Implements the task zeitwerk:check

[Matilda Smeds & Xavier Noria]

3 files changed, 90 insertions, 0 deletions

diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index e5f9e37c33..84974d8f03 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -1,3 +1,14 @@
+*   Applications upgrading to Rails 6 can run the command
+
+    ```
+    bin/rails zeitwerk:check
+    ```
+
+    to check if the project structure they were using with the classic
+    autoloader is compatible with `:zeitwerk` mode.
+
+    *Matilda Smeds* & *Xavier Noria*
+
 *   Allow loading seeds without ActiveJob.
 
     Fixes #35782
diff --git a/railties/lib/rails/tasks.rb b/railties/lib/rails/tasks.rb
index 2f644a20c9..0e592e15c3 100644
--- a/railties/lib/rails/tasks.rb
+++ b/railties/lib/rails/tasks.rb
@@ -15,6 +15,7 @@ require "rake"
   routes
   tmp
   yarn
+  zeitwerk
 ).tap { |arr|
   arr << "statistics" if Rake.application.current_scope.empty?
 }.each do |task|
diff --git a/railties/lib/rails/tasks/zeitwerk.rake b/railties/lib/rails/tasks/zeitwerk.rake
new file mode 100644
index 0000000000..cf84bb6948
--- /dev/null
+++ b/railties/lib/rails/tasks/zeitwerk.rake
@@ -0,0 +1,78 @@
+# frozen_string_literal: true
+
+ensure_classic_mode = ->() do
+  if Rails.autoloaders.zeitwerk_enabled?
+    abort <<~EOS
+      Please, enable temporarily :classic mode:
+
+        # config/application.rb
+        config.autoloader = :classic
+
+      and try again. When all is good, you can delete that line.
+    EOS
+  end
+end
+
+eager_load = ->() do
+  Rails.configuration.eager_load_namespaces.each(&:eager_load!)
+end
+
+mismatches = []
+check_directory = ->(directory, parent) do
+  # test/mailers/previews might not exist.
+  return unless File.exists?(directory)
+
+  Dir.foreach(directory) do |entry|
+    next if entry.start_with?(".")
+    next if parent == Object && entry == "concerns"
+
+    abspath = File.join(directory, entry)
+
+    if File.directory?(abspath) || abspath.end_with?(".rb")
+      print "."
+      cname = File.basename(abspath, ".rb").camelize.to_sym
+      if parent.const_defined?(cname, false)
+        if File.directory?(abspath)
+          check_directory[abspath, parent.const_get(cname)]
+        end
+      else
+        mismatches << [abspath, parent, cname]
+      end
+    end
+  end
+end
+
+report = ->() do
+  puts
+  if mismatches.empty?
+    puts "All is good!"
+    puts "Please, remember to delete `config.autoloader = :classic` from config/application.rb."
+  else
+    mismatches.each do |abspath, parent, cname|
+      relpath = abspath.sub(%r{\A#{Regexp.escape(Rails.root.to_path)}/}, "")
+      cpath = parent == Object ? cname : "#{parent.name}::#{cname}"
+      puts "expected #{relpath} to define #{cpath}"
+    end
+    puts
+    puts <<~EOS
+      Please revise the reported mismatches. You can normally fix them by adding
+      acronyms to config/initializers/inflections.rb or renaming the constants.
+    EOS
+  end
+end
+
+namespace :zeitwerk do
+  desc "Checks project structure for Zeitwerk compatibility"
+  task check: :environment do
+    ensure_classic_mode[]
+    eager_load[]
+
+    $stdout.sync = true
+    ActiveSupport::Dependencies.autoload_paths.each do |autoload_path|
+      check_directory[autoload_path, Object]
+    end
+    puts
+
+    report[]
+  end
+end