parser: disallow `for mut in range` (fix #12234) (#12277)

vlib/builtin/js/array.js.v

@@ -15,7 +15,7 @@ struct array_buffer {
 fn (mut a array_buffer) make_copy() {
 	if a.index_start != 0 || a.has_slice {
 		mut new_arr := JS.makeEmtpyJSArray()
-		for mut i in 0 .. a.len {
+		for i in 0 .. a.len {
 			#new_arr.push(a.val.get(i))
 
 			mut x := i

vlib/v/parser/for.v

@@ -153,6 +153,9 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 				return p.error_with_pos('cannot declare index variable with range `for`',
 					key_var_pos)
 			}
+			if val_is_mut {
+				return p.error_with_pos('variable in range `for` cannot be mut', mut_pos)
+			}
 		} else {
 			// this type will be set in checker
 			p.scope.register(ast.Var{

vlib/v/parser/tests/for.out

@@ -1,4 +0,0 @@
-vlib/v/parser/tests/for.vv:1:5: error: cannot declare index variable with range `for`
-    1 | for i, k in 0..5 {
-      |     ^
-    2 | }

vlib/v/parser/tests/for_index_in_range.out

@@ -0,0 +1,4 @@
+vlib/v/parser/tests/for_index_in_range.vv:1:5: error: cannot declare index variable with range `for`
+    1 | for i, k in 0..5 {
+      |     ^
+    2 | }

vlib/v/parser/tests/for_mut_in_range.out

@@ -0,0 +1,4 @@
+vlib/v/parser/tests/for_mut_in_range.vv:1:5: error: variable in range `for` cannot be mut
+    1 | for mut i in 0..5 {
+      |     ~~~
+    2 | }

vlib/v/parser/tests/for_mut_in_range.vv

@@ -0,0 +1,2 @@
+for mut i in 0..5 {
+}