From 6438512529a6ba784decd62ae9ae92703c2d0bfa Mon Sep 17 00:00:00 2001
From: yuyi <yuyi98@163.com>
Date: Fri, 31 Dec 2021 15:54:06 +0800
Subject: [PATCH] checker: add error for `for mut var in string {` (fix #12998)
 (#13000)

---
 vlib/v/checker/checker.v                   |  2 ++
 vlib/v/checker/tests/for_in_mut_string.out | 14 ++++++++++++++
 vlib/v/checker/tests/for_in_mut_string.vv  | 22 ++++++++++++++++++++++
 3 files changed, 38 insertions(+)
 create mode 100644 vlib/v/checker/tests/for_in_mut_string.out
 create mode 100644 vlib/v/checker/tests/for_in_mut_string.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 2ad90a7977..43fd827491 100644
--- a/vlib/v/checker/checker.v
+++ b/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(
diff --git a/vlib/v/checker/tests/for_in_mut_string.out b/vlib/v/checker/tests/for_in_mut_string.out
new file mode 100644
index 0000000000..989006ae91
--- /dev/null
+++ b/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 | }
diff --git a/vlib/v/checker/tests/for_in_mut_string.vv b/vlib/v/checker/tests/for_in_mut_string.vv
new file mode 100644
index 0000000000..d9779394cd
--- /dev/null
+++ b/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)
+	}
+}