Redmine

+1 Thank you!

Pavel, do you have any working patch for this?

sry, I don't. This is a pretty heavy change and Redmine has a lot of technical debt in this area. But I have a few hints based on my findings:

Zeitwerk depends on Rails conventions, especially the folder structure.
1/ initializers
/master/config/initializers/10-patches.rb
depends on

include Redmine::I18n

2/ there're multiple module definitions in a single file (many places), that don't match the filename and the expected folder structure.
3/ Ruby / Rails patches are global from the pre Ruby 2.0 era. They're patched non-deterministically in some random file

module ActiveRecord
  class Base
    def self.human_attribute_name(attr, options = {})
      ...
    end
  end
end

or a require chain like this
/master/lib/redmine/core_ext/string.rb

this issue can be avoided by using manual require at startup time, but you may consider using refinements instead
https://www.cloudbees.com/blog/ruby-refinements/

4/ require_dependency should be removed
/master/app/models/principal.rb#L221

here's a Rails guide and there's also an example of how to deal with STI
https://edgeguides.rubyonrails.org/autoloading_and_reloading_constants.html

5/ autoloads based on #constantize
/master/lib/redmine/wiki_formatting.rb#L42

6/ a folder structure violations, a few examples
/master/lib/diff.rb#L3

module RedmineDiff
  class Diff
  end
end

class Diff
end

module Redmine
  class Diff
  end
end

7/ module names should use camel-case, upper-case is reserved for constants
/master/lib/redmine/helpers/url.rb#L24
/master/lib/redmine/version.rb#L7

I think now it's pretty clear this is an extremely breaking change. Redmine is a part 1, but even if we fix all these issues, it's impossible to keep compatibility for plugins, but it's well, unavoidable...

Zeitwerk mode can be enabled by putting

config.load_defaults 6.1

and tested by

rails zeitwerk:check

Zeitwerk mode will become mandatory in Rails 7.

I think that decisions like module names and folder structure should be discussed with your core team. It's hard to accept a small PR without a confirmation it fixes anything and on the other hand, a big PR might include changes you won't be eventually willing to accept. I would recommend you to investigate these problems yourself, split them into smaller tasks and comment them with your expectations. The community can help you solve it this way, but this task is too big with many open questions.

I had to remove some references due to your spam protection :)

I haven't tried it, it seems that problem 7 can be avoided by writing config/initializers/inflations.rb.
https://github.com/fxn/zeitwerk#inflection