Better print for AnyKeyPath in Swift 5.8

Before Swift 5.8, if you try to print a keypath via a print() or po command, it will print out general output for a Swift class.

struct User {
    let firstName: String
    let lastName: String
    var nickName: String
}

print(\User.firstName)
// Swift.KeyPath<ModuleName.User, Swift.String>

print(\User.lastName)
// Swift.KeyPath<ModuleName.User, Swift.String>

print(\User.nickName)
// Swift.WritableKeyPath<ModuleName.User, Swift.String>

In this example, Swift will print a class name and a type of that key path.

This isn't very useful since you can't distinguish one property from another.

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Add CustomDebugStringConvertible conformance to AnyKeyPath

Swift 5.8 ([SE-0369]) make AnyKeyPath conform to CustomDebugStringConvertible protocol and provide more useful information when print.

The followings are the output for the same keypaths.

print(\User.firstName)
// \User.firstName

print(\User.lastName)
// \User.lastName

print(\User.nickName)
// \User.nickName

As you can see, we know at a glance what the keypaths are referring to.