VM regression fixes (#8146)

compiler/vm.nim

@@ -209,7 +209,7 @@ proc writeField(n: var PNode, x: TFullReg) =
   of rkNone: discard
   of rkInt: n.intVal = x.intVal
   of rkFloat: n.floatVal = x.floatVal
-  of rkNode: n = x.node
+  of rkNode: n = copyValue(x.node)
   of rkRegisterAddr: writeField(n, x.regAddr[])
   of rkNodeAddr: n = x.nodeAddr[]
 
@@ -912,6 +912,7 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       if a.kind == nkSym:
         regs[ra].node = if a.sym.ast.isNil: newNode(nkNilLit)
                         else: copyTree(a.sym.ast)
+        regs[ra].node.flags.incl nfIsRef
       else:
         stackTrace(c, tos, pc, "node is not a symbol")
     of opcEcho:
@@ -1462,6 +1463,7 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       else:
         regs[ra].node = newNodeI(nkIdent, c.debug[pc])
         regs[ra].node.ident = getIdent(c.cache, regs[rb].node.strVal)
+        regs[ra].node.flags.incl nfIsRef
     of opcSetType:
       if regs[ra].kind != rkNode:
         internalError(c.config, c.debug[pc], "cannot set type")

tests/vm/tvmmisc.nim

@@ -90,3 +90,33 @@ block:
       var some = newStringOfCap(size)
       result = size
     doAssert f(4) == 4
+
+# #7871
+static:
+  type Obj = object
+    field: int
+  var s = newSeq[Obj](1)
+  var o = Obj()
+  s[0] = o
+  o.field = 2
+  doAssert s[0].field == 0
+
+# #8125
+static:
+   let def_iter_var = ident("it")
+
+# #8142
+static:
+  type Obj = object
+    names: string
+
+  proc pushName(o: var Obj) =
+    var s = ""
+    s.add("FOOBAR")
+    o.names.add(s)
+
+  var o = Obj()
+  o.names = ""
+  o.pushName()
+  o.pushName()
+  doAssert o.names == "FOOBARFOOBAR"