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


> If the TP is not needed ever, it is not even enabled. If at some point you needed it, but no longer, Zeitwerk disables it.

True, but what worries me would be gems using Zeitwerk and not fully eagerloading, there's even an explicit API for doing just that highligthed in the readme: https://github.com/fxn/zeitwerk#eager-loading

```ruby
db_adapters = "#{__dir__}/my_gem/db_adapters"
loader.do_not_eager_load(db_adapters)
loader.setup
loader.eager_load # won't eager load the database adapters
```

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.

On the other hand, assigning constants is extremely rare past the boot phase, so I'm much less worried about it.

But again all this is hard to quantify and a bit "handwavy". I believe the cleanliness of the API alone should be the main argument, as well as not interfering with other TracePoint usages.

----------------------------------------
Feature #17881: Add a Module#const_added callback
https://bugs.ruby-lang.org/issues/17881#change-92117

* 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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