After reformatting your code, copied from "lib/system/arc.nim" I guess:
proc nimDecRefIsLast(p: pointer): bool {.compilerRtl, inl.} =
if p != nil:
var cell = head(p)
if (cell.rc and not rcMask) == 0:
result = true
when traceCollector:
cprintf("[ABOUT TO DESTROY] %pn", cell)
But your line numbers match neither "devel" nor the 1.4 release. Try the latest 1.4 release. And then post more details if it's still a problem.