Officially supported ports of Swift currently include those for Apple’s platforms and also Linux, which is what the company promised back in June when it made the original announcement. That covers a lot of ground for Swift developers (who will generally already be running OS X) and developers in general (who probably use Linux more widely than the general populace). Apple’s Linux tools include support for the nascent package manager, the LLDB debugger, and the REPL command-line environment.
As for other platforms, including Windows, at least initially Apple is going to lean on the community to provide that support if there’s demand for it.
“[Windows support] wasn’t something that in terms of us catalyzing this initial release that we wanted to take on directly,” Federighi told us. “We think Linux and of course our platforms provide a great foundation to get started. But we’re very open to ports to other platforms being contributed to the core project, and certainly given that LLVM, Clang, and LLDB, which are foundational technologies to Swift, are already available, already ported to Windows. I think it’s foreseeable that someone in the community, whether led by Microsoft or others, would be doing that port.”