do_feed()

function do_feed() {
	global $wp_query;

	$feed = get_query_var( 'feed' );

	// Remove the pad, if present.
	$feed = preg_replace( '/^_+/', '', $feed );

	if ( $feed == '' || $feed == 'feed' )
		$feed = get_default_feed();

	if ( ! has_action( "do_feed_{$feed}" ) ) {
		wp_die( __( 'ERROR: This is not a valid feed template.' ), '', array( 'response' => 404 ) );
	}

	/**
	 * Fires once the given feed is loaded.
	 *
	 * The dynamic portion of the hook name, `$feed`, refers to the feed template name.
	 * Possible values include: 'rdf', 'rss', 'rss2', and 'atom'.
	 *
	 * @since WP-2.1.0
	 * @since WP-4.4.0 The `$feed` parameter was added.
	 *
	 * @param bool   $is_comment_feed Whether the feed is a comment feed.
	 * @param string $feed            The feed name.
	 */
	do_action( "do_feed_{$feed}", $wp_query->is_comment_feed, $feed );
}