I only want one feature from Swift: faster type-checking. If you write a few lines of math, or a moderately complex SwiftUI expression, suddenly the typechecker starts timing out. Swift is the only language I've ever used with this problem.
Chris Lattner recently explained that because of function overloads (and other features), type inference is really hard to make fast in Swift, and is why he says that "using Hindley-Milner was a mistake". [0]
It's a fair point, but radically different 8 years on.
Swift type inference is two-way, so you can often improve things by annotating types instead of using inference.
Also, it very easy to have complex builders and nested closures that complicate inferencing, so it's a bit of a side-effect of how much people use the powerful features.
The big compile-time gotcha now is using macros. To be safe, each macro runs in its own process hogging cores, and the new Xcode compile scheduler isn't quite sensitive to that, so sometimes command-line builds with less parallelism are a fair bit faster. The solution there is judicious use, particularly of libraries depending heavily on macros.
I can only imagine how terrible things must have been if the current state is radically improved. I can lock up xcode by doing nothing more than backspacing over a few characters.
I wouldn't expect it, ever, frankly. Seen multiple Apple people come and go while explaining how they were going to solve that (c.f. 2016(!) for link #2)
https://hachyderm.io/@evanw/109859384302551859
https://www.cocoawithlove.com/blog/2016/07/12/type-checker-i...
https://danielchasehooper.com/posts/why-swift-is-slow/
This isn't mentioned at all in the announcement so I'm kind of disappointed.