From 8f78c8de60b75e7f4528fe815f2fc3a9a4c1a803 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Sat, 4 Jul 2026 05:14:13 +0800
Subject: [PATCH] fixes #25956; mapIt pointlessly does extra zeroing which,
 e.g., newSeqWith often avoids (#25957)

fixes #25956
---
 lib/pure/collections/sequtils.nim | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/lib/pure/collections/sequtils.nim b/lib/pure/collections/sequtils.nim
index 299349a05a..8d24d7d492 100644
--- a/lib/pure/collections/sequtils.nim
+++ b/lib/pure/collections/sequtils.nim
@@ -121,6 +121,13 @@ template unCheckedInc(x) =
   inc(x)
   {.pop.}
 
+template newSeqForOverwrite(T: typedesc; len: int): untyped =
+  ## Allocates a fixed-length seq whose elements will be assigned by index.
+  when supportsCopyMem(T) and declared(newSeqUninit):
+    newSeqUninit[T](len)
+  else: # TODO: use `newSeqUnsafe` when that's available
+    newSeq[T](len)
+
 func concat*[T](seqs: varargs[seq[T]]): seq[T] =
   ## Takes several sequences' items and returns them inside a new sequence.
   ## All sequences must be of the same type.
@@ -1105,7 +1112,7 @@ template mapIt*(s: typed, op: untyped): untyped =
         evalOnceAs(s2, s, compiles((let _ = s)))
 
         var i = 0
-        var result = newSeq[OutType](s2.len)
+        var result = newSeqForOverwrite(OutType, s2.len)
         for it {.inject.} in s2:
           result[i] = op
           i += 1
@@ -1171,10 +1178,7 @@ template newSeqWith*(len: int, init: untyped): untyped =
     assert seqRand[0] != seqRand[1]
   type T = typeof(init)
   let newLen = len
-  when supportsCopyMem(T) and declared(newSeqUninit):
-    var result = newSeqUninit[T](newLen)
-  else: # TODO: use `newSeqUnsafe` when that's available
-    var result = newSeq[T](newLen)
+  var result = newSeqForOverwrite(T, newLen)
   for i in 0 ..< newLen:
     result[i] = init
   move(result) # refs bug #7295