From: "byroot (Jean Boussier)" <noreply@...>
Date: 2021-12-14T11:42:22+00:00
Subject: [ruby-core:106654] [Ruby master Feature#17881] Add a Module#const_added callback

Issue #17881 has been updated by byroot (Jean Boussier).


> So, I think we should not add this until we have a good use case.

Yeah, this is no longer for Zeitwerk, since TracePoint reentrancy was made possible by @ko1.

We still want it though for https://github.com/Shopify/tapioca, cc @ufuk

From our conversation:

> Yeah, all consts would be very handy for my use case indeed. I can already get class/module definitions/reopens with TracePoint, but cannot catch other constant definitions. We end up having to query const_source_location for those.

The `super` issue is totally acceptable, that's already a common issue with `inherited` and such, and it's very easily caught by static checkers.

----------------------------------------
Feature #17881: Add a Module#const_added callback
https://bugs.ruby-lang.org/issues/17881#change-95326

* Author: byroot (Jean Boussier)
* Status: Feedback
* Priority: Normal
----------------------------------------
### Use case

Autoloaders like `zeitwerk` need a callback when a new class or module is registered in the constant table.

Currently this is implemented with TracePoint's `:class` event. It works, but it is a bit unfortunate to have to use an API intended for debugging to implement production features. It doesn't feel "conceptually clean". 

It also [doesn't play well with MJIT](https://k0kubun.medium.com/ruby-3-jit-can-make-rails-faster-756310f235a), even though it's more of an MJIT limitation.

Additionally this usage of TracePoint cause [some incompatibilities with some debuggers like `byebug`](https://github.com/deivid-rodriguez/byebug/issues/564) (even though others don't have this issue). 

### Proposal

I believe that if Ruby was to call `Module#const_added` when a constant is registered, Zeitwerk could get rid of TracePoint.

For now I implemented it as: `const_added(const_name)` for similarity with `method_added`. But maybe it could make sense to have the signature be `const_added(const_name, const_value)`.

Also since `method_removed` exists, maybe `const_removed` would need to be added for consistency.

### Links

Patch: https://github.com/ruby/ruby/pull/4521
Zeitwerk side discussion: https://github.com/fxn/zeitwerk/issues/135

cc @k0kubun



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