add_action( 'init', 'ASHmostrecent_init' );
function ASHmostrecent_init(){
add_rewrite_rule( 'last/?$', 'index.php?post_type=post&posts_per_page=1');
}
A query already pulls up posts starting from the latest to the oldest, so all you have to do is limit what’s returned by one. posts_per_page does this; limiting by one will give you the latest post. Edit the post_type accordingly.