better error message for subtle nil errors

compiler/cgen.nim

@@ -346,6 +346,8 @@ proc resetLoc(p: BProc, loc: var TLoc) =
     else:
       linefmt(p, cpsStmts, "$1 = 0;$n", rdLoc(loc))
   else:
+    if optNilCheck in p.options:
+      linefmt(p, cpsStmts, "#chckNil((void*)$1);$n", addrLoc(loc))
     if loc.s != OnStack:
       linefmt(p, cpsStmts, "#genericReset((void*)$1, $2);$n",
               addrLoc(loc), genTypeInfo(p.module, loc.t))

compiler/options.nim

@@ -92,7 +92,7 @@ var
   gOptions*: TOptions = {optObjCheck, optFieldCheck, optRangeCheck, 
                          optBoundsCheck, optOverflowCheck, optAssert, optWarns, 
                          optHints, optStackTrace, optLineTrace,
-                         optPatterns}
+                         optPatterns, optNilCheck}
   gGlobalOptions*: TGlobalOptions = {optThreadAnalysis}
   gExitcode*: int8
   gCmd*: TCommands = cmdNone  # the command