fixes #15130 (#15141)

* fixes #15130

* you really have to copy from cursors

compiler/dfa.nim

@@ -628,7 +628,7 @@ proc isAnalysableFieldAccess*(orig: PNode; owner: PSym): bool =
   # XXX Allow closure deref operations here if we know
   # the owner controlled the closure allocation?
   result = n.kind == nkSym and n.sym.owner == owner and
-    sfGlobal notin n.sym.flags and
+    {sfGlobal, sfThread, sfCursor} * n.sym.flags == {} and
     (n.sym.kind != skParam or isSinkParam(n.sym)) # or n.sym.typ.kind == tyVar)
   # Note: There is a different move analyzer possible that checks for
   # consume(param.key); param.key = newValue  for all paths. Then code like

compiler/varpartitions.nim

@@ -48,7 +48,7 @@ type
     s: seq[VarIndex]
     graphs: seq[MutationInfo]
     unanalysableMutation, performCursorInference: bool
-    inAsgnSource: int
+    inAsgnSource, inConstructor: int
 
 proc `$`*(config: ConfigRef; g: MutationInfo): string =
   result = ""
@@ -264,8 +264,10 @@ proc analyseAsgn(c: var Partitions; dest: var VarIndex; n: PNode) =
       dest.flags.incl ownsData
 
   of nkCurly, nkBracket, nkPar, nkTupleConstr:
+    inc c.inConstructor
     for son in n:
       analyseAsgn(c, dest, son)
+    dec c.inConstructor
     if n.typ.skipTypes(abstractInst).kind == tySequence:
       # you must destroy a sequence:
       dest.flags.incl ownsData
@@ -278,6 +280,8 @@ proc analyseAsgn(c: var Partitions; dest: var VarIndex; n: PNode) =
       else:
         # otherwise it's just a dependency, nothing to worry about:
         connect(c, dest.sym, n.sym, n.info)
+        # but a construct like ``[symbol]`` is dangerous:
+        if c.inConstructor > 0: dest.flags.incl ownsData
 
   of nkDotExpr, nkBracketExpr, nkHiddenDeref, nkDerefExpr,
       nkObjUpConv, nkObjDownConv, nkCheckedFieldExpr, nkAddr, nkHiddenAddr: