get_post_comments_feed_link( int $post_id, string $feed = '' )

function get_post_comments_feed_link( $post_id = 0, $feed = '' ) {
	$post_id = absint( $post_id );

	if ( ! $post_id )
		$post_id = get_the_ID();

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

	$post = get_post( $post_id );
	$unattached = 'attachment' === $post->post_type && 0 === (int) $post->post_parent;

	if ( '' != get_option('permalink_structure') ) {
		if ( 'page' == get_option('show_on_front') && $post_id == get_option('page_on_front') )
			$url = _get_page_link( $post_id );
		else
			$url = get_permalink($post_id);

		if ( $unattached ) {
			$url =  home_url( '/feed/' );
			if ( $feed !== get_default_feed() ) {
				$url .= "$feed/";
			}
			$url = add_query_arg( 'attachment_id', $post_id, $url );
		} else {
			$url = trailingslashit($url) . 'feed';
			if ( $feed != get_default_feed() )
				$url .= "/$feed";
			$url = user_trailingslashit($url, 'single_feed');
		}
	} else {
		if ( $unattached ) {
			$url = add_query_arg( array( 'feed' => $feed, 'attachment_id' => $post_id ), home_url( '/' ) );
		} elseif ( 'page' == $post->post_type ) {
			$url = add_query_arg( array( 'feed' => $feed, 'page_id' => $post_id ), home_url( '/' ) );
		} else {
			$url = add_query_arg( array( 'feed' => $feed, 'p' => $post_id ), home_url( '/' ) );
		}
	}

	/**
	 * Filters the post comments feed permalink.
	 *
	 * @since WP-1.5.1
	 *
	 * @param string $url Post comments feed permalink.
	 */
	return apply_filters( 'post_comments_feed_link', $url );
}