Unfortunately, you have to use C or ObjC (or at least the ObjC runtime) as the middleman.
Now that there is a stable Swift ABI, at least it's theoretically possible to add a first-class Swift interoperability support to Rust, but it's a quite a chunk of work that nobody has attempted yet.
Currently the practical approach is to write the UI in Swift and Rust on the back-end, e.g. gif.ski app does it this way: https://github.com/sindresorhus/Gifski
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Great work! Is it easy to interact between Swift and Rust or UIKit libraries?
Unfortunately, you have to use C or ObjC (or at least the ObjC runtime) as the middleman.
Now that there is a stable Swift ABI, at least it's theoretically possible to add a first-class Swift interoperability support to Rust, but it's a quite a chunk of work that nobody has attempted yet.
Currently the practical approach is to write the UI in Swift and Rust on the back-end, e.g. gif.ski app does it this way: https://github.com/sindresorhus/Gifski