[ruby-core:109385] [Ruby master Feature#18930] Officially deprecate class variables

Issue #18930 has been updated by austin (Austin Ziegler).


If we’re going to deprecate them, we need something that effectively replaces them—and class instance variables aren’t it. I was experimenting with something along the lines of:

```ruby
class Base
  def self.dependents
    @dependents ||= {}
  end

  def self.inherited(klass)
     return unless klass.name.match(/V(\d+)\z/)
     name = $1.to_i
     dependents[name] = klass
  end
end

class Rand < Base
end

class Timestamp < Base
end

class V1 < Rand
end

class V2 < Timestamp
end

class V3 < Timestamp
end

class V4 < Timestamp
end
```

This results in `Base.dependents` producing `{}`, but `Rand.dependents` producing `{1=>V1}` and `Timestamp.dependents` producing `{2=>V2,3=>V3,4=>V4}`.

Switching to a class variable produces the result expected.

I’ve managed to do this a different way, but automatic collection of all dependents in a hierarchy only at the top level requires a bit of a hacky workaround (`Base.dependents[name] = klass`).

Yes, in this case, I could *probably* switch to a constant (even a private constant), but that also feels wrong.

----------------------------------------
Feature #18930: Officially deprecate class variables
https://bugs.ruby-lang.org/issues/18930#change-98535

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
----------------------------------------
Ruby's class variables are very confusing, and it seem many people agree they should be avoided (#18927).

How about we deprecate them officially?

Concretely:
* Mention in the documentation that class variables are deprecated and should be avoided/should not be used.
* Add a parse-time deprecation warning, now that we only see those with `Warning[:deprecation] = true` it seems reasonable to add.



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