put the new for loop macros under an experimental switch named 'forLoopMacros'

compiler/options.nim

@@ -111,14 +111,16 @@ type
     ideNone, ideSug, ideCon, ideDef, ideUse, ideDus, ideChk, ideMod,
     ideHighlight, ideOutline, ideKnown, ideMsg
 
-  Feature* = enum  ## experimental features
+  Feature* = enum  ## experimental features; DO NOT RENAME THESE!
     implicitDeref,
     dotOperators,
     callOperator,
     parallel,
     destructor,
     notnil,
-    dynamicBindSym
+    dynamicBindSym,
+    forLoopMacros
+    #caseStmtMacros
 
   SymbolFilesOption* = enum
     disabledSf, writeOnlySf, readOnlySf, v2Sf

compiler/semstmts.nim

@@ -720,8 +720,9 @@ proc handleForLoopMacro(c: PContext; n: PNode): PNode =
 proc semFor(c: PContext, n: PNode): PNode =
   checkMinSonsLen(n, 3, c.config)
   var length = sonsLen(n)
-  result = handleForLoopMacro(c, n)
-  if result != nil: return result
+  if forLoopMacros in c.features:
+    result = handleForLoopMacro(c, n)
+    if result != nil: return result
   openScope(c)
   result = n
   n.sons[length-2] = semExprNoDeref(c, n.sons[length-2], {efWantIterator})