StringインスタンスをStaticStringとして使う `func withStaticString<T>(@NoEscape f: StaticString -> T)`

String+StaticString.swift

import Foundation

extension String {
    func withStaticString(@noescape f: StaticString -> Void) {
        withCString {
            let rawPointer = $0._rawValue
            let byteSize = lengthOfBytesUsingEncoding(NSUTF8StringEncoding)._builtinWordValue
            let isASCII = true._getBuiltinLogicValue()
            let staticString = StaticString(_builtinStringLiteral: rawPointer, byteSize: byteSize, isASCII: isASCII)
            f(staticString)
        }
    }
}