Last time I looked the automatic differentiation was in a compiler branch with no immediate plans to merge in master. But overall it is promising. I even installed Swift on Linux to play with it, didn't get to ML as I have an AMD GPU and this is a can of worms. Hope it's finished one day. I would prefer for Julia ml libraries to become mainstream. But, it is what it is. Also, the ideal for me would be Rust for tensor…
* as you said, auto-diff is in a branch or the Google fork of the project * the only pre-built images are for Ubuntu 18.04 * on Linux, the REPL seems to somewhat broken * many libraries are assuming OSX or iOS
I don't feel a lot of hope for adoption of Swift on Linux. Apple obviously is not working on that (fair, they have no reason to do so) and the Swift community also has no focus on Linux, since ... they are in the Apple ecosystem. Meanwhile, the Open Source community is much more interested in Rust than Swift.
For the differentiable programming - this is what got me excited, but after trying, I was a bit underwhelmed. Not that it isn't great technology, its just not figured out yet. I tried to come up with a use case outside of ML and the one I tried wasn't really applicable.
I do feel however that someone will come up with something and that it will have quite some impact.