When following the instructions in the book, you have to add swift_bridge code before running.

[kshehata]

I'm trying to follow the instructions in the book for XCode + Cargo, and I was pulling my hair out getting undefined symbols at the linker stage at the first simulator run. Turns out you just have to add a bit of swift-bridge code to lib.rs in order for the symbols to be generated.

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        fn add(left: u64, right: u64) -> u64;
    }
}

It would be really helpful for newbies to add this to the book, or at least leave it as an issue here for people to find :)