I know that but if I decide to use Rust instead of Swift, why would I be wrapping Rust in Swift through C? And still it doesn't solve a problem how Rust can call anything from Cocoa or my existing code.

> if I decide to use Rust instead of Swift, why would I be wrapping Rust in Swift through C?

To avoid the unnecessary overhead of dynamically dispatched Objective-C method calls.

To me, it seems more sensible to go

  Rust -> C -> Swift

vs

  Rust -> C -> Objective-C -> Swift

Rust can call anything that exposes a C-compatible ABI, and Rust code can expose a C-compatible ABI so any language that can call into C (i.e. all languages) can call into Rust.