[ruby-core:93071] [Ruby trunk Bug#15916] Memory leak in Regular Expression interpolation

Issue #15916 has been reported by mltsy (Joe Marty).

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Bug #15916: Memory leak in Regular Expression interpolation
https://bugs.ruby-lang.org/issues/15916

* Author: mltsy (Joe Marty)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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When interpolating a string inside a Regexp literal, if the string contains a multibyte character loaded from a file (not sure if this covers all the cases, but this is what triggers it for me), Ruby leaks memory.

The code below reproduces the problem, while outputting the process memory usage as it rises (get_process_mem gem is required).

Ways to avoid the memory leak (although I don't know why) include:
1. Using the string literal to define `PATTERN` directly (Not loading it from a file)
2. Using `Regexp.new` instead of a literal interpolation (`/#{...}/`)
3. Shortening the string to just a few characters (maybe small enough to fit inside a single RVALUE?)

``` ruby
require 'get_process_mem'

str = "String that doesn't fit into a single RVALUE, with a multibyte char:" + 160.chr(Encoding::UTF_8)
File.write('weirdstring.txt', str)

class Leak
  PATTERN = File.read("weirdstring.txt").freeze

  def test
    100_000.times { /#{PATTERN}/i }
  end
end

t = Leak.new

loop do
  print "Running... "

  t.test

  puts " process mem: #{GetProcessMem.new.mb.to_i}MB"
end

```

Expected Result:
Constant memory usage (avoiding the leak produces constant memory usage between 10-20MB)

Actual Result:
Continual memory growth (it only takes 60 seconds or so to consume 500MB)



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