XWayland is the compatibility layer for running legacy X11 applications atop Wayland. The XWayland code has been baking for a while and as of a few hours ago the initial support was finally merged. This XWayland merging came just in time as the merge window for the six-month update, X.Org Server 1.16, is soon closing.
For those curious about the initial XWayland details for X.Org Server 1.16, read my article last week. The code merged is for XWayland as its own DDX component rather than xorg-server module. This is good for being universal amongst the open-source drivers in using GLAMOR for 2D acceleration, but it might cause problems for NVIDIA's binary driver.
X.Org Server 1.16 is anticipated for release in July. Merged along with the initial XWayland DDX support were also many GLAMOR acceleration improvements.