[ruby-core:107108] [Ruby master Feature#17881] Add a Module#const_added callback

Issue #17881 has been updated by mame (Yusuke Endoh).

Assignee set to byroot (Jean Boussier)
Status changed from Feedback to Assigned

@byroot Do you merge your pull request to the HEAD? This proposal is already approved by matz.

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Feature #17881: Add a Module#const_added callback
https://bugs.ruby-lang.org/issues/17881#change-95954

* Author: byroot (Jean Boussier)
* Status: Assigned
* Priority: Normal
* Assignee: byroot (Jean Boussier)
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### 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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