parser: check for_in mut key (#8235)

vlib/v/parser/for.v

@@ -83,6 +83,7 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 	{
 		// `for i in vals`, `for i in start .. end`, `for mut user in users`, `for i, mut user in users`
 		mut val_is_mut := p.tok.kind == .key_mut
+		mut_pos := p.tok.position()
 		if val_is_mut {
 			p.next()
 		}
@@ -91,6 +92,9 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 		mut key_var_name := ''
 		mut val_var_name := p.check_name()
 		if p.tok.kind == .comma {
+			if val_is_mut {
+				p.error_with_pos('index of array or key of map cannot be mutated', mut_pos)
+			}
 			p.next()
 			if p.tok.kind == .key_mut {
 				// `for i, mut user in users {`

vlib/v/parser/tests/for_in_mut_index_of_array.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/for_in_mut_index_of_array.vv:3:6: error: index of array or key of map cannot be mutated
+    1 | fn main() {
+    2 |     mut m := [1, 2, 3]
+    3 |     for mut i, _ in m {
+      |         ~~~
+    4 |         println(i)
+    5 |     }

vlib/v/parser/tests/for_in_mut_index_of_array.vv

@@ -0,0 +1,6 @@
+fn main() {
+	mut m := [1, 2, 3]
+	for mut i, _ in m {
+		println(i)
+	}
+}

vlib/v/parser/tests/for_in_mut_key_of_map.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/for_in_mut_key_of_map.vv:3:6: error: index of array or key of map cannot be mutated
+    1 | fn main() {
+    2 |     mut m := {'foo': 1, 'bar': 2}
+    3 |     for mut k, _ in m {
+      |         ~~~
+    4 |         println(k)
+    5 |     }

vlib/v/parser/tests/for_in_mut_key_of_map.vv

@@ -0,0 +1,6 @@
+fn main() {
+	mut m := {'foo': 1, 'bar': 2}
+	for mut k, _ in m {
+		println(k)
+	}
+}