cgen: fix fn mut argument of interface type (#13616)

vlib/v/gen/c/fn.v

@@ -1709,6 +1709,11 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 				g.write('&/*sum*/')
 				g.expr(arg.expr)
 				return
+			} else if arg_sym.kind == .interface_ && exp_sym.kind == .interface_
+				&& (arg.expr is ast.Ident || arg.expr is ast.SelectorExpr) {
+				g.write('&/*iface*/')
+				g.expr(arg.expr)
+				return
 			}
 		}
 		if !g.is_json_fn {

vlib/v/tests/fn_mut_arg_of_interface_test.v

@@ -0,0 +1,27 @@
+interface TheInterface {
+mut:
+	an_interface() ?
+}
+
+struct Implementation {
+}
+
+fn (mut i Implementation) an_interface() ? {
+	return
+}
+
+fn maker() ?TheInterface {
+	inner := Implementation{}
+	return inner
+}
+
+fn do(mut inter TheInterface) string {
+	return 'ok'
+}
+
+fn test_fn_mut_arg_of_interface() ? {
+	mut inner := maker() ?
+	ret := do(mut inner)
+	println(ret)
+	assert ret == 'ok'
+}