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Feature #17881

Updated by byroot (Jean Boussier) 4 months ago

### 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 has performance implications (albeit relatively minor), and [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. MJIT](https://k0kubun.medium.com/ruby-3-jit-can-make-rails-faster-756310f235a). 

 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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