MacRumors:

As discovered in the first beta of iOS 13.4, Apple is working on a new “CarKey” feature that will allow an iPhone or an Apple Watch to unlock, lock, and start NFC-compatible vehicles.

Very excited about this feature. Use my AppleWatch to unlock and start my car? Sold. But there’s more.

CarKey “keys” live in the Wallet app and as we found in the first beta, can be shared with other people so you can allow others temporary access to your car. In the second beta of iOS 13.4, there’s updated wording concerning sharing, which makes it clear that digital car keys will be able to be sent to people using the Messages app.

So I can Message my key to someone granting them temporary access to my car (think a valet or loaning the car to a friend without having to hand them my physical keys).

Not clear if and when this will roll out in a public release, but I do like the concept.