Source

File: wp-includes/link-template.php

function get_feed_link( $feed = '' ) {
	global $wp_rewrite;

	$permalink = $wp_rewrite->get_feed_permastruct();

	if ( '' !== $permalink ) {
		if ( false !== strpos( $feed, 'comments_' ) ) {
			$feed      = str_replace( 'comments_', '', $feed );
			$permalink = $wp_rewrite->get_comment_feed_permastruct();
		}

		if ( get_default_feed() == $feed ) {
			$feed = '';
		}

		$permalink = str_replace( '%feed%', $feed, $permalink );
		$permalink = preg_replace( '#/+#', '/', "/$permalink" );
		$output    = home_url( user_trailingslashit( $permalink, 'feed' ) );
	} else {
		if ( empty( $feed ) ) {
			$feed = get_default_feed();
		}

		if ( false !== strpos( $feed, 'comments_' ) ) {
			$feed = str_replace( 'comments_', 'comments-', $feed );
		}

		$output = home_url( "?feed={$feed}" );
	}

	/**
	 * Filters the feed type permalink.
	 *
	 * @since 1.5.0
	 *
	 * @param string $output The feed permalink.
	 * @param string $feed   The feed type. Possible values include 'rss2', 'atom',
	 *                       or an empty string for the default feed type.
	 */
	return apply_filters( 'feed_link', $output, $feed );
}