Issue #17881 has been updated by fxn (Xavier Noria).


> If I'm not mistaken, such usage could cause the TracePoint to stay active at runtime, which isn't a huge deal, but will still trigger every time a singleton_class or anonymous class is created, which isn't a good practice at runtime, but that I nonetheless see happen.

For anonymous classes, in the (rare?) eventual case that the tracer remains enabled [we return immediately](https://github.com/fxn/zeitwerk/blob/e576c60d3e9f4d60f9a630e0aac3d248d6e776c5/lib/zeitwerk/explicit_namespace.rb#L63). The cost of that has to be absolutely negligible in practice.

Regarding anonymous classes and modules, remember TP does NOT trigger `:class` on `Class.new` or `Module.new`, which is perfect for our use case, since Zeitwerk is only interested in classes/modules with names.

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Feature #17881: Add a Module#const_added callback
https://bugs.ruby-lang.org/issues/17881#change-92133

* Author: byroot (Jean Boussier)
* Status: Open
* 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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