mutable args: don't allow primitives

compiler/tests/fn_test.v

@@ -71,7 +71,7 @@ fn modify_array(a mut []int) {
 	a << 888 
 } 
 
-fn test_mut() {
+fn test_mut_array() {
 	mut nums := [1, 2, 3] 
 	modify_array(mut nums) 
 	assert nums.len == 4 
@@ -81,6 +81,31 @@ fn test_mut() {
 	assert nums[3] == 888 
 } 
 
+fn mod_struct(user mut User) {
+	user.age++ 
+}
+
+struct User {
+mut: 
+	age int 
+} 
+
+fn test_mut_struct() {
+	mut user := User{18} 
+	mod_struct(mut user)
+	assert user.age == 19 
+} 
+
+fn mod_ptr(buf mut byteptr) {
+	buf[0] = 77 
+} 
+
+fn test_mut_ptr() {
+	buf := malloc(10) 
+	mod_ptr(mut buf) 
+	assert buf[0] == 77 
+} 
+
 fn test_fns() {
 	// no asserts for now, just test function declarations above 
 }