` helper method can be included anywhere in your tests to
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 7e4152aa51..290aabc6f9 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -133,6 +133,223 @@ Action Cable JavaScript API:
   +    ActionCable.logger.enabled = false
   ```
 
+### Autoloading
+
+The default configuration for Rails 6
+
+```ruby
+# config/application.rb
+
+config.load_defaults "6.0"
+```
+
+enables `zeitwerk` autoloading mode on CRuby. In that mode, autoloading, reloading, and eager loading are managed by [Zeitwerk](https://github.com/fxn/zeitwerk).
+
+#### Public API
+
+In general, applications do not need to use the API of Zeitwerk directly. Rails sets things up according to the existing contract: `config.autoload_paths`, `config.cache_classes`, etc.
+
+While applications should stick to that interface, the actual Zeitwerk loader object can be accessed as
+
+```ruby
+Rails.autoloaders.main
+```
+
+That may be handy if you need to preload STIs or configure a custom inflector, for example.
+
+#### Project Structure
+
+If the application being upgraded autoloads correctly, the project structure should be already mostly compatible.
+
+However, `classic` mode infers file names from missing constant names (`underscore`), whereas `zeitwerk` mode infers constant names from file names (`camelize`). These helpers are not always inverse of each other, in particular if acronyms are involved. For instance, `"FOO".underscore` is `"foo"`, but `"foo".camelize` is `"Foo"`, not `"FOO"`. Compatibility can be checked by setting `classic` mode first temporarily:
+
+```ruby
+# config/application.rb
+
+config.load_defaults "6.0"
+config.autoloader = :classic
+```
+
+and then running
+
+```
+bin/rails zeitwerk:check
+```
+
+When all is good, you can delete `config.autoloader = :classic`.
+
+#### require_dependency
+
+All known use cases of `require_dependency` have been eliminated, you should grep the project and delete them.
+
+In the case of STIs with a hierarchy of more than two levels, you can preload the leaves of the hierarchy in an initializer:
+
+```ruby
+# config/initializers/preload_stis.rb
+
+# By preloading leaves, the entire hierarchy is loaded upwards following
+# the references to superclasses in the class definitions.
+sti_leaves = %w(
+  app/models/leaf1.rb
+  app/models/leaf2.rb
+  app/models/leaf3.rb
+)
+Rails.autoloaders.main.preload(sti_leaves)
+```
+
+#### Qualified names in class and module definitions
+
+You can now robustly use constant paths in class and module definitions:
+
+```ruby
+# Autoloading in this class' body matches Ruby semantics now.
+class Admin::UsersController < ApplicationController
+  # ...
+end
+```
+
+A gotcha to be aware of is that, depending on the order of execution, the classic autoloader could sometimes be able to autoload `Foo::Wadus` in
+
+```ruby
+class Foo::Bar
+  Wadus
+end
+```
+
+That does not match Ruby semantics because `Foo` is not in the nesting, and won't work at all in `zeitwerk` mode. If you find such corner case you can use the qualified name `Foo::Wadus`:
+
+```ruby
+class Foo::Bar
+  Foo::Wadus
+end
+```
+
+or add `Foo` to the nesting:
+
+```ruby
+module Foo
+  class Bar
+    Wadus
+  end
+end
+```
+
+#### Concerns
+
+You can autoload and eager load from a standard structure like
+
+```
+app/models
+app/models/concerns
+```
+
+In that case, `app/models/concerns` is assumed to be a root directory (because it belongs to the autoload paths), and it is ignored as namespace. So, `app/models/concerns/foo.rb` should define `Foo`, not `Concerns::Foo`.
+
+The `Concerns::` namespace worked with the classic autoloader as a side-effect of the implementation, but it was not really an intended behavior. An application using `Concerns::` needs to rename those classes and modules to be able to run in `zeitwerk` mode.
+
+#### Autoloaded Constants and Explicit Namespaces
+
+If a namespace is defined in a file, as `Hotel` is here:
+
+```
+app/models/hotel.rb         # Defines Hotel.
+app/models/hotel/pricing.rb # Defines Hotel::Pricing.
+```
+
+the `Hotel` constant has to be set using the `class` or `module` keywords. For example:
+
+```ruby
+class Hotel
+end
+```
+
+is good.
+
+Alternatives like
+
+```ruby
+Hotel = Class.new
+```
+
+or
+
+```ruby
+Hotel = Struct.new
+```
+
+won't work, child objects like `Hotel::Pricing` won't be found.
+
+This restriction only applies to explicit namespaces. Classes and modules not defining a namespace can be defined using those idioms.
+
+#### Spring and the `test` Environment
+
+Spring reloads the application code if something changes. In the `test` environment you need to enable reloading for that to work:
+
+```ruby
+# config/environments/test.rb
+
+config.cache_classes = false
+```
+
+Otherwise you'll get this error:
+
+```
+reloading is disabled because config.cache_classes is true
+```
+
+#### Bootsnap
+
+Bootsnap should be at least version 1.4.2.
+
+In addition to that, Bootsnap needs to disable the iseq cache due to a bug in the interpreter if running Ruby 2.5. Please make sure to depend on at least Bootsnap 1.4.4 in that case.
+
+#### `config.add_autoload_paths_to_load_path`
+
+The new configuration point
+
+```ruby
+config.add_autoload_paths_to_load_path
+```
+
+is `true` by default for backwards compatibility, but allows you to opt-out from adding the autoload paths to `$LOAD_PATH`.
+
+This makes sense in most applications, since you never should require a file in `app/models`, for example, and Zeitwerk only uses absolute file names internally.
+
+By opting-out you optimize `$LOAD_PATH` lookups (less directories to check), and save Bootsnap work and memory consumption, since it does not need to build an index for these directories.
+
+#### Thread-safety
+
+In classic mode constant autoloading is not thread-safe, though Rails has locks in place for example to make web requests thread-safe when autoloading is enabled, as it is common in `development` mode.
+
+Constant autoloading is thread-safe in `zeitwerk` mode. For example, you can now autoload in multi-threaded scripts executed by the `runner` command.
+
+#### Globs in config.autoload_paths
+
+Beware of configurations like
+
+```ruby
+config.autoload_paths += Dir["#{config.root}/lib/**/"]
+```
+
+Every element of `config.autoload_paths` should represent the top-level namespace (`Object`) and they cannot be nested in consequence (with the exception of `concerns` directories explained above).
+
+To fix this, just remove the wildcards:
+
+```ruby
+config.autoload_paths << "#{config.root}/lib"
+```
+
+#### How to Use the Classic Autoloader in Rails 6
+
+Applications can load Rails 6 defaults and still use the classic autoloader by setting `config.autoloader` this way:
+
+```ruby
+# config/application.rb
+
+config.load_defaults "6.0"
+config.autoloader = :classic
+```
+
 Upgrading from Rails 5.1 to Rails 5.2
 -------------------------------------