fix(preprocess): avoid shadowing of iota

gnovm/pkg/gnolang/preprocess.go

@@ -1295,6 +1295,17 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 					n.Path = NewValuePathBlock(0, 0, blankIdentifier)
 					return n, TRANS_CONTINUE
 				case "iota":
+					// iota is a non-shadowable builtin; reject declaring a new
+					// identifier named "iota" (`iota := 5` or `for iota := range x`)
+					// instead of falling through to the constant-only check below.
+					isDefine := ftype == TRANS_RANGE_KEY || ftype == TRANS_RANGE_VALUE
+					if as, ok := ns[len(ns)-1].(*AssignStmt); ok {
+						isDefine = isDefine || (ftype == TRANS_ASSIGN_LHS && as.Op == DEFINE)
+					}
+					if isDefine {
+						panic(fmt.Sprintf("builtin identifiers cannot be shadowed: %s", n.Name))
+					}
+
 					pd := lastDecl(ns)
 					valueDecl, ok := pd.(*ValueDecl)
 					if !ok || !valueDecl.Const {

gnovm/tests/files/iota_identifier.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	iota := 5 // ERROR "builtin identifiers cannot be shadowed"
+	_ = iota
+}
+
+// Error:
+// main/iota_identifier.gno:4:2-6: builtin identifiers cannot be shadowed: iota

gnovm/tests/files/iota_identifier_range.gno

@@ -0,0 +1,11 @@
+package main
+
+func main() {
+	s := []int{1, 2, 3}
+	for iota := range s { // ERROR "builtin identifiers cannot be shadowed"
+		_ = iota
+	}
+}
+
+// Error:
+// main/iota_identifier_range.gno:5:6-10: builtin identifiers cannot be shadowed: iota