It's worth noting that, if Data.description returned a hex dump (or some other complete representation of the instance's contents), then we could provide a compatible Data.init(_: String) and then conform Data to LosslessStringConvertible.

We do have a bug in radar for this (15719610).

Casting to NSData does not help any more in Xcode 10/Swift 4.2:

    let data1 = Data(bytes: [1, 2, 3])
    print(data1 as NSData) // Foundation._NSSwiftData
    
    let data2 = "Hello world".data(using: .utf8)!
    print(data2 as NSData) // _NSInlineData

I'd be open to accepting a PR that improves struct Data's debugDescription to print out some actual bytes.

On macOS 10.15 (Catalina) beta, the NSData output is now truncated:

let data = Data(1...255)
// 255 bytes
print(data as NSData)
// {length = 255, bytes = 0x01020304 05060708 090a0b0c 0d0e0f10 ... f8f9fafb fcfdfeff }

so that this workaround does not help anymore to get the (full) content of a Data value (for debugging purposes).