Outputs rel=canonical for singular queries.

function rel_canonical() {
	if ( ! is_singular() ) {
		return;
	}

	$id = get_queried_object_id();

	if ( 0 === $id ) {
		return;
	}

	$url = wp_get_canonical_url( $id );

	if ( ! empty( $url ) ) {
		echo '' . "n";
	}
}