get_search_feed_link( string $search_query = '', string $feed = '' )

function get_search_feed_link($search_query = '', $feed = '') {
	global $wp_rewrite;
	$link = get_search_link($search_query);

	if ( empty($feed) )
		$feed = get_default_feed();

	$permastruct = $wp_rewrite->get_search_permastruct();

	if ( empty($permastruct) ) {
		$link = add_query_arg('feed', $feed, $link);
	} else {
		$link = trailingslashit($link);
		$link .= "feed/$feed/";
	}

	/**
	 * Filters the search feed link.
	 *
	 * @since WP-2.5.0
	 *
	 * @param string $link Search feed link.
	 * @param string $feed Feed type.
	 * @param string $type The search type. One of 'posts' or 'comments'.
	 */
	return apply_filters( 'search_feed_link', $link, $feed, 'posts' );
}