Hello there!

I've found this one while adding JRuby 9.2 to concurrent-ruby's travis CI config, see ħere and ruby-concurrency/concurrent-ruby#733 .

Environment

• JRuby: jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 OpenJDK 64-Bit Server VM 25.71-b00 on 1.8.0-internal-jenkins_2017_07_04_16_04-b00 [linux-x86_64]
• Kernel: Linux maruhime 4.13.0-45-generic #50-Ubuntu SMP Wed May 30 08:23:18 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
• Distro: Ubuntu 17.10

Expected Behavior

On MRI when getting the to_s of a class or module, the resulting string is a copy and so is safe to be mutated at will, and changing it will have no impact on the to_s of the class/module.

Testcase:

puts RUBY_DESCRIPTION

class A
end

A.to_s.downcase!

puts A.to_s

MRI output:

ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
A

Actual Behavior

On JRuby 9.2, mutating the result of to_s wrongly changes the actual to_s of the class/module.

JRuby output:

jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 OpenJDK 64-Bit Server VM 25.71-b00 on 1.8.0-internal-jenkins_2017_07_04_16_04-b00 [linux-x86_64]
a