An optimizer for ARC (#14962)

* WIP: an optimizer for ARC
* do not optimize away destructors in 'finally' if unstructured control flow is involved
* optimized the optimizer
* minor code cleanup
* first steps to .cursor inference
* cursor inference: big steps to a working solution
* baby steps
* better .cursor inference
* new feature: expandArc for easy inspection of the AST after ARC transformations
* added topt_cursor test
* adapt tests
* cleanups, make tests green
* optimize common traversal patterns
* moved test case
* fixes .cursor inference so that npeg compiles once again
* cursor inference: more bugfixes

Co-authored-by: Clyybber <darkmine956@gmail.com>

tests/destructor/tdestructor3.nim

@@ -1,5 +1,6 @@
 discard """
-  output: '''assign
+  output: '''
+assign
 destroy
 destroy
 5
@@ -104,12 +105,12 @@ test()
 #------------------------------------------------------------
 # Issue #12883
 
-type 
+type
   TopObject = object
     internal: UniquePtr[int]
 
 proc deleteTop(p: ptr TopObject) =
-  if p != nil:    
+  if p != nil:
     `=destroy`(p[]) # !!! this operation used to leak the integer
     deallocshared(p)
 
@@ -117,12 +118,12 @@ proc createTop(): ptr TopObject =
   result = cast[ptr TopObject](allocShared0(sizeof(TopObject)))
   result.internal = newUniquePtr(1)
 
-proc test2() = 
+proc test2() =
   let x = createTop()
   echo $x.internal
   deleteTop(x)
 
-echo "---------------"  
+echo "---------------"
 echo "app begin"
 test2()
 echo "app end"

tests/destructor/tmisc_destructors.nim

@@ -21,11 +21,13 @@ proc `=sink`(dest: var Foo, src: Foo) =
 proc `=`(dest: var Foo, src: Foo) =
   assign_counter.inc
 
+proc createFoo(): Foo = Foo(boo: 0)
+
 proc test(): auto =
-  var a, b: Foo
+  var a, b = createFoo()
   return (a, b, Foo(boo: 5))
 
-var (a, b, _) = test()
+var (ag, bg, _) = test()
 
 doAssert assign_counter == 0
 doAssert sink_counter == 0

tests/destructor/tuse_result_prevents_sinks.nim

@@ -17,15 +17,20 @@ proc `=sink`(self: var Foo; other: Foo) =
 
 proc `=destroy`(self: var Foo) = discard
 
+template preventCursorInference(x) =
+  let p = unsafeAddr(x)
+
 proc test(): Foo =
   result = Foo()
   let temp = result
+  preventCursorInference temp
   doAssert temp.i > 0
   return result
 
 proc testB(): Foo =
   result = Foo()
   let temp = result
+  preventCursorInference temp
   doAssert temp.i > 0
 
 discard test()