I don’t see the value in this. Swift as a language is a far better fit for Apple APIs than Rust. A lot of important Apple technologies like Core Data, Cocoa bindings etc relies on dynamic dispatch which Rust does not support.

Rust has a useful niche, but a language which aims to cover a broad set of developers need a much simpler model for managing memory than what Rust offers IMHO. Swift already does so many of the things Rust offer, while being a more approachable language which also matches Apple API. Suddenly going for Rust would have been a very strange and I strategic choice. That would have been wasting tons of resources spent developing Swift.

Why have both Swift and Rust? The features of both languages overlap too much for it to make any sense to build a technology stack based on both.

Apple has a large existing ecosystem to cater to. Swift is thus the better choice as Apple has full control and can adjust it to fit their technology needs.

The needs of the Rust community and the needs of Apple diverge too much. Rust is primarily a systems programming language. Apple needs a language which is also a good application programming language, and Swift outshines Rust as an Application programming language.

If you look at Swift you will notice a lot of features which exist to make tools integrate well with it. This makes designing GUIs great with Swift. Rust was not designed to be used for GUI apps.

Also Rust had made some serious mistakes such as crating a design with very slow compile times. That scales poorly for large projects and teams. And especially for the kind of rapid iteration you want to do when developing apps.

It is far more sensible for Apple to copy Rust features into Swift than to suddenly switch to an entirely new language. They already got the designer of Rust working on Swift so that should not be hard.