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

fixes #22664

compiler/liftdestructors.nim

@@ -553,17 +553,28 @@ 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:
  body.add setLenSeqCall(c, t, x, y)
  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)]"