dfa.nim: track object/tuple field accesses more precisely; sink(o.x); sink(o.y) needs to compile; activate the tuple unpacking transf.nim bugfix

tests/destructor/tobjfield_analysis.nim

@@ -0,0 +1,35 @@
+discard """
+  output: '''works'''
+"""
+
+type
+  MyVal = object
+    f: ptr float
+
+proc `=destroy`(x: var MyVal) =
+  if x.f != nil:
+    dealloc(x.f)
+
+proc `=sink`(x1: var MyVal, x2: Myval) =
+  if x1.f != x2.f:
+    `=destroy`(x1)
+    x1.f = x2.f
+
+proc `=`(x1: var MyVal, x2: Myval) {.error.}
+
+proc newVal(x: float): MyVal =
+  result.f = create(float)
+  result.f[] = x
+
+proc sinkMe(x: sink MyVal) =
+  discard
+
+proc main =
+  var y = (newVal(3.0), newVal(4.0))
+
+  sinkMe y[0]
+  sinkMe y[1]
+  echo "works"
+
+main()
+