mutable args: don't allow primitives

compiler/fn.v

@@ -626,17 +626,21 @@ fn (p mut Parser) fn_args(f mut Fn) {
 		if is_mut {
 			p.next()
 		}
-		mut typ2 := p.get_type()
+		mut typ := p.get_type()
+		if is_mut && is_primitive_type(typ) {
+			p.error('mutable arguments are only allowed for arrays, maps, and structs.' + 
+			'\nreturn values instead: `foo(n mut int)` => `foo(n int) int`') 
+		} 
 		for name in names {
-			if !p.first_run() && !p.table.known_type(typ2) {
-				p.error('fn_args: unknown type $typ2')
+			if !p.first_run() && !p.table.known_type(typ) { 
+				p.error('fn_args: unknown type $typ')
 			}
 			if is_mut { 
-				typ2 += '*'
+				typ += '*'
 			}
 			v := Var {
 				name: name
-				typ: typ2
+				typ: typ 
 				is_arg: true
 				is_mut: is_mut
 				ptr: is_mut

compiler/table.v

@@ -113,6 +113,10 @@ fn is_float_type(typ string) bool {
 	return FLOAT_TYPES.contains(typ)
 }
 
+fn is_primitive_type(typ string) bool {
+	return is_number_type(typ) || typ == 'string' 
+} 
+
 fn new_table(obfuscate bool) *Table {
 	mut t := &Table {
 		obf_ids: map[string]int{}

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 
 }