There's little hope that IntelliJ could extend their own tooling to Swift's evolving language. Indeed, Swift has compile-time-checked regular expressions, concretely-typed generics, and so much more not seen in other languages.
Swift is catching up to Rust's memory/ownership model and building out both C and C++ interoperation, making them a powerful solution for safe systems programming.
Refactorings across a class hierarchy don't help Swift much since value types are preferred.
What's really missing from XCode or Swift-LS is partial parsing: effective feedback when your code is incomplete or broken, and good feedback about the mini-build that parses the Package.swift declaration.
>Swift has compile-time-checked regular expressions
And IntelliJ has had inspections over regular expressions for a decade, nothing special about it, or hard to implement.
> concretely-typed generics
Handled for years by IntelliJ.
> so much more not seen in other languages.
I'd be curious to see a list, because there's not much that is very special about Swift.
The biggest problem that AppCode faced is that even when they open source things, unless it grows way beyond Apple like LLVM did, Apple still only does things for Apple. They have no incentive of making Swift-Syntax easy to consume and use. For the longest time, they would push gigantic dumps of commits to the various Swift repos on release date, meaning that Xcode would already have everything integrated, and others are left to play catch-up. This is entirely done on purpose to kneecap competitors.