fixes #24806; don't elide wasMoved when syms are used in blocks (#24831)

fixes #24806

Blocks don't merge symbols that are used before destruction to the
parent scope, which causes `wasMoved; destroy` to elide incorrectly

Author: ringabout

compiler/optimizer.nim

@@ -28,11 +28,14 @@ type
     hasReturn, hasBreak: bool
     label: PSym # can be nil
     parent: ptr BasicBlock
+    symToDel: seq[PNode]
 
   Con = object
     somethingTodo: bool
     inFinally: int
 
+proc invalidateWasMoved(c: var BasicBlock; x: PNode)
+
 proc nestedBlock(parent: var BasicBlock; kind: TNodeKind): BasicBlock =
   BasicBlock(wasMovedLocs: @[], kind: kind, hasReturn: false, hasBreak: false,
     label: nil, parent: addr(parent))
@@ -62,6 +65,10 @@ proc mergeBasicBlockInfo(parent: var BasicBlock; this: BasicBlock) {.inline.} =
   if this.hasReturn:
     parent.wasMovedLocs.setLen 0
     parent.hasReturn = true
+  elif this.symToDel.len > 0:
+    parent.symToDel = this.symToDel
+    for i in this.symToDel:
+      invalidateWasMoved(parent, i)
 
 proc wasMovedTarget(matches: var IntSet; branch: seq[PNode]; moveTarget: PNode): bool =
   result = false
@@ -149,6 +156,7 @@ proc analyse(c: var Con; b: var BasicBlock; n: PNode) =
     # any usage of the location before destruction implies we
     # cannot elide the 'wasMoved(x)':
     b.invalidateWasMoved n
+    b.symToDel.add n
 
   of nkNone..pred(nkSym), succ(nkSym)..nkNilLit, nkTypeSection, nkProcDef, nkConverterDef,
       nkMethodDef, nkIteratorDef, nkMacroDef, nkTemplateDef, nkLambda, nkDo,

tests/arc/t24806.nim

@@ -0,0 +1,39 @@
+discard """
+  matrix: "-d:useMalloc;"
+"""
+
+type
+  GlobFilter* = object
+    incl*: bool
+    glob*: string
+
+  GlobState* = object
+    one: int
+    two: int
+
+proc aa() =
+  let filters = @[GlobFilter(incl: true, glob: "**")]
+  var wbg = newSeqOfCap[GlobState](1)
+  wbg.add GlobState()
+  var
+    dirc = @[wbg]
+  while true:
+    wbg = dirc[^1]
+    dirc.add wbg
+    break
+
+var handlerLocs = newSeq[string]()
+handlerLocs.add "sammich"
+aa()
+aa()
+
+block: # bug #24806
+  proc aa() =
+    var
+      a = @[0]
+      b = @[a]
+    block:
+      a = b[0]
+      b.add a
+
+  aa()