fixes #22664; guard against potential seqs self assignments (#22671)

fixes #22664

compiler/liftdestructors.nim

@@ -553,6 +553,14 @@ proc forallElements(c: var TLiftCtx; t: PType; body, x, y: PNode) =
   else:
     body.sons.setLen counterIdx
 
+proc checkSelfAssignment(c: var TLiftCtx; t: PType; body, x, y: PNode) =
+  var cond = callCodegenProc(c.g, "sameSeqPayload", c.info,
+      newTreeIT(nkAddr, c.info, makePtrType(c.fn, x.typ, c.idgen), x),
+      newTreeIT(nkAddr, c.info, makePtrType(c.fn, y.typ, c.idgen), y)
+      )
+  cond.typ = getSysType(c.g, c.info, tyBool)
+  body.add genIf(c, cond, newTreeI(nkReturnStmt, c.info, newNodeI(nkEmpty, c.info)))
+
 proc fillSeqOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
   case c.kind
   of attachedDup:
@@ -560,10 +568,13 @@ proc fillSeqOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
     forallElements(c, t, body, x, y)
   of attachedAsgn, attachedDeepCopy:
     # we generate:
+    # if x.p == y.p:
+    #   return
     # setLen(dest, y.len)
     # var i = 0
     # while i < y.len: dest[i] = y[i]; inc(i)
     # This is usually more efficient than a destroy/create pair.
+    checkSelfAssignment(c, t, body, x, y)
     body.add setLenSeqCall(c, t, x, y)
     forallElements(c, t, body, x, y)
   of attachedSink:

lib/system/seqs_v2.nim

@@ -32,6 +32,10 @@ type
     len: int
     p: ptr NimSeqPayload[T]
 
+  NimRawSeq = object
+    len: int
+    p: pointer
+
 const nimSeqVersion {.core.} = 2
 
 # XXX make code memory safe for overflows in '*'
@@ -139,6 +143,8 @@ proc newSeq[T](s: var seq[T], len: Natural) =
   shrink(s, 0)
   setLen(s, len)
 
+proc sameSeqPayload(x: pointer, y: pointer): bool {.compilerproc, inline.} =
+  result = cast[ptr NimRawSeq](x)[].p == cast[ptr NimRawSeq](y)[].p
 
 template capacityImpl(sek: NimSeqV2): int =
   if sek.p != nil: (sek.p.cap and not strlitFlag) else: 0

tests/arc/tarcmisc.nim

@@ -614,3 +614,24 @@ method process*(self: App): Option[Event] {.base.} =
 type Test2 = ref object of RootObj
 
 method bug(t: Test2): seq[float] {.base.} = discard
+
+block: # bug #22664
+  type
+    ElementKind = enum String, Number
+    Element = object
+      case kind: ElementKind
+      of String:
+        str: string
+      of Number:
+        num: float
+    Calc = ref object
+      stack: seq[Element]
+
+  var calc = new Calc
+
+  calc.stack.add Element(kind: Number, num: 200.0)
+  doAssert $calc.stack == "@[(kind: Number, num: 200.0)]"
+  let calc2 = calc
+  calc2.stack = calc.stack # This nulls out the object in the stack
+  doAssert $calc.stack == "@[(kind: Number, num: 200.0)]"
+  doAssert $calc2.stack == "@[(kind: Number, num: 200.0)]"