checker: disallow for mut letter in 'abc' { (#17234)

vlib/v/checker/for.v

@@ -113,6 +113,9 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 				}
 				else {}
 			}
+		} else if node.val_is_mut {
+			c.error('string type is immutable, it cannot be changed', node.pos)
+			return
 		}
 		if sym.kind == .struct_ {
 			// iterators
@@ -139,8 +142,6 @@ 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 == .any {
 			node.cond_type = typ
 			node.kind = sym.kind

vlib/v/checker/tests/for_in_mut_string.out

@@ -5,10 +5,10 @@ vlib/v/checker/tests/for_in_mut_string.vv:19:6: error: string type is immutable,
       |         ~~~
    20 |         println(ch)
    21 |     }
-vlib/v/checker/tests/for_in_mut_string.vv:20:3: error: `println` can not print void expressions
+vlib/v/checker/tests/for_in_mut_string.vv:23:6: error: string type is immutable, it cannot be changed
-   18 | fn wrap_text(mut gv Grid) {
-   19 |     for mut ch in gv.header {
-   20 |         println(ch)
-      |         ~~~~~~~~~~~
    21 |     }
-   22 | }
+   22 |
+   23 |     for mut letter in 'abc' {
+      |         ~~~
+   24 |         bit := letter - 1
+   25 |         println(bit)

vlib/v/checker/tests/for_in_mut_string.vv

@@ -19,4 +19,9 @@ fn wrap_text(mut gv Grid) {
 	for mut ch in gv.header {
 		println(ch)
 	}
+
+	for mut letter in 'abc' {
+		bit := letter - 1
+		println(bit)
+	}
 }