make 'not nil' experimental

compiler/condsyms.nim

@@ -116,3 +116,4 @@ proc initDefines*() =
   defineSymbol("nimVmEqIdent")
   defineSymbol("nimNoNil")
   defineSymbol("nimNoZeroTerminator")
+  defineSymbol("nimNotNil")

compiler/options.nim

@@ -109,7 +109,8 @@ type
     dotOperators,
     callOperator,
     parallel,
-    destructor
+    destructor,
+    notnil
 
   ConfigRef* = ref object ## eventually all global configuration should be moved here
     cppDefines*: HashSet[string]

compiler/semtypes.nim

@@ -1389,6 +1389,8 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
               n.sons[2].kind == nkNilLit:
             result = freshType(result, prev)
             result.flags.incl(tfNotNil)
+            if notnil notin c.features:
+              localError(n.info, "enable the 'not nil' annotation with {.experimental: \"notnil\".}")
           else:
             localError(n.info, errGenerated, "invalid type")
         of 2: