We have been told a couple of times now that a “pro” EOS R mirrorless camera that is in the works will have some kind of a hybrid EF/RF lens mount. So this basically means that you'd be able to mount both types of lenses without the use of an adapter.
We were also told that this was being developed for the “EOS-1 style EOS R camera”, which would make a lot of sense. As I don't believe the reduction of size for an “EOS-1” style mirrorless camera is all that important.
This is definitely a bit of a strange rumour, but if Canon could pull this off, it would be great for Canon shooters.
We'll have some more information on this soon.