SwiftUI: How to create a binding to a property of an environment object?

The following minimal code toggles the display of details by pressing a button.

struct ContentView: View {
    @State var showDetails: Bool = false
    var body: some View {
        VStack {
            DetailsButton(showDetails: $showDetails)  // 1
            if showDetails {
                Text("This is my message!")
            }
        }
    }
}

struct DetailsButton: View {
    @Binding var showDetails: Bool
    var body: some View {
        Button("\(showDetails ? "Hide" : "Show") Details") {
            showDetails.toggle()
        }
    }
}

I would like to achieve the same thing, but moving the showDetails property into a ViewModel which I pass as an environment variable:

(Note that I'm using the new Observation framework of Swift 5.9 and iOS 17 / macOS 14.)

@Observable class ViewModel {
    var showDetails: Bool
}

Ordinarily I would initialize it in the App class:

struct TestApp: App {
    @State var vm = ViewModel()
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(vm)
        }
    }
}

and passing the ViewModel into ContentView as an environment variable:

@Environment(ViewModel.self) private var vm

In this scenario, how can I pass the binding for vm.showDetails from ContentView to DetailsButton as I did in the original example? (See the line marked "1")

To use your vm.showDetails in DetailsButton try this approach, using a @Bindable

 import SwiftUI
 import Observation
 
 @main
 struct TestApp: App {
     @State private var vm = ViewModel()
     
     var body: some Scene {
         WindowGroup {
             ContentView()
                 .environment(vm)
         }
     }
 }
  
 
@Observable class ViewModel {
    var showDetails: Bool = false // <--- here
}

struct ContentView: View {
    @Environment(ViewModel.self) var vm  
    
    var body: some View {
        @Bindable var vm = vm  // <--- here
        VStack {
            DetailsButton(showDetails: $vm.showDetails) 
            if vm.showDetails {   // <--- here
                Text("This is my message!")
            }
        }
    }
}

struct DetailsButton: View {
    @Binding var showDetails: Bool  
    
    var body: some View {
        Button("\(showDetails ? "Hide" : "Show") Details") {  
            showDetails.toggle()  
        }
    }
}