Swift 5.9: Noncopyable struct

noncopyable.swift

// Noncopyable struct: passed by reference
// and only one variable can hold the reference.
struct User: ~Copyable {
    var name: String

    // Read-only
    func read() {
        print(self.name)
        // self.name = "BBBBB" // Error: 'self' is immutable
    }

    // Read-write
    mutating func readWrite() {
        print(self.name)
        self.name = "CCCCC"
    }

    // Read-write, and doesn't return ownership afterwards
    consuming func finalize() {
        print(self.name)
        self.name = "DDDDD"
    }
}

// ERROR: Noncopyable parameter must specify its ownership
// func implicitUser(_ user: User) {
//     print(self.name)
// }


// Read-only, temporary ownership
func borrowUser(_ user: borrowing User) {
    print(user.name)
    // user.name = "Hello"   // Error: 'user' is a 'let' constant
}

// Read-write, permanent ownership
func consumeUser(_ user: consuming User) {
    print(user.name)
    user.name = "EEEEE"      // OK
}

// --- Usage examples --- //

let user1 = User(name: "AAAAA")
print(user1.name)       // OK

// Transfers the reference to `user2`
var user2 = user1
// print(user1.name)    // Error: 'user1' used after consume
print(user2.name)       // OK

// `borrowing` temporarily transfers ownership during function call.
borrowUser(user2)
print(user2.name)       // OK
borrowUser(user2)
print(user2.name)       // OK

user2.read()
print(user2.name)       // OK
user2.read()
print(user2.name)       // OK

user2.readWrite()
print(user2.name)       // OK
user2.readWrite()
print(user2.name)       // OK

// `consuming` permanently transfers ownership.
// consumeUser(user2)
// print(user2.name)    // Error: 'user2' used after consume

user2.finalize()
// print(user2.name)    // Error: 'user2' used after consume
// user2.finalize()     // Error: 'user2' consumed more than once