ARC related bugfixes and refactorings (#12781)

tests/destructor/tgcleak4.nim

@@ -0,0 +1,47 @@
+discard """
+  outputsub: "no leak: "
+  cmd: "nim c --gc:arc $file"
+"""
+# bug #12758
+type
+  TExpr {.inheritable.} = object ## abstract base class for an expression
+  PLiteral = ref TLiteral
+  TLiteral = object of TExpr
+    x: int
+    op1: string
+  TPlusExpr = object of TExpr
+    a, b: ref TExpr
+    op2: string
+
+method eval(e: ref TExpr): int {.base.} =
+  # override this base method
+  quit "to override!"
+
+method eval(e: ref TLiteral): int = return e.x
+
+method eval(e: ref TPlusExpr): int =
+  # watch out: relies on dynamic binding
+  return eval(e.a) + eval(e.b)
+
+proc newLit(x: int): ref TLiteral =
+  new(result)
+  result.x = x
+  result.op1 = $getOccupiedMem()
+
+proc newPlus(a, b: ref TExpr): ref TPlusExpr =
+  new(result)
+  result.a = a
+  result.b = b
+  result.op2 = $getOccupiedMem()
+
+const Limit = when compileOption("gc", "markAndSweep") or compileOption("gc", "boehm"): 5*1024*1024 else: 500_000
+
+for i in 0..100_000:
+  var s: array[0..11, ref TExpr]
+  for j in 0..high(s):
+    s[j] = newPlus(newPlus(newLit(j), newLit(2)), newLit(4))
+    if eval(s[j]) != j+6:
+      quit "error: wrong result"
+  if getOccupiedMem() > Limit: quit("still a leak!")
+
+echo "no leak: ", getOccupiedMem()