Make newSeqOfCap not initialize memory. (#21842)

It's used in `newSeqUninitialized`.

---------

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

compiler/ccgexprs.nim

@@ -1463,7 +1463,7 @@ proc genNewSeqOfCap(p: BProc; e: PNode; d: var TLoc) =
   var a: TLoc = initLocExpr(p, e[1])
   if optSeqDestructors in p.config.globalOptions:
     if d.k == locNone: d = getTemp(p, e.typ, needsInit=false)
-    linefmt(p, cpsStmts, "$1.len = 0; $1.p = ($4*) #newSeqPayload($2, sizeof($3), NIM_ALIGNOF($3));$n",
+    linefmt(p, cpsStmts, "$1.len = 0; $1.p = ($4*) #newSeqPayloadUninit($2, sizeof($3), NIM_ALIGNOF($3));$n",
       [d.rdLoc, a.rdLoc, getTypeDesc(p.module, seqtype.lastSon),
       getSeqPayloadType(p.module, seqtype),
     ])

lib/system.nim

@@ -925,7 +925,7 @@ proc reset*[T](obj: var T) {.noSideEffect.} =
       obj = default(typeof(obj))
 
 proc setLen*[T](s: var seq[T], newlen: Natural) {.
-  magic: "SetLengthSeq", noSideEffect.}
+  magic: "SetLengthSeq", noSideEffect, nodestroy.}
   ## Sets the length of seq `s` to `newlen`. `T` may be any sequence type.
   ##
   ## If the current length is greater than the new length,

lib/system/mm/go.nim

@@ -109,7 +109,7 @@ proc newSeqRC1(typ: PNimType, len: int): pointer {.compilerRtl.} =
   writebarrierptr(addr(result), newSeq(typ, len))
 
 proc nimNewSeqOfCap(typ: PNimType, cap: int): pointer {.compilerproc.} =
-  result = newObj(typ, align(GenericSeqSize, typ.base.align) + cap * typ.base.size)
+  result = newObjNoInit(typ, align(GenericSeqSize, typ.base.align) + cap * typ.base.size)
   cast[PGenericSeq](result).len = 0
   cast[PGenericSeq](result).reserved = cap
   cast[PGenericSeq](result).elemSize = typ.base.size

lib/system/seqs_v2.nim

@@ -50,6 +50,15 @@ proc newSeqPayload(cap, elemSize, elemAlign: int): pointer {.compilerRtl, raises
   else:
     result = nil
 
+proc newSeqPayloadUninit(cap, elemSize, elemAlign: int): pointer {.compilerRtl, raises: [].} =
+  # Used in `newSeqOfCap()`.
+  if cap > 0:
+    var p = cast[ptr NimSeqPayloadBase](alignedAlloc(align(sizeof(NimSeqPayloadBase), elemAlign) + cap * elemSize, elemAlign))
+    p.cap = cap
+    result = p
+  else:
+    result = nil
+
 template `+!`(p: pointer, s: int): pointer =
   cast[pointer](cast[int](p) +% s)
 
@@ -125,7 +134,7 @@ proc add*[T](x: var seq[T]; y: sink T) {.magic: "AppendSeqElem", noSideEffect, n
     # We also save the `wasMoved + destroy` pair for the sink parameter.
     xu.p.data[oldLen] = y
 
-proc setLen[T](s: var seq[T], newlen: Natural) =
+proc setLen[T](s: var seq[T], newlen: Natural) {.nodestroy.} =
   {.noSideEffect.}:
     if newlen < s.len:
       shrink(s, newlen)