fixes cast expressions introduces unnecessary copies (#24004)

It speeds up
```nim
proc foo =
  let piece = cast[seq[char]](newSeqUninit[uint8](5220600386'i64))

foo()
```

Notes that `cast[ref](...)` is excluded because we need to keep the ref
alive if the parameter is something with pointer types (e.g.
`cast[ref](pointer)`or `cast[ref](makePointer(...))`)

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

compiler/injectdestructors.nim

@@ -501,7 +501,7 @@ proc containsConstSeq(n: PNode): bool =
     return true
   result = false
   case n.kind
-  of nkExprEqExpr, nkExprColonExpr, nkHiddenStdConv, nkHiddenSubConv:
+  of nkExprEqExpr, nkExprColonExpr, nkHiddenStdConv, nkHiddenSubConv, nkCast:
     result = containsConstSeq(n[1])
   of nkObjConstr, nkClosure:
     for i in 1..<n.len:
@@ -829,6 +829,9 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode; tmpFlags = {sfSing
     elif n.kind in {nkObjDownConv, nkObjUpConv}:
       result = copyTree(n)
       result[0] = p(n[0], c, s, sinkArg)
+    elif n.kind == nkCast and n.typ.skipTypes(abstractInst).kind in {tyString, tySequence}:
+      result = copyTree(n)
+      result[1] = p(n[1], c, s, sinkArg)
     elif n.typ == nil:
       # 'raise X' can be part of a 'case' expression. Deal with it here:
       result = p(n, c, s, normal)

tests/destructor/tatomicptrs.nim

@@ -143,11 +143,13 @@ proc `=sink`*[T](m: var MySeq[T], m2: MySeq[T]) {.inline.} =
       `=destroy`(m)
     m.len = m2.len
     m.data = m2.data
+    m.refcount = m2.refcount
 
 proc len*[T](m: MySeq[T]): int {.inline.} = m.len
 
 proc newMySeq*[T](size: int, initial_value: T): MySeq[T] =
   result.len = size
+  result.refcount = 1
   if size > 0:
     result.data = cast[ptr UncheckedArray[T]](allocShared(sizeof(T) * size))
 

tests/destructor/tv2_cast.nim

@@ -10,69 +10,41 @@ destroying O1'''
 var
   data
   :tmpD
-  :tmpD_1
-  :tmpD_2
-data =
-  :tmpD = `=dup`(cast[string](
-    :tmpD_2 = encode(cast[seq[byte]](
-      :tmpD_1 = newString(100)
-      :tmpD_1))
-    :tmpD_2))
-  :tmpD
-`=destroy`(:tmpD_2)
-`=destroy_1`(:tmpD_1)
-`=destroy_1`(data)
+data = cast[string](encode(cast[seq[byte]](
+  :tmpD = newString(100)
+  :tmpD)))
+`=destroy`(:tmpD)
+`=destroy`(data)
 -- end of expandArc ------------------------
 --expandArc: main1
 
 var
   s
   data
-  :tmpD
-  :tmpD_1
 s = newString(100)
-data =
-  :tmpD = `=dup`(cast[string](
-    :tmpD_1 = encode(toOpenArrayByte(s, 0, len(s) - 1))
-    :tmpD_1))
-  :tmpD
-`=destroy`(:tmpD_1)
-`=destroy_1`(data)
-`=destroy_1`(s)
+data = cast[string](encode(toOpenArrayByte(s, 0, len(s) - 1)))
+`=destroy`(data)
+`=destroy`(s)
 -- end of expandArc ------------------------
 --expandArc: main2
 
 var
   s
   data
-  :tmpD
-  :tmpD_1
 s = newSeq(100)
-data =
-  :tmpD = `=dup`(cast[string](
-    :tmpD_1 = encode(s)
-    :tmpD_1))
-  :tmpD
-`=destroy`(:tmpD_1)
-`=destroy_1`(data)
-`=destroy`(s)
+data = cast[string](encode(s))
+`=destroy`(data)
+`=destroy_1`(s)
 -- end of expandArc ------------------------
 --expandArc: main3
 
 var
   data
   :tmpD
-  :tmpD_1
-  :tmpD_2
-data =
-  :tmpD = `=dup`(cast[string](
-    :tmpD_2 = encode do:
-      :tmpD_1 = newSeq(100)
-      :tmpD_1
-    :tmpD_2))
-  :tmpD
-`=destroy`(:tmpD_2)
-`=destroy`(:tmpD_1)
+data = cast[string](encode do:
+  :tmpD = newSeq(100)
+  :tmpD)
+`=destroy`(:tmpD)
 `=destroy_1`(data)
 -- end of expandArc ------------------------
 '''