fixes #25925 (#25926)

Author: Araq

compiler/cgen.nim

@@ -838,8 +838,12 @@ proc assignGlobalVar(p: BProc, n: PNode; value: Rope) =
       else:
         initializer = value
       genGlobalVarDecl(p.module.s[cfsVars], p, n, td, initializer = initializer)
-  if p.withinLoop > 0 and value == "":
+  if p.withinLoop > 0 and value == "" and
+      s.loc.t.skipTypes(abstractInst).kind notin {tyVar, tyLent}:
     # fixes tests/run/tzeroarray:
+    # Don't reset borrowed references (var/lent): the pointer itself is still
+    # uninitialized here, so resetLoc would dereference garbage. Such variables
+    # (e.g. the loop var of `mitems`) are always assigned before use anyway.
     backendEnsureMutable s
     resetLoc(p, s.locImpl)
 

tests/arc/tmitems_loopvar.nim

@@ -0,0 +1,28 @@
+discard """
+  output: '''a
+b
+c
+a
+b
+c'''
+"""
+
+# A `var T` loop variable (here from `mitems`) declared inside an enclosing
+# loop must not be reset by dereferencing: at module scope it is emitted as a
+# global, and the in-loop reset path used `resetLoc`, which dereferenced the
+# still-uninitialized borrowed pointer and crashed with a SIGSEGV.
+
+for p in @["abc", "123"]:
+  var testA = @["a", "b", "c"]
+  for l in testA.mitems:
+    echo(l)
+
+# also exercise actual mutation through the borrowed reference
+block:
+  var ok = true
+  for p in @["x", "y"]:
+    var s = @[1, 2, 3]
+    for l in s.mitems:
+      l += 10
+    if s != @[11, 12, 13]: ok = false
+  doAssert ok