But...

1. I do not want to use the rackup CLI.
2. Installing rackup gem conflicts with rack < 3 - and rack 2.x is still required by many other apps.
3. Puma still places its file in rack/handler and not rackup/handler

In Rack 2.x, the rackup CLI was included regardless of whether you wanted it, and now it's optional (in a separate gem). The server and handler code is not strictly part of "Rack the interface" that's why it's now independent. The rackup gem has compatibility files in place for rack/handler so it continues to work.

I understand this isn't exactly the answer you want, so feel free to help shape the future of Rack and Rackup by proposing a better way forward. If it makes sense, I don't see why we wouldn't consider it.

Well, first of all, thanks for the discussion.

Since I am not involved in rack development, and the decisions and discussions that led to this situation, I am not in any position to offer something better. That said, I think there are problems with the current situation.

Even when including rackup in the above example, I am getting a similar error, now from the rackup gem:

➜ ruby server.rb
/home/vagrant/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rackup-0.1.0/lib/ra
ckup/handler.rb:3:in `require': cannot load such file -- rack/handler (LoadError)
        from /home/vagrant/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rackup-0.1.0/lib/rackup/handler.rb:3:in `<top (required)>'
        from /home/vagrant/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rackup-0.1.0/lib/rackup.rb:1:in `require_relative'
        from /home/vagrant/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rackup-0.1.0/lib/rackup.rb:1:in `<top (required)>'

This even happens with just bundling and doing nothing else:

# server.rb
require 'bundler/inline'

gemfile do
  source "https://rubygems.org"

  gem "rack", "= 3.0.0"
  gem "puma", "= 6.0.0"
  gem "rackup"
end

require 'rackup/handler/puma'

and it is only passing the error after installing webrick.
After that, the above file still fails with

cannot load such file -- rackup/handler/puma (LoadError)

replacing it to rack/handler... shows a deprecation error...

It feels like I am chasing a ghost.