cgen: fix fn call with mut reference args (fix #21265) (#21719)

vlib/v/gen/c/fn.v

@@ -2589,6 +2589,11 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 	mut needs_closing := false
 	if arg.is_mut && !exp_is_ptr {
 		g.write('&/*mut*/')
+	} else if arg.is_mut && arg_typ.is_ptr() && expected_type.is_ptr()
+		&& g.table.sym(arg_typ).kind == .struct_ && expected_type == arg_typ.ref() {
+		g.write('&/*mut*/')
+		g.expr(arg.expr)
+		return
 	} else if exp_is_ptr && !arg_is_ptr && !(arg_sym.kind == .alias
 		&& g.table.unaliased_type(arg_typ).is_pointer() && expected_type.is_pointer()) {
 		if arg.is_mut {

vlib/v/tests/fn_call_mut_ref_args_test.v

@@ -0,0 +1,23 @@
+@[heap]
+struct Client {
+mut:
+	next &Client = unsafe { nil }
+	prev &Client = unsafe { nil }
+}
+
+fn init_vm1(mut head &Client) {
+	for c := head; c; c = c.next {
+	}
+}
+
+fn init_vm2(mut head &Client) {
+	for c := head; c == unsafe { nil }; c = c.next {
+	}
+}
+
+fn test_fn_call_mut_ref_args() {
+	mut head := &Client{}
+	init_vm1(mut head)
+	init_vm2(mut head)
+	assert true
+}

vlib/v/tests/in_expression_test.v

@@ -337,6 +337,6 @@ fn in_both_mut_ref(mut arr []&Bar, mut bar &Bar) {
 
 fn test_in_both_mut_and_ref() {
 	mut arr := []&Bar{}
-	mut bar := Bar{}
-	in_both_mut_ref(mut &arr, mut &bar)
+	mut bar := &Bar{}
+	in_both_mut_ref(mut &arr, mut bar)
 }