Well Swift already builds on Windows, but obviously that does not come with much of what you might use to write a Mac app. The biggest holes to fill are those not covered by the open source version of Foundation (which thankfully, Apple is working on filling) and those left by the absence of AppKit/SwiftUI. So most of their hole-plugging is going to be centered around SwiftUI and whatever bits of AppKit beyond SwiftUI they may be using.

The SwiftUI API is mostly "just" a DSL built with result builders[0] and should be reproducible without too much trouble. The harder part is reimplementing all of the behavior concerning diffing, rendering, etc, and actually drawing the widgets. Last I knew, their plan is to use community built WinRT/WinUI Swift bindings (perhaps this[1]) as a starting point to marry native Windows widgets to their recreated API. It may be necessary to write some widgets from scratch though, because there are several types that WinUI currently lacks.

It's a gargantuan task they've taken on, but I'm watching intently because despite being the world's most popular platform, the native Windows dev story is currently pretty underwhelming.

[0]: https://www.swiftbysundell.com/articles/deep-dive-into-swift... [1]: https://github.com/ericsink/SwiftWinRT