Enable Zeitwerk by default on TruffleRuby

[eregon]

Summary

Since the Zeitwerk test suite now passes on TruffleRuby, I think it is time to enable it by default when running Rails on TruffleRuby.

I generated a new empty Rails app with this change and it worked fine.

In general, I'd like to have as few differences as possible between running Rails on TruffleRuby and running Rails on CRuby.
So that way, it would be possible to switch between both at any time without changing anything (notably, no need to change the Gemfile, configuration, etc), and have minimal behavior changes too.

cc @fxn

[fxn]

Fantastic! 🎉

[fxn]

Would that need a release I guess?

[eregon]

@fxn A release of TruffleRuby or a release of Rails?

Re TruffleRuby the next release is planned on Nov 17. I don't think we need to worry if people use the 19.2 release instead, the basics of Zeitwerk should still work fine, and for Rails we'd still recommend to use truffleruby-head for now as there are lots of performance changes in master recently that matter for Rails.

Re Rails releases I think we can just wait for the next release :)

[fxn]

Question basically is: This patch enables the mode for any release of TruffleRuby. So, people upgrading to 6.1 applications running on a stable release of TruffleRuby today will get the mode silently switched. People creating new 6.1 apps with a previous version of TruffleRuby will have the mode enabled in a version that does not pass the suite.

However, I guess TruffleRuby users are vigilant enough that I was thinking about adding a CHANGELOG entry like:

zeitwerk mode is enabled by default with TruffleRuby. Please make sure you are running at least TruffleRuby such and such.

On the assumption that in practice nobody would be running on a previous version. But do not know what to say instead of "such and such".

Alternatively, we could revert this patch and document that you can enable zeitwerk mode yourself if running truffleruby-head.

[eregon]

I think the CHANGELOG entry makes sense, I'll make a PR.

[eregon]

#40144

[eregon]

We could also check the TruffleRuby version if you want, like here.
But it makes it harder for the user to know which autoloader is used.

IMHO almost every behavior difference between running on TruffleRuby and CRuby is essentially a bug. So here we are fixing that behavior difference, and previous Rails versions simply had the unexpected difference.

[fxn]

We could also check the TruffleRuby version if you want, like here.
But it makes it harder for the user to know which autoloader is used.

Yeah.

On a second thought, perhaps it is too early to enable this. Maybe a better first step would be to document that you can enable zeitwerk mode if running truffleruby-head, and wait just a bit until everything is more mature.

[eregon]

On a second thought, perhaps it is too early to enable this. Maybe a better first step would be to document that you can enable zeitwerk mode if running truffleruby-head, and wait just a bit until everything is more mature.

I don't like that, because it makes a significant difference between running on CRuby and on TruffleRuby, and Rails 6 users typically can assume Zeitwerk is used and don't manually set the autoloader. Also very few people would actually find that in the docs.

I can make a PR with the version check if you want to make sure nothing changes when running on older TruffleRuby.

[fxn]

I don't like that, because it makes a significant difference between running on CRuby and on TruffleRuby, and Rails 6 users typically can assume Zeitwerk is used and don't manually set the autoloader. Also very few people would actually find that in the docs.

Well, the fact that Zeitwerk works only in CRuby is well documented in the upgrade notes and the autoloading guide. It does not need a version check because Zeitwerk supports the Ruby version required by Rails 6.

Therefore, if you run Rails 6 on JRuby and think you're running in zeitwerk mode, that is not something we can do much about. And what we cannot certainly do is to let you run on zeitwerk mode by default on an interpreter that does not support it. On the other hand, you have the means to override the default.

I merged assuming a release was about to be out and we could document or something, but if this is only to be valid on TruffleRuby's main branch for a while, I believe I need to ponder this again. Let me think.

[eregon]

Let's check the TruffleRuby version and be on the safe side then: #40145

[fxn]

Re TruffleRuby the next release is planned on Nov 17. I don't think we need to worry if people use the 19.2 release instead, the basics of Zeitwerk should still work fine, and for Rails we'd still recommend to use truffleruby-head for now as there are lots of performance changes in master recently that matter for Rails.

Based on this comment, and after thinking about this for a while, I believe I prefer to leave the check as is. In normal circumstances, one should play by the book and control things precisely. But, due to the nature of the project and usage patterns, I think we can be more optimistic with TruffleRuby and keep it simple.

The only tweak I've done is moving the check to the 6.1 defaults.