If the plan is to make a native UI for each platform, good approach.
If it is to stay macOS only, also a possible decision not everything needs to be cross platform, why bring Rust into the picture?
If it is to stay macOS only, also a possible decision not everything needs to be cross platform, why bring Rust into the picture?
Integrating object code into Objective C or Swift is easy, commonplace, people do it all the time.
Rust can generate object code all day, you can insist on the C ABI anywhere you need to, and then you can bring in anything from cargo you want. CocoaPods is... not cargo.
If the target is to be platform specific, then reducing the amount of compiler toolchains, IDE/editor plugins, and language knowledge required across team members is a good thing.
If the purpose is to go cross platform, then it is another matter.
Then again this is a matter of opinion, feel free to disagree, I am not here to change your mind.
In this case, they can use a fast Rust backend for multiple platforms (or just one) and then use Swift to make a nice, native front-end for MacOS and other native UI languages for other platforms if they wish.
They also know Rust so if they want to use Rust, what's the problem in using it? They don't seem to have any problem with building a UI on top, it's built and looks/works fine.