Improve vm support for ref types

tests/vm/tref.nim

@@ -9,4 +9,49 @@ static:
 
   b[5] = 'c'
   doAssert a[] == "Hellocworld"
-  doAssert b[] == "Hellocworld"
+  doAssert b[] == "Hellocworld"
+
+  proc notGlobal() =
+    var
+      a: ref string
+      b: ref string
+    new a
+
+    a[] = "Hello world"
+    b = a
+
+    b[5] = 'c'
+    doAssert a[] == "Hellocworld"
+    doAssert b[] == "Hellocworld"
+  notGlobal()
+
+static: # bug 6081
+  block:
+    type Obj = object
+      field: ref int
+    var i: ref int
+    new(i)
+    var r = Obj(field: i)
+    var rr = r
+    r.field = nil
+    doAssert rr.field != nil
+
+  proc foo() = # Proc to avoid special global logic
+    var s: seq[ref int]
+    var i: ref int
+    new(i)
+    s.add(i)
+    var head = s[0]
+    s[0] = nil
+    doAssert head != nil
+
+  foo()
+
+static:
+
+  block: # global alias
+    var s: ref int
+    new(s)
+    var ss = s
+    s[] = 1
+    doAssert ss[] == 1