swift - Warning: Initialization of 'UnsafeBufferPointer<T>' results in a dangling buffer pointer

Question

After update to Swift 5.2 / Xcode 11.4 got a warning to following code:

extension Data {

    init<T>(from value: T) {
        var value = value
        let pointer = UnsafeBufferPointer(start: &value, count: 1)
        self.init(buffer: pointer)
    }

    func to<T>(type: T.Type) -> T {
        return self.withUnsafeBytes { $0.load(as: T.self) }
    }
}

On line let pointer = UnsafeBufferPointer(start: &value, count: 1) I got

Initialization of 'UnsafeBufferPointer' results in a dangling buffer pointer

I can use @silenceWarning but it's dirty solution. Maybe I need to store pointer somewhere and clean it in the future?

Answer 1

I also met these annoying warnings.

var str = "aaaaabbbbbccccc"
var num1 = 1
var num2 = 22

var data = Data()
// Initialization of 'UnsafeBufferPointer<String>' results in a dangling buffer pointer
data.append(UnsafeBufferPointer(start: &str, count: 1)) 
// Initialization of 'UnsafeBufferPointer<Int>' results in a dangling buffer pointer
data.append(UnsafeBufferPointer(start: &num1, count: 1))
// Initialization of 'UnsafeBufferPointer<Int>' results in a dangling buffer pointer 
data.append(UnsafeBufferPointer(start: &num2, count: 1)) 

Considering @greg's answer, I put the Data.append into withUnsafePointer's closure, and it does not show warnings anymore.

withUnsafePointer(to: &str) { data.append(UnsafeBufferPointer(start: $0, count: 1)) } // ok
withUnsafePointer(to: &num1) { data.append(UnsafeBufferPointer(start: $0, count: 1)) } // ok
withUnsafePointer(to: &num2) { data.append(UnsafeBufferPointer(start: $0, count: 1)) } // ok

Here is the extension

extension Data {
    init<T>(value: T) {
        self = withUnsafePointer(to: value) { (ptr: UnsafePointer<T>) -> Data in
            return Data(buffer: UnsafeBufferPointer(start: ptr, count: 1))
        }
    }

    mutating func append<T>(value: T) {
        withUnsafePointer(to: value) { (ptr: UnsafePointer<T>) in
            append(UnsafeBufferPointer(start: ptr, count: 1))
        }
    }
}