make move-analysis smarter; see tuse_ownedref_after_move test case

tests/destructor/tuse_ownedref_after_move.nim

@@ -0,0 +1,60 @@
+discard """
+  cmd: '''nim c --newruntime $file'''
+  errormsg: "'=' is not available for type <owned Widget>; requires a copy because it's not the last read of ':env.b1()'; another read is done here: tuse_ownedref_after_move.nim(53, 4)"
+  line: 49
+"""
+
+import core / allocators
+import system / ansi_c
+
+type
+  Widget* = ref object of RootObj
+    drawImpl: owned(proc (self: Widget))
+
+  Button* = ref object of Widget
+    caption: string
+    onclick: owned(proc())
+
+  Window* = ref object of Widget
+    elements: seq[owned Widget]
+
+
+proc newButton(caption: string; onclick: owned(proc())): owned Button =
+  proc draw(self: Widget) =
+    let b = Button(self)
+    echo b.caption
+
+  result = Button(drawImpl: draw, caption: caption, onclick: onclick)
+
+proc newWindow(): owned Window =
+  proc draw(self: Widget) =
+    let w = Window(self)
+    for e in w.elements:
+      if not e.drawImpl.isNil: e.drawImpl(e)
+
+  result = Window(drawImpl: draw, elements: @[])
+
+proc draw(w: Widget) =
+  if not w.drawImpl.isNil: w.drawImpl(w)
+
+proc add*(w: Window; elem: owned Widget) =
+  w.elements.add elem
+
+proc main =
+  var w = newWindow()
+
+  var b = newButton("button", nil)
+  b.onclick = proc () =
+    b.caption = "clicked!"
+  w.add b
+
+  w.draw()
+  # simulate button click:
+  b.onclick()
+
+  w.draw()
+
+main()
+
+let (a, d) = allocCounters()
+discard cprintf("%ld %ld  new: %ld\n", a, d, allocs)