bring #21802 back; fixes #21753 [backport] (#21815)

* bring #21802 back; fixes #21753 [backport]

* adds tests and multiple fixes

* add test cases

* refactor and remove startId

* fixes custom hooks and adds tests

* handle tyUncheckedArray better

1 files changed, 123 insertions, 0 deletions

diff --git a/tests/types/tcyclic.nim b/tests/types/tcyclic.nim
new file mode 100644
index 000000000..fc2852c49
--- /dev/null
+++ b/tests/types/tcyclic.nim
@@ -0,0 +1,123 @@
+## todo publish the `isCyclic` when it's mature.
+proc isCyclic(t: typedesc): bool {.magic: "TypeTrait".} =
+  ## Returns true if the type can potentially form a cyclic type
+
+template cyclicYes(x: typed) =
+  doAssert isCyclic(x)
+
+template cyclicNo(x: typed) =
+  doAssert not isCyclic(x)
+
+# atomic types are not cyclic
+cyclicNo(int)
+cyclicNo(float)
+cyclicNo(string)
+cyclicNo(char)
+cyclicNo(void)
+
+type
+  Object = object
+  Ref = ref object
+
+cyclicNo(Object)
+cyclicNo(Ref)
+
+type
+  Data1 = ref object
+  Data2 = ref object
+    id: Data1
+
+cyclicNo(Data2)
+
+type
+  Cyclone = ref object
+    data: Cyclone
+
+  Alias = Cyclone
+
+  Acyclic {.acyclic.} = ref object
+    data: Acyclic
+
+  LinkedNode = object
+    next: ref LinkedNode
+
+  LinkedNodeWithCursor = object
+    next {.cursor.} : ref LinkedNodeWithCursor
+
+cyclicYes(Cyclone)
+cyclicYes(Alias)
+cyclicNo(seq[Cyclone])
+cyclicNo((Cyclone, ))
+cyclicNo(Acyclic)
+
+cyclicYes(LinkedNode)
+
+when false:
+  # todo fix me
+  cyclicNo(LinkedNodeWithCursor)
+
+type
+  ObjectWithoutCycles = object
+    data: seq[ObjectWithoutCycles]
+
+cyclicNo(ObjectWithoutCycles)
+
+
+block:
+  type
+    Try = object
+      id: Best
+    Best = object
+      name: ref Try
+    Best2 = ref Best
+
+  cyclicYes(Best)
+  cyclicYes(Try)
+  cyclicNo(Best2)
+
+  type
+    Base = object
+      data: ref seq[Base]
+    Base2 = ref Base
+
+  cyclicYes(Base)
+  cyclicNo(Base2)
+
+
+  type
+    Base3 = ref object
+      id: Base3
+
+    Base4 = object
+      id: ref Base4
+
+  cyclicYes(Base3)
+  cyclicYes(Base4)
+  cyclicYes(ref Base4)
+
+block:
+  type Cyclic2 = object
+    x: ref (Cyclic2, int)
+
+  cyclicYes (Cyclic2, int)
+  cyclicYes (ref (Cyclic2, int))
+
+block:
+  type
+    myseq[T] = object
+      data: ptr UncheckedArray[T]
+    Node = ref object
+      kids: myseq[Node]
+
+  cyclicNo(Node)
+
+block:
+  type
+    myseq[T] = object
+      data: ptr UncheckedArray[T]
+    Node = ref object
+      kids: myseq[Node]
+
+  proc `=trace`(x: var myseq[Node]; env: pointer) = discard
+
+  cyclicYes(Node)