From 99b09e6609e5012a237b7ea34aed86c67328cf3f Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Tue, 9 Sep 2025 22:16:12 +0800
Subject: [PATCH] fixes #24093; Dereferencing result of cast in single
 expression triggers unnecessary copy (#25143)

fixes #24093

transforms
```nim
let a = new array[1000, byte]
block:
  for _ in cast[typeof(a)](a)[]:
    discard
```
into
```nim
let a = new array[1000, byte]
block:
  let temp = cast[typeof(a)](a)
  for _ in temp[]:
    discard
```
So it keeps the same behavior with the manual version

(cherry picked from commit 08d74a1c272c25cbc7a0b5c2a9288a06cc23f619)
---
 compiler/transf.nim | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/compiler/transf.nim b/compiler/transf.nim
index c6f0dbb332..197e073477 100644
--- a/compiler/transf.nim
+++ b/compiler/transf.nim
@@ -828,12 +828,20 @@ proc transformFor(c: PTransf, n: PNode): PNode =
         t = formal.ast.typ # better use the type that actually has a destructor.
       elif t.destructor == nil and arg.typ.destructor != nil:
         t = arg.typ
-      # generate a temporary and produce an assignment statement:
-      var temp = newTemp(c, t, formal.info)
-      #incl(temp.sym.flags, sfCursor)
-      addVar(v, temp)
-      stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg, true))
-      newC.mapping[formal.itemId] = temp
+
+      if arg.kind in {nkDerefExpr, nkHiddenDeref}:
+        # optimizes for `[]` # bug #24093
+        var temp = newTemp(c, arg[0].typ, formal.info)
+        addVar(v, temp)
+        stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg[0], true))
+        newC.mapping[formal.itemId] = newDeref(temp)
+      else:
+        # generate a temporary and produce an assignment statement:
+        var temp = newTemp(c, t, formal.info)
+        #incl(temp.sym.flags, sfCursor)
+        addVar(v, temp)
+        stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg, true))
+        newC.mapping[formal.itemId] = temp
     of paVarAsgn:
       assert(skipTypes(formal.typ, abstractInst).kind in {tyVar, tyLent})
       newC.mapping[formal.itemId] = arg