Source

File: wp-includes/class-wp-feed-cache-transient.php

public function save( $data ) {
		if ( $data instanceof SimplePie ) {
			$data = $data->data;
		}

		set_transient( $this->name, $data, $this->lifetime );
		set_transient( $this->mod_name, time(), $this->lifetime );
		return true;
	}