fixes #25469; Conversion from distinct in for forces a copy of underlying instance

compiler/transf.nim

@@ -702,6 +702,11 @@ proc putArgInto(arg: PNode, formal: PType): TPutArgInto =
   of nkAddr, nkHiddenAddr:
     result = putArgInto(arg[0], formal)
     if result == paViaIndirection: result = paFastAsgn
+  of nkHiddenStdConv, nkHiddenSubConv, nkConv:
+    if compareTypes(arg.typ, arg[1].typ, dcEqIgnoreDistinct, {IgnoreRangeShallow}):
+      result = putArgInto(arg[1], formal)
+    else:
+      result = paFastAsgn
   of nkCurly, nkBracket:
     for i in 0..<arg.len:
       if putArgInto(arg[i], formal) != paDirectMapping:

tests/arc/tdistinct_for_nodup.nim

@@ -0,0 +1,41 @@
+discard """
+  cmd: '''nim c --mm:arc --expandArc:foo $file'''
+  nimout: '''
+--expandArc: foo
+
+var broken_cursor
+block :tmp:
+  var i
+  var i_1 = 0
+  let L = len(seq[Large](broken_cursor))
+  block :tmp_1:
+    while i_1 < L:
+      i = seq[Large](broken_cursor)[i_1]
+      discard i
+      {.push, overflowChecks: false.}
+      inc(i_1, 1)
+      {.pop.}
+      const
+        loc`gensym1 = (filename: "iterators.nim", line: 254, column: 10)
+        ploc`gensym1 = "iterators.nim(254, 11)"
+      bind instantiationInfo
+      mixin failedAssertImpl
+      {.line: (filename: "iterators.nim", line: 254, column: 10).}:
+        if not (len(seq[Large](broken_cursor)) == L):
+          failedAssertImpl("iterators.nim(254, 11) `len(a) == L` the length of the seq changed while iterating over it")
+-- end of expandArc ------------------------
+'''
+"""
+
+
+type
+  Large = array[1024, byte]
+  List = distinct seq[Large]
+
+proc foo =
+  var
+    broken: List
+  for i in seq[Large](broken):
+    discard i
+
+foo()