I believe it will be possible (I'm expecting Apple to open source it after it is out of beta as they do with their LLVM changes) and that it could then be used

I believe it will be possible (I'm expecting Apple to open source it after it is out of beta as they do with their LLVM changes) and that it could then be used on any platform LLVM supports. That doesn't mean that I think that it will be widely used, the Objective C compatibility won't be very helpful. It does integrate cleanly with C but C++ libraries need to be wrapped in C/Obj-C at the moment. I do like Swift and it quite possible it will be used on other platforms but I'm not convinced. I haven't really checked out Rust and Go properly yet but compared with C/C++ Swift might be tempting for a new project on other platforms.