Error: 'XXX' is not GC-safe as it accesses 'YYY' which is a global using GC'ed memory

[monster]

Hi,

I can't seem to get around this error:
    
    
    Error: 'mapPID2Address' is not GC-safe as it accesses 'mapper' which is a 
global using GC'ed memory
    

with code that basically does this:
    
    
    import asyncdispatch
    import asyncnet
    
    type
      ProcessID* = uint32
      NotAString = array[256,char]
      ProcessIDToAddress* = proc (pid: ProcessID, port: var Port, address: var 
NotAString): void {.nimcall.}
    
    var myProcessMapper: pointer
    var myClusterPort: uint16
    
    proc mapPID2Address(pid: ProcessID): (string, Port) {.gcSafe.} =
      let mapper = cast[ProcessIDToAddress](myProcessMapper)
      var port = Port(myClusterPort)
      var address: NotAString
      mapper(pid, port, address)
      let alen = address.find(char(0)) + 1
      var address2 = newStringOfCap(alen)
      for i in 0..alen:
        address2.add(address[i])
      result = (address2, port)
    
    proc openPeerConnection(pid: ProcessID): Future[AsyncSocket] {.async.} =
      ## Opens a connection to another peer.
      let (address, port) = mapPID2Address(pid)
      result = await asyncnet.dial(address, port)
    
    asyncCheck openPeerConnection(ProcessID(0))
    

Unfortunately, this example doesn't produce the error; I can't see where the 
difference is, but "mapper(pid, port, address)" is what bothers the compiler in 
my real code.

Initially, I wanted ProcessIDToAddress to return a string, but after getting 
the error, I decided to use an array as var parameter. My understanding is that 
the array is a stack object, and not a GC object, and so mapper doesn't 
actually access any GCed memory, and should not produce the error.