Fix unsound transform pass (#8633)

When a `var openArray[T]` function parameter goes trough the
`transformAddrDeref` pass we may lose the `var` specifier, leading to
nasty crashes at runtime.

compiler/ast.nim

@@ -1674,6 +1674,14 @@ proc skipStmtList*(n: PNode): PNode =
   else:
     result = n
 
+proc toVar*(typ: PType): PType =
+  ## If ``typ`` is not a tyVar then it is converted into a `var <typ>` and
+  ## returned. Otherwise ``typ`` is simply returned as-is.
+  result = typ
+  if typ.kind != tyVar:
+    result = newType(tyVar, typ.owner)
+    rawAddSon(result, typ)
+
 proc toRef*(typ: PType): PType =
   ## If ``typ`` is a tyObject then it is converted into a `ref <typ>` and
   ## returned. Otherwise ``typ`` is simply returned as-is.

compiler/astalgo.nim

@@ -412,6 +412,8 @@ proc debugTree(conf: ConfigRef; n: PNode, indent: int, maxRecDepth: int;
         else:
           addf(result, ",$N$1\"ident\": null", [istr])
       else:
+        if renderType and n.typ != nil:
+          addf(result, ",$N$1\"typ\": $2", [istr, debugType(conf, n.typ, 2)])
         if sonsLen(n) > 0:
           addf(result, ",$N$1\"sons\": [", [istr])
           for i in countup(0, sonsLen(n) - 1):

compiler/transf.nim

@@ -369,6 +369,8 @@ proc transformAddrDeref(c: PTransf, n: PNode, a, b: TNodeKind): PTransNode =
       result = PTransNode(n.sons[0])
       if n.typ.skipTypes(abstractVar).kind != tyOpenArray:
         PNode(result).typ = n.typ
+      elif n.typ.skipTypes(abstractInst).kind in {tyVar}:
+        PNode(result).typ = toVar(PNode(result).typ)
   of nkHiddenStdConv, nkHiddenSubConv, nkConv:
     var m = n.sons[0].sons[1]
     if m.kind == a or m.kind == b:
@@ -377,6 +379,8 @@ proc transformAddrDeref(c: PTransf, n: PNode, a, b: TNodeKind): PTransNode =
       result = PTransNode(n.sons[0])
       if n.typ.skipTypes(abstractVar).kind != tyOpenArray:
         PNode(result).typ = n.typ
+      elif n.typ.skipTypes(abstractInst).kind in {tyVar}:
+        PNode(result).typ = toVar(PNode(result).typ)
   else:
     if n.sons[0].kind == a or n.sons[0].kind == b:
       # addr ( deref ( x )) --> x

tests/ccgbugs/topenarraycast.nim

@@ -0,0 +1,8 @@
+proc foo[T](s: var openArray[T]): T =
+  for x in s: result += x
+
+proc bar(xyz: var seq[int]) =
+  doAssert 6 == (seq[int](xyz)).foo()
+
+var t = @[1,2,3]
+bar(t)