[ruby-core:104868] [Ruby master Feature#18035] Introduce general model/semantic for immutable by default.

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


Many things discussed in the description here.

I think it's important to differentiate shallow frozen (`Kernel#frozen?`) and deep frozen (= immutable), and not try to change their meaning.
So for example overriding freeze to deep freeze does not seem good.

There was a suggestion for `deep_freeze` in #17145, which IMHO would be a good addition.

> Objects created by literal are immutable.

I don't agree, for instance `[]` and `{}` should not be frozen, that would just be counter-productive in many cases.

Maybe `CONSTANT = value` should `.deep_freeze` the value, this was discussed with `Ractor.make_shareable` but that was rejected (#17273).

There is also the question of how to mark a class as creating immutable objects.
And potentially still allow to subclass it, and what it should do with initialize_copy, allocate, etc.
That's illustrated with the `Immutable` above but otherwise not much discussed.
I think that's probably worth its own ticket, because it's a big enough subject of its own, I'll try to make one.

> copy-on-write for Array

That's required for efficient Array#shift so you can assume it's there on all major Ruby implementations.

----------------------------------------
Feature #18035: Introduce general model/semantic for immutable by default.
https://bugs.ruby-lang.org/issues/18035#change-93224

* 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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