While there is a perfectly fine answer, FireStore has the functionality you need built in, and it doesn't require two calls to the database. In fact, it doesn't require any calls to the database.

Here's an example

    let testRef = self.db.collection("test_node")
    let someData = [
        "child_key": "child_value"
    ]

    let aDoc = testRef.document() //this creates a document with a documentID
    print(aDoc.documentID) //prints the documentID, no database interaction
    //you could add the documentID to an object etc at this point
    aDoc.setData(someData) //stores the data at that documentID

See the documentation Add a Document for more info.

In some cases, it can be useful to create a document reference with an auto-generated ID, then use the reference later. For this use case, you can call doc():

You may want to consider a slightly different approach. You can obtain the document ID in the closure following the write as well. So let's give you a cool Ride (class)

class RideClass {
    var availableSeats: Int
    var carType: String
    var dateCreated: String
    var ID: String

    init(seats: Int, car: String, createdDate: String) {
        self.availableSeats = seats
        self.carType = car
        self.dateCreated = createdDate
        self.ID = ""
    }

    func getRideDict() -> [String: Any] {
        let dict:[String: Any] = [
            "availableSeats": self.availableSeats,
            "carType": self.carType,
            "dateCreated": self.dateCreated
        ]
        return dict
    }
}

and then some code to create a ride, write it out and leverage it's auto-created documentID

    var aRide = RideClass(seats: 3, car: "Lincoln", createdDate: "20190122")

    var ref: DocumentReference? = nil
    ref = db.collection("rides").addDocument(data: aRide.getRideDict() ) { err in
        if let err = err {
            print("Error adding document: \(err)")
        } else {
            aRide.ID = ref!.documentID
            print(aRide.ID) //now you can work with the ride and know it's ID
        }
    }

I believe that if you use Swift's inbuilt ID generator, called UUID, provided by the Foundation Framework, this will let you do what you want to do. Please see the below code for my recommended changes. Also by doing it this way, when you first initialise your "Ride" struct, you can generate its ID variable then, instead of doing it inside the function. This is the way I generate unique ID's throughout my applications and it works perfectly! Hope this helps!

struct Ride {
    var availableSeats: Int
    var carType: String
    var dateCreated: Timestamp
    var ID: String
}


func createRide(ride: Ride, completion: @escaping(_ rideID: String, _ error: Error?) -> Void) {
    // Firebase setup
    settings.areTimestampsInSnapshotsEnabled = true
    db.settings = settings

    // Add a new document with a generated ID
    var ref: DocumentReference? = nil
    let newDocumentID = UUID().uuidString
    ref = db.collection("rides").document(newDocumentID).setData([
        "availableSeats": ride.availableSeats,
        "carType": ride.carType,
        "dateCreated": ride.dateCreated,
        "ID": newDocumentID,
    ], merge: true) { err in
        if let err = err {
            print("Error adding ride: \(err)")
            completion(nil, err)
        } else {
            print("Ride added with ID: \(newDocumentID)")
            completion(newDocumentID, nil)
        }
    }
}

This is my solution which works like a charm

    let opportunityCollection = db.collection("opportunities")
    let opportunityDocument = opportunityCollection.document()
    let id = opportunityDocument.documentID

    let data: [String: Any] = ["id": id,
                               "name": "Kelvin"]

    opportunityDocument.setData(data) { (error) in
        if let error = error {
            completion(.failure(error))
        } else {
            completion(.success(()))
        }
    }