fixes #26143; Possible memory error (#26154)

fixes #26143

follows up https://github.com/nim-lang/Nim/pull/20307

(cherry picked from commit c36c527db3)

compiler/injectdestructors.nim

@@ -412,6 +412,20 @@ proc genDefaultCall(t: PType; c: Con; info: TLineInfo): PNode =
   result.add(newSymNode(createMagic(c.graph, c.idgen, "default", mDefault)))
   result.typ() = t
 
+proc stabilizeBracketIndex(n: PNode; c: var Con; body: var PNode): PNode =
+  ## Evaluate a side-effecting index once and return the stable access.
+  doAssert n.kind == nkBracketExpr and not isAtom(n[1])
+  let temp = newSym(skLet, getIdent(c.graph.cache, "bracketTmp"), c.idgen,
+                    c.owner, n[1].info)
+  temp.typ = n[1].typ
+  let tempAsNode = newSymNode(temp)
+  body.add newTree(nkLetSection, n[1].info,
+                   newTree(nkIdentDefs, tempAsNode,
+                           newNodeI(nkEmpty, tempAsNode.info), n[1]))
+  result = copyNode(n)
+  result.add n[0]
+  result.add tempAsNode
+
 proc destructiveMoveVar(n: PNode; c: var Con; s: var Scope): PNode =
   # generate: (let tmp = v; reset(v); tmp)
   if (not hasDestructor(c, n.typ)) and c.inEnsureMove == 0:
@@ -423,6 +437,10 @@ proc destructiveMoveVar(n: PNode; c: var Con; s: var Scope): PNode =
   else:
     result = newNodeIT(nkStmtListExpr, n.info, n.typ)
 
+    var n = n
+    if n.kind == nkBracketExpr and not isAtom(n[1]):
+      n = stabilizeBracketIndex(n, c, result)
+
     var temp = newSym(skLet, getIdent(c.graph.cache, "blitTmp"), c.idgen, c.owner, n.info)
     temp.typ = n.typ
     var v = newNodeI(nkLetSection, n.info)
@@ -1139,24 +1157,11 @@ proc sameLocation*(a, b: PNode): bool =
     else: false
 
 proc genFieldAccessSideEffects(c: var Con; s: var Scope; dest, ri: PNode; flags: set[MoveOrCopyFlag] = {}): PNode =
-  # with side effects
-  var temp = newSym(skLet, getIdent(c.graph.cache, "bracketTmp"), c.idgen, c.owner, ri[1].info)
-  temp.typ = ri[1].typ
-  var v = newNodeI(nkLetSection, ri[1].info)
-  let tempAsNode = newSymNode(temp)
-
-  var vpart = newNodeI(nkIdentDefs, tempAsNode.info, 3)
-  vpart[0] = tempAsNode
-  vpart[1] = newNodeI(nkEmpty, tempAsNode.info)
-  vpart[2] = ri[1]
-  v.add(vpart)
-
-  var newAccess = copyNode(ri)
-  newAccess.add ri[0]
-  newAccess.add tempAsNode
-
-  var snk = c.genSink(s, dest, newAccess, flags)
-  result = newTree(nkStmtList, v, snk, c.genWasMoved(newAccess))
+  result = newNodeI(nkStmtList, ri.info)
+  let newAccess = stabilizeBracketIndex(ri, c, result)
+  let snk = c.genSink(s, dest, newAccess, flags)
+  result.add snk
+  result.add c.genWasMoved(newAccess)
 
 proc ownsData(c: var Con; s: var Scope; orig: PNode; flags: set[MoveOrCopyFlag]): PNode =
   var n = orig

tests/arc/tarc_orc.nim

@@ -35,6 +35,20 @@ proc bug20303() =
 
 bug20303()
 
+block: # bug #26143
+  var indexCalls = 0
+
+  proc nextIndex(): int =
+    result = indexCalls
+    inc indexCalls
+
+  proc consume(value: sink string) =
+    doAssert value == "A"
+
+  var values = @["A", "B"]
+  consume(values[nextIndex()])
+  doAssert indexCalls == 1
+
 proc main() = # todo bug with templates
   block: # bug #11267
     var a: seq[char] = block: @[]