ARC: fixes memory leaks with newSeq used in a loop [backport:1.4] (#18040)

* ARC: fixes memory leaks with newSeq used in a loop [backport:1.4]
* Update tests/arc/tnewseq_legacy.nim

Author: Araq

compiler/ccgexprs.nim

@@ -2319,7 +2319,12 @@ proc genMagicExpr(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
       gcUsage(p.config, e)
     else:
       genNewFinalize(p, e)
-  of mNewSeq: genNewSeq(p, e)
+  of mNewSeq:
+    if optSeqDestructors in p.config.globalOptions:
+      e[1] = makeAddr(e[1], p.module.idgen)
+      genCall(p, e, d)
+    else:
+      genNewSeq(p, e)
   of mNewSeqOfCap: genNewSeqOfCap(p, e, d)
   of mSizeOf:
     let t = e[1].typ.skipTypes({tyTypeDesc})

lib/system/seqs_v2.nim

@@ -124,3 +124,7 @@ proc setLen[T](s: var seq[T], newlen: Natural) =
       if xu.p == nil or xu.p.cap < newlen:
         xu.p = cast[typeof(xu.p)](prepareSeqAdd(oldLen, xu.p, newlen - oldLen, sizeof(T), alignof(T)))
       xu.len = newlen
+
+proc newSeq[T](s: var seq[T], len: Natural) =
+  shrink(s, 0)
+  setLen(s, len)

tests/arc/tnewseq_legacy.nim

@@ -0,0 +1,13 @@
+discard """
+  output: "(allocCount: 201, deallocCount: 201)"
+  cmd: "nim c --gc:orc -d:nimAllocStats $file"
+"""
+
+proc main(prefix: string) =
+  var c: seq[string]
+  for i in 0..<100:
+    newSeq(c, 100)
+    c[i] = prefix & $i
+
+main("abc")
+echo getAllocStats()