checker: add error for for mut var in string { (fix #12998) (#13000)

vlib/v/checker/checker.v

@@ -2590,6 +2590,8 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 			node.kind = sym.kind
 			node.val_type = val_type
 			node.scope.update_var_type(node.val_var, val_type)
+		} else if sym.kind == .string && node.val_is_mut {
+			c.error('string type is immutable, it cannot be changed', node.pos)
 		} else {
 			if sym.kind == .map && !(node.key_var.len > 0 && node.val_var.len > 0) {
 				c.error(

vlib/v/checker/tests/for_in_mut_string.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/for_in_mut_string.vv:19:6: error: string type is immutable, it cannot be changed
+   17 |
+   18 | fn wrap_text(mut gv Grid) {
+   19 |     for mut ch in gv.header {
+      |         ~~~
+   20 |         println(ch)
+   21 |     }
+vlib/v/checker/tests/for_in_mut_string.vv:20:3: error: `println` can not print void expressions
+   18 | fn wrap_text(mut gv Grid) {
+   19 |     for mut ch in gv.header {
+   20 |         println(ch)
+      |         ~~~~~~~~~~~
+   21 |     }
+   22 | }

vlib/v/checker/tests/for_in_mut_string.vv

@@ -0,0 +1,22 @@
+module main
+
+[heap]
+pub struct Grid {
+pub mut:
+	header string
+}
+
+fn main() {
+	h := 'yo'
+
+	mut grid := Grid{
+		header: h
+	}
+	wrap_text(mut grid)
+}
+
+fn wrap_text(mut gv Grid) {
+	for mut ch in gv.header {
+		println(ch)
+	}
+}