extension
Data
{
    init(unsafeUninitializedCapacity inCapacity: Int,
            initializingWith initializer: (inout UnsafeMutableRawBufferPointer, inout Int) throws -> Void)
        rethrows
    {
        let buffer = UnsafeMutableRawPointer.allocate(byteCount: inCapacity, alignment: MemoryLayout<UInt8>.alignment)
        var bp = UnsafeMutableRawBufferPointer(start: buffer, count: inCapacity)
        let originalAddress = bp.baseAddress
        var count: Int = 0
        defer
        {
            precondition(count <= inCapacity, "Initialized count set to greater than specified capacity.")
            precondition(bp.baseAddress == originalAddress, "Can't reassign buffer in Array(unsafeUninitializedCapacity:initializingWith:)")
        }
        
        do
        {
            try initializer(&bp, &count)
        }
        
        catch (let e)
        {
            self = Data(bytesNoCopy: buffer, count: count, deallocator: .custom({ b,c in b.deallocate() }))
            throw e
        }
        
        self = Data(bytesNoCopy: buffer, count: count, deallocator: .custom({ b,c in b.deallocate() }))
    }
}