simple 'var openarray[char]' assignment crash when the openarray source is a local string and using gc:arc

[jangko]

This simple nonce[1] = char(65) crash. it works with default gc but not with gc:arc

Example

proc cryptCTR*(nonce: var openArray[char]) =
  nonce[1] = char(65)

proc main() =
  var nonce = "0123456701234567"
  cryptCTR(nonce.toOpenArray(0, nonce.len-1))
  debugEcho nonce
  
main()

Current Output

SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

0A23456701234567

$ nim -v
Nim Compiler Version 1.3.1 [Windows: amd64]
Compiled at 2020-04-09
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: b4e9f8e814373fc38741736197d88475663ce758
active boot switches: -d:release

[cooldome]

Needs PrepareForMutation before calling a function cryptCTR. I guess codegen can do it.