By how much? Will extra language codebase justify the speed increase (if any)?
Say you're developing some mobile app, why wouldn't you want to use a single unified codebase for front & backends instead of splitting it across Rust and Swift?
Anyway, if 5% improvement is all that Rust will have over Swift, it's as dead as Lisp.
When Swift is finally open-sourced, there will be a huge exodus from Rust community and move to Swift. This is not survivable. Rust will quickly lose the little momentum that it has right now and it will be game over.
Personally I believe that any investment into codebases using Rust is a huge mistake and a waste of money at this point in time.
Swift also is annoyingly special-cased, with things like Options being special in the language, and if let only working on options (if i understand correctly). Rust took syntax like if let from swift, and then made it general.
Swift also has to deal with Obj-C interop which is not great. I imagine Swift being as popular as Objective-C in terms of non-mobile apps.