Motorola rolled out Marshmallow to the 2015 Moto X Pure a few months ago, but now the first factory image is available for download. Well, "available" might not be the right word. We're under the impression that it exists, and you can theoretically obtain it, but you have to ask Motorola for the privilege.
The new Marshmallow factory image has build number MPH24.49-18_18, which is the same as the regular OTA'd device. Like Nexus factory images, you can use Motorola's image to flash your device to the stock software if you've done something to mess it up or if you installed one of those leaked soak tests that didn't have an upgrade path. You will have to unlock your bootloader, which requires getting a code from Motorola.
To get the factory image for Marshmallow, you need to log in with a Motorola or Google account, then request access to a Google Drive folder. It takes anywhere from a few hours to a few days for Motorola to approve access. I have no idea why they do it this way, but it's been like this since the OG Moto X. If you think you'll need it at some point, you might as well request access now.