Retrieves a trailing-slashed string if the site is set for adding trailing slashes.

Conditionally adds a trailing slash if the permalink structure has a trailing
slash, strips the trailing slash if not. The string is passed through the
{@see ‘user_trailingslashit’} filter. Will remove trailing slash from string, if
site is not set to have them.

function user_trailingslashit( $string, $type_of_url = '' ) {
	global $wp_rewrite;
	if ( $wp_rewrite->use_trailing_slashes ) {
		$string = trailingslashit( $string );
	} else {
		$string = untrailingslashit( $string );
	}

	/**
	 * Filters the trailing-slashed string, depending on whether the site is set to use trailing slashes.
	 *
	 * @since 2.2.0
	 *
	 * @param string $string      URL with or without a trailing slash.
	 * @param string $type_of_url The type of URL being considered. Accepts 'single', 'single_trackback',
	 *                            'single_feed', 'single_paged', 'commentpaged', 'paged', 'home', 'feed',
	 *                            'category', 'page', 'year', 'month', 'day', 'post_type_archive'.
	 */
	return apply_filters( 'user_trailingslashit', $string, $type_of_url );
}