Naked Unicode surrogates in Swift

surrogates.swift

// Making strings of "normal" Unicode is pretty easy:

// Unicode Scalar literal
"\u{2603}" // "☃"

// From int
String(UnicodeScalar(0x2603)) // "☃"

// Maybe you want to test how your app handles weird Unicode input, like naked surrogates.
// But Swift makes it pretty hard to create such strings.

"\u{d800}" // Invalid unicode scalar

UnicodeScalar(0xd800) // EXC_BAD_INSTRUCTION

var gen = GeneratorOfOne(0xd800 as UInt16)
var utf16 = UTF16()
let result = utf16.decode(&gen) // Error

// It turns out you can do it with NSString!

let cc: [unichar] = [0xd800]
NSString(characters: cc, length: cc.count) // "�"