[ruby-core:116199] [Ruby master Feature#18035] Introduce general model/semantic for immutability.

Issue #18035 has been updated by ioquatix (Samuel Williams).


> If this is accepted, should Data classes all be immutable? To quote the docs:

By default, it does not look like Data is considered immutable as nested values can mutate.

However, this proposal would allow for immutable Data instances, e.g.

```ruby
irb(main):001> Person = Data.define(:name, :age, :hobbies)
=> Person
irb(main):002> bob = Person.new("Bob", 20, ["Ruby"])
=> #<data Person name="Bob", age=20, hobbies=["Ruby"]>
irb(main):003> bob.hobbies << "Fishing"
=> ["Ruby", "Fishing"]
irb(main):004> bob
=> #<data Person name="Bob", age=20, hobbies=["Ruby", "Fishing"]>
irb(main):005> Immutable bob
=> #<data Person name="Bob", age=20, hobbies=["Ruby", "Fishing"]>
irb(main):006> bob.hobbies << "Biking"
(irb):6:in `<main>': can't modify frozen Array: ["Ruby", "Fishing"] (FrozenError)
	from <internal:kernel>:187:in `loop'
	from -e:1:in `<main>'
```


----------------------------------------
Feature #18035: Introduce general model/semantic for immutability.
https://bugs.ruby-lang.org/issues/18035#change-106215

* 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.

Proposed PR: https://github.com/ruby/ruby/pull/4879



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