What to learn next, Swift or Rust

For example if I want an HTTP server, I’d choose JS, Kotlin, Rust, etc.

I wouldn't. Swift is definitely better than any of those choices... and I say that as someone with decades of experience writing HTTP services.

I don't currently use Swift for any of my HTTP servers - but only because it's a relatively immature for that task and I'm generally a late adopter (also, I work in an industry where bugs are painfully expensive). But I do use Swift client side, and I definitely intend to switch over to Swift for my server side work at some point in the near future and it's what I recommend for someone starting out today.

By far - my favourite feature in Swift is the memory manager. It uses an "Automatic Reference Counter" which is essentially old school C or Assembly style memory management... except the compiler writes all of the memory management code for you. This often results in your code using significantly less RAM and better sustained performance than other languages and it's also just plain easier to work with - as an experienced developer I can look at Swift and know what it's going to do at a low level with the memory. In modern garbage collected languages, even though I have plenty of experience with those, I don't really know what it's doing under the hood and often I'm surprised by how much memory it uses. On server side code, memory is expensive and traffic can burst to levels drastically higher than your typical baseload activity levels, using less memory and using predictable amounts of memory is really really nice.

At one point, years ago, Apple had a compiler flag to use Garbage Collection or Automatic Reference Counting. The Garbage Collector worked just as well as in any other language... but there was no situation, ever, where it worked better than ARC so Apple killed their GC implementation. ARC is awesome and I don't understand why it's uniquely an Apple thing. Now that Swift is open source, it's available everywhere. Yay.

I find compared to every other language I've ever used, with Swift I tend to catch mistakes while writing the code instead of while testing the code, because the language has been carefully designed to ensure as many common mistakes are compile time errors or at least warnings which require an extra step (often just a single operator) to tell the compiler that, yes, you did intend to write it like that.

That feature is especially beneficial to an inexperienced developer like OP.

The other thing I love about swift is how flexible it is. For example, compare these two blocks of code - they basically do the same thing and they are both Swift:

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Create text field
        let textField = UITextField(frame: CGRect(x: 20, y: 100, width: 300, height: 40))
        textField.placeholder = "Enter text"
        textField.borderStyle = .roundedRect
        view.addSubview(textField)

        // Create button
        let button = UIButton(frame: CGRect(x: 20, y: 200, width: 300, height: 50))
        button.setTitle("Tap Me", for: .normal)
        button.backgroundColor = .blue
        button.addTarget(self, action: #selector(buttonTapped), for: .touchUpInside)
        view.addSubview(button)
    }
}

struct ContentView: View {
    @State private var text = ""
    
    var body: some View {
        VStack(spacing: 20) {
            // Text Field
            TextField("Enter text", text: $text)
                .padding()
                .textFieldStyle(RoundedBorderTextFieldStyle())
            
            // Button
            Button("Tap Me") {
                print("Button was tapped!")
            }
            .padding()
            .background(Color.blue)
            .foregroundColor(.white)
            .cornerRadius(8)
        }
        .padding()
    }
}

Perhaps garbage collection is the wrong term to use as it dosen't happen at runtime (I wasn't sure what other term to call what Rust does). But Rust does provide a abstraction over manual manual memory management and if you are experienced with Rust sure you can probably visualize where the compiler would put the malloc and free calls so it is kind of a mix where you do technically have control it is just hidden from you.

Edit: It seems the term is just compile-time garbage collection so maybe you could consider it falling under garbage collection as an umbrella term.

Essentially although there are a few key differences:

• In Rust there always only one owner while in C++ you can leak ownership if you are using shared_ptr.
• In Rust you can borrow references you do not own safely and in C++ there is no gurantee a unique_ptr can be shared safely.
• In Rust, A lot more compile time optimization for the borrow checker is available whereas in C++ the type system dosen't always let the compiler know for sure when an object goes out of scope, is moved, or is destroyed and so you miss out on a lot of optimization that would be trivial with Rust like syntax.

Pretty much, with some atomic additions like "you cannot mutate a reference when it is borrowed immutably elsewhere" or "you cannot borrow a reference mutably multiple times".

Language absolutely is a marketable skill because most companies are looking to hire someone who can start working day one not someone they'll have to train for weeks or even months in a new language that heavily relies on some specific framework.

Sensational answer! Thank you.

Python actually isn't my first language, just my current choice. I've programmed in Basic, Pascal, Fortran, PL-SQL, Prolog and C at various times in the past. My question was more about which is likely to scale over time to be the more popular ML language.

Python also sucks for MacOS gui apps, so I was contemplating building MacOS/iOs apps for myself as a side quest.

I'm trying to work out which one will have better for ML in a couple of years time.

Julia looks like it is pointed towards ML programming and is fast, but I don't see the same level of potential in a few years that Rust and Swift seem to have.

Rust seems to generating a lot more buzz and I've been seeing posts about Swifts ML libraries that look interesting. My crystal ball seems to be saying that Rust will follow a similar arc that Python took and gain some serious ML creds through libraries built by community/industry. I think Swift will also gain some credible ML capabilities too because it has the Apple behemoth behind it.