Fix infinite recursion when using json.to on ref with cycle.

lib/pure/json.nim

@@ -1651,6 +1651,13 @@ import options
 proc workaroundMacroNone[T](): Option[T] =
   none(T)
 
+proc depth(n: NimNode, current = 0): int =
+  result = 1
+  for child in n:
+    let d = 1 + child.depth(current + 1)
+    if d > result:
+      result = d
+
 proc createConstructor(typeSym, jsonNode: NimNode): NimNode =
   ## Accepts a type description, i.e. "ref Type", "seq[Type]", "Type" etc.
   ##
@@ -1660,6 +1667,9 @@ proc createConstructor(typeSym, jsonNode: NimNode): NimNode =
   # echo("--createConsuctor-- \n", treeRepr(typeSym))
   # echo()
 
+  if depth(jsonNode) > 150:
+    error("The `to` macro does not support ref objects with cycles.", jsonNode)
+
   case typeSym.kind
   of nnkBracketExpr:
     var bracketName = ($typeSym[0]).normalize

tests/stdlib/tjsonmacro.nim

@@ -354,6 +354,4 @@ when isMainModule:
     let dataDeser = to(dataParsed, Test)
     doAssert dataDeser.name == "FooBar"
     doAssert dataDeser.fallback.kind == JFloat
-    doAssert dataDeser.fallback.getFloat() == 56.42
+    doAssert dataDeser.fallback.getFloat() == 56.42
-
-  # TODO: Cycles lead to infinite loops.

tests/stdlib/tjsonmacro_reject2.nim

@@ -0,0 +1,21 @@
+discard """
+  file: "tjsonmacro_reject2.nim"
+  line: 10
+  errormsg: "The `to` macro does not support ref objects with cycles."
+"""
+import json
+
+type
+  Misdirection = object
+    cycle: Cycle
+
+  Cycle = ref object
+    foo: string
+    cycle: Misdirection
+
+let data = """
+  {"cycle": null}
+"""
+
+let dataParsed = parseJson(data)
+let dataDeser = to(dataParsed, Cycle)