SwiftUI Extract @ToolbarContent to its own var

You don't actually need to create another struct - instead you can use @ToolbarContentBuilder. It's a @ViewBuilder equivalent for ToolbarContent:

struct ContentView: View {
    var body: some View {
        NavigationView {
            List {}
                .toolbar(content: myToolBarContent)
        }
    }
    
    @ToolbarContentBuilder
    func myToolBarContent() -> some ToolbarContent {
        ToolbarItem(placement: .principal) {
            Text("Hi")
        }
        ToolbarItem(placement: .navigationBarTrailing) {
            Text("Ho")
        }
    }
}

If you want to separate the toolbar from the body, then you need to create a separate struct for Toolbar content. Here is a possible demo.

struct ContentView: View {
    
    var body: some View {
        NavigationView {
            List {
            }
            .toolbar{MyToolBarContent()}
        }
    }
    
    struct MyToolBarContent: ToolbarContent {
        var body: some ToolbarContent {
            ToolbarItem(placement: .principal) {
                Text("Hi")
            }
            ToolbarItem(placement: .navigationBarTrailing) {
                Text("Ho")
            }
        }
    }
}

Worth adding on to pawello2222 answer by mentioning that on macOS, it doesn't take much more to enable nifty user customisable toolbars, e.g.

@main
struct myHappyAppyApp: App {

    var body: some Scene {
        WindowGroup {
            ContentView()
        .commands {
            ToolbarCommands() //<- Turns on the toolbar customization
        }
    }
}



struct ContentView: View {
    var body: some View {
        NavigationView {
            List {}
            .toolbar(id: "hihobar", content: myToolBarContent)
        }
    }

    @ToolbarContentBuilder
    func myToolBarContent() -> some CustomizableToolbarContent {
        ToolbarItem(id: "hitag") {
            Text("Hi")
        }
        ToolbarItem(id: "hotag") {
            Text("Ho")
        }
        ToolbarItem(id: "spacertag") {
            Spacer()
        }
    }
}