You wouldn't be able to compile applications/libraries made specifically for Apple platforms using open-source Swift. They use libraries such as Cocoa, UIKit, etc which are only available on Apple platforms.
@PulkoMandy I think cocotron aims to be the open-source reimplementation of the Cocoa APIs. However it uses the Objective-C runtime, so that won't be useful for the Swift port anyway.
Also, open-source Swift has no way of directly calling C++ code, so you can't use the Haiku APIs without some sort of C interface to them, like libcharlemagne or Habid.
Although these libraries are very outdated, the former did have a working demo of calling the Haiku API from C and then to Lua.