[ruby-core:81158] [Ruby trunk Bug#13564] Exception message management

Issue #13564 has been reported by ko1 (Koichi Sasada).

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Bug #13564: Exception message management
https://bugs.ruby-lang.org/issues/13564

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
* ruby -v: 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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We can modify `Exception#message` if given String is not frozen.
Should we continue this specification?

Now, we can modify exception message with message string modification.

```
begin
  raise 'foo'
ensure
  $!.message.replace 'bar' #=> Exception with message "bar"
end
```

However, if we pass frozen string, it is not allowed.

```
begin
  raise 'foo'.freeze
ensure
  $!.message.replace 'bar' #=> `replace': can't modify frozen String (RuntimeError)
end
```

Furthermore, `# frozen_string_literal: true` freeze all of string literals.

```
# frozen_string_literal: true

begin
  raise 'foo'
ensure
  $!.message.replace 'bar' #=> `replace': can't modify frozen String (RuntimeError)
end
```

# Background and motivation 

I want to add Exception message on ensure clause like the code in previous section. Just now, we need to re-raise another exception (with `raise($!.class, new_msg, $!.backtrace)`). I tried to modify `$!.message` and it works on small script. However, I try it on production (*1), it doesn't work because of `frozen_string_literal: true`.

*1: https://svn.ruby-lang.org/cgi-bin/viewvc.cgi/trunk/test/rubygems/test_gem_gem_runner.rb?r1=58723&r2=58722&pathrev=58723

I think current behavior (specification?) is easy to misusing.

# Ideas

* (1) To prevent such behavior
  * (1-1) Freeze message strings at initialize
  * (1-2) Return copy string at `Exception#message`
* (2) Provide `Exception#message = `
  * And (1-1) or (1-2)
* (3) Allow such behavior. If a frozen message is given, dup it and set as modifiable.





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