[ruby-core:97745] [Ruby master Bug#16769] Struct.new(..., immutable: true)

Issue #16769 has been reported by k0kubun (Takashi Kokubun).

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Bug #16769: Struct.new(..., immutable: true)
https://bugs.ruby-lang.org/issues/16769

* Author: k0kubun (Takashi Kokubun)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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## Background
We've discussed interface to pass Struct attributes (like `immutable: true`, which is actually not added yet) at once. But I believe just adding `immutable: true` alone is really helpful in various cases. Thus I've spun out this ticket only for `immutable: true` from [Feature #16122].

## Proposal

```rb
Post = Struct.new(:id, :name, immutable: true)

post = Post.new(1, "hello world")
post.id = 2 # NoMethodError (undefined method `id=' for #<struct Post id=1, name="hello world">)
```

Given `immutable: true`, an instance returned by `.new` is frozen, and writer methods are not defined.

## Use case

* Allow using Struct's nice features when we need an immutable model, instead of defining a normal class with `attr_reader`s and methods to support the Struct's features.
  * If it were a Struct, `to_s`, `inspect`, `==`, and a bunch of other methods are nicely defined by default. Deconstructing a Struct on Pattern Matching is also available. 
     * This level of support from the entire ecosystem may not be available if it's just a third-party library.
  * We could achieve a similar thing if we call `Post.new(...).freeze` or override `#initialize` to call `freeze` inside it, but it is not fun and feels like a workaround.
     * Today I suggested to use Struct for a model class to take advantage of the above benefits in a code review, but the implementation stuck with a bare class with `attr_reader`s because the author didn't want writer methods to be defined (of course we don't want to manually undef them from a Struct class either) and calling `freeze` to workaround it seems tricky. I strongly desired Ruby's Struct is useful enough to cover this use case.



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