withUnsafeBufferPointer

// Declaration
func withUnsafeBufferPointer(_ body: (UnsafeBufferPointer) hrows -> R) rethrows -> R

let numbers = [1, 2, 3, 4, 5]
let sum = numbers.withUnsafeBufferPointer { buffer -> Int in
    var result = 0
    for i in stride(from: buffer.startIndex, to: buffer.endIndex, by: 2) {
        result += buffer[i]
    }
    return result
}
// 'sum' == 9