The core language has been open source for a while. Windows support was added a few days ago. It will not allow you to develop iOS apps on Windows, as the tools

The core language has been open source for a while. Windows support was added a few days ago. It will not allow you to develop iOS apps on Windows, as the tools to do that are part of Xcode, which is not open source and not cross-platform. This announcement concerns Swift System, which provides a wrapper over UNIX functions like `open'. Since those functions come from C, they do things like using integers to represent files (``file descriptors'') and setting errno to indicate failure. System wraps them to make them into proper Swift functions that throw exceptions and make full use of the type system. It has been open-sourced and Linux support has been added.