[ruby-core:105697] [Ruby master Feature#18035] Introduce general model/semantic for immutable by default.

Issue #18035 has been updated by Eregon (Benoit Daloze).


Some notes from the discussion on Slack:

In general shareable (from Ractor.make_shareable) != immutable, notably for special shareable objects like Ractor, and potentially Ractor::Array (which would share any element) or so. So the deep_frozen/immutable flag should be something different.
For the special case of Module/Class I believe nobody wants them frozen for obj.deep_freeze or so, so that's a case of shareable and "deep freezing/immutable" agree, but not for other special-shareable objects.
Immutable does imply shareable, but not the other way around.

Re half-frozen if some override of `freeze` raises an exception, that's fine, the bug should be fixed in the bad freeze, it was already discussed extensively when adding Ractor.make_shareable.
The exception will prevent `freeze/deep_freeze/Immutable#new` to return which is good enough to indicate the problem.

Implementation-wise, I guess we can reuse quite a bit from `Ractor.make_shareable`, but it needs to be a different flag, and it needs to freeze more than `Ractor.make_shareable` (absolutely everything except Module/Class).

----------------------------------------
Feature #18035: Introduce general model/semantic for immutable by default.
https://bugs.ruby-lang.org/issues/18035#change-94196

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
It would be good to establish some rules around mutability, immutability, frozen, and deep frozen in Ruby.

I see time and time again, incorrect assumptions about how this works in production code. Constants that aren't really constant, people using `#freeze` incorrectly, etc.

I don't have any particular preference but:

- We should establish consistent patterns where possible, e.g.
  - Objects created by `new` are mutable.
  - Objects created by literal are immutable.

We have problems with how `freeze` works on composite data types, e.g. `Hash#freeze` does not impact children keys/values, same for Array. Do we need to introduce `freeze(true)` or `#deep_freeze` or some other method?

Because of this, frozen does not necessarily correspond to immutable. This is an issue which causes real world problems.

I also propose to codify this where possible, in terms of "this class of object is immutable" should be enforced by the language/runtime, e.g.


```ruby
module Immutable
  def new(...)
    super.freeze
  end
end

class MyImmutableObject
  extend Immutable

  def initialize(x)
    @x = x
  end
  
  def freeze
    return self if frozen?
    
    @x.freeze
    
    super
  end
end

o = MyImmutableObject.new([1, 2, 3])
puts o.frozen?
```

Finally, this area has an impact to thread and fiber safe programming, so it is becoming more relevant and I believe that the current approach which is rather adhoc is insufficient.

I know that it's non-trivial to retrofit existing code, but maybe it can be done via magic comment, etc, which we already did for frozen string literals.



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