return gc traced object from dynlib cause crash when orc/arc enabled

[codehz]

Example

# nim c --gc:arc --debugger:native --cc:clang_cl --debuginfo:on a.nim
proc create(): ref int {.dynlib: "b.dll", importc.}

block:
  var x = create()
  echo cast[int](x) # check the pointer
echo "here"
GC_fullCollect()

# nim c --gc:arc --debugger:native --cc:clang_cl --debuginfo:on --app:lib b.nim
proc create(): ref int{.dynlib, exportc.} =
  new int

Current Output

2317140426832
Traceback (most recent call last)
D:\Temp\nimdll\a.nim(4)  a
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\arc.nim(161) nimRawDispose
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\alloc.nim(972) dealloc
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\alloc.nim(857) rawDealloc
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\alloc.nim(518) listRemove
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

no output and no crash

Possible Solution

• In file xyz there is a call that might be the cause of it.

Additional Information

$ nim -v
Nim Compiler Version 1.4.2 [Windows: amd64]
Compiled at 2020-11-30
Copyright (c) 2006-2020 by Andreas Rumpf

active boot switches: -d:release

[cooldome]

add -d:useMalloc to Nim command line

[timotheecour]

add -d:useMalloc to Nim command line

that's hardly a viable workaround in the general case though; also user passed --gc:arc, not --newruntime so docs are either incorrect or imprecise

``useMalloc``            Makes Nim use C's `malloc`:idx: instead of Nim's
                         own memory manager, albeit prefixing each allocation with
                         its size to support clearing memory on reallocation.
                         This only works with ``gc:none`` and
                         with ``--newruntime``.