[ruby-core:90881] [Ruby trunk Feature#15504] Freeze all Range object

Issue #15504 has been reported by ko1 (Koichi Sasada).

----------------------------------------
Feature #15504: Freeze all Range object
https://bugs.ruby-lang.org/issues/15504

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
# Abstrcat

Range is now non-frozen. How about to freeze all of Range objects?

# Background

We freeze some type of objects, Numerics (r47523) and Symbols [Feature #8906].
I believe making objects immutable solves some kind of programming difficulties.

`Range` is mutable, at least it is written in Range literal. So we can write the following weird program. 

```
2.times{
  r = (1..3)
  p r.instance_variable_get(:@foo)
  #=> 1st time: nil
  #=> 2nd time: :bar
  r.instance_variable_set(:@foo, :bar)
}
```

in `range.c`, there is a comment (thanks znz-san):

```
static void
range_modify(VALUE range)
{
    rb_check_frozen(range);
    /* Ranges are immutable, so that they should be initialized only once. */
    if (RANGE_EXCL(range) != Qnil) {
	rb_name_err_raise("`initialize' called twice", range, ID2SYM(idInitialize));
    }
}
```

# Patch

```
Index: range.c
===================================================================
--- range.c	(リビジョン 66699)
+++ range.c	(作業コピー)
@@ -45,6 +45,8 @@
     RANGE_SET_EXCL(range, exclude_end);
     RANGE_SET_BEG(range, beg);
     RANGE_SET_END(range, end);
+
+    rb_obj_freeze(range);
 }
 
 VALUE
```

# Discussion

There are several usage of mutable Range in tests.

* (1) taint-flag
* (2) add singleton methods.
* (3) subclass with mutable states

Maybe (2) and (3) are points.

Thanks,
Koichi



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