Lattner had a vision for a lower-level Swift, which wouldn't need the support Swift needs today - it didn't end up happening and I suspect is not practical. He talked about it in several places, but obviously by 2018 or so what Chris Lattner thinks about the future of Swift doesn't matter very much.

This "lower-level Swift" felt like a similar problem to what Carbon or Herb Sutter's Cpp2 have. They've got something that's unsafe, and they want to somehow build a safe abstraction, but that's a foundation layer, and you've already built a tower of stuff above that, so you need to build it underneath all the stuff you have, which will be way harder than what Rust did where they begin at the bottom.

Is it impossible? No. But it might well be too expensive to be pulled off in practice, especially given that you need the result to pay back your expense over and above what exists today e.g. Rust.