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>

1 files changed, 4 insertions, 1 deletions

diff --git a/compiler/injectdestructors.nim b/compiler/injectdestructors.nim
index 92c823f37..c0bf374d0 100644
--- a/compiler/injectdestructors.nim
+++ b/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)