don't require an implementation for procs marked with .error; activate the move optimizer for destructors

tests/destructor/tprevent_assign.nim

@@ -0,0 +1,33 @@
+discard """
+  errormsg: "'=' is not available for type <Foo>; requires a copy because it's not the last read of 'otherTree'"
+  line: 29
+"""
+
+type
+  Foo = object
+    x: int
+
+proc `=destroy`(f: var Foo) = f.x = 0
+proc `=`(a: var Foo; b: Foo) {.error.} # = a.x = b.x
+proc `=sink`(a: var Foo; b: Foo) = a.x = b.x
+
+proc createTree(x: int): Foo =
+  Foo(x: x)
+
+proc take2(a, b: sink Foo) =
+  echo a.x, " ", b.x
+
+proc allowThis() =
+  # all these temporary lets are harmless:
+  let otherTree = createTree(44)
+  let b = otherTree
+  let c = b
+  take2(createTree(34), c)
+
+proc preventThis() =
+  let otherTree = createTree(44)
+  let b = otherTree
+  take2(createTree(34), otherTree)
+
+allowThis()
+preventThis()