Getting string from Swift 4 new key path syntax?

Short answer: you can't. The KeyPath abstraction is designed to encapsulate a potentially nested property key path from a given root type. As such, exporting a single String value might not make sense in the general case.

For instance, should the hypothetically exported string be interpreted as a property of the root type or a member of one of its nested types? At the very least a string array-ish would need to be exported to address such scenarios...

Per type workaround. Having said that, given that KeyPath conforms to the Equatable protocol, you can provide a custom, per type solution yourself. For instance:

struct Auth {
    var email: String
    var password: String
}
struct User {
    var name: String
    var auth: Auth
}

provide an extension for User-based key paths:

extension PartialKeyPath where Root == User {
    var stringValue: String {
        switch self {
        case \User.name: return "name"
        case \User.auth: return "auth"
        case \User.auth.email: return "auth.email"
        case \User.auth.password: return "auth.password"
        default: fatalError("Unexpected key path")
    }
}

usage:

let name:  KeyPath<User, String> = \User.name
let email: KeyPath<User, String> = \User.auth.email
print(name.stringValue)  /* name */
print(email.stringValue) /* auth.email */

I wouldn't really recommend this solution for production code, given the somewhat high maintenance, etc. But since you were curious this, at least, gives you a way forward ;)

A bit late to the party, but I've stumbled upon a way of getting a key path string from NSObject subclasses at least:

NSExpression(forKeyPath: \UIView.bounds).keyPath