fixes #25251; SIGBUS with iterator over const Table lookup - premature temporary destruction (#25255)

fixes #25251

enforce a copy if the arg is a deref of a lent pointer since the arg
could be a temporary that will go out of scope

(cherry picked from commit 6f73094263)

compiler/transf.nim

@@ -828,8 +828,11 @@ proc transformFor(c: PTransf, n: PNode): PNode =
       elif t.destructor == nil and arg.typ.destructor != nil:
         t = arg.typ
 
-      if arg.kind in {nkDerefExpr, nkHiddenDeref}:
+      if arg.kind in {nkDerefExpr, nkHiddenDeref} and
+          arg[0].typ.skipTypes(abstractInst).kind != tyLent:
         # optimizes for `[]` # bug #24093
+        # bug #25251: enforce a copy if the arg is a deref of a lent pointer
+        # since the arg could be a temporary that will go out of scope
         var temp = newTemp(c, arg[0].typ, formal.info)
         addVar(v, temp)
         stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg[0], true))

tests/iter/titer_issues.nim

@@ -432,3 +432,28 @@ block:
   let x = cast[typeof(aaa)](aaa)   # not even var
   for _ in x[]:
     discard
+
+import std/[tables, unicode, sequtils]
+
+const
+    myTable = {
+        "en": "abcdefghijklmnopqrstuvwxyz",
+    }.toTable
+
+proc buggyVersion(locale: string): seq[Rune] =
+    result = toSeq(runes(myTable[locale]))
+
+proc workingVersion(locale: string): seq[Rune] =
+    # string lifetime is extended
+    let str = myTable[locale]
+    result = toSeq(runes(str))
+
+# echo "Testing working version..."
+let runes2 = workingVersion("en")
+# echo "Got ", runes2.len, " runes"
+
+# echo "Testing buggy version..."
+let runes1 = buggyVersion("en")  # <-- CRASHES HERE
+
+doAssert runes1.len == runes2.len
+# echo "Got ", runes1.len, " runes"