It also lets one avoid reference-counting traffic, which can be significant for values that contain multiple ARC/COW things (such as strings and arrays), and is

It also lets one avoid reference-counting traffic, which can be significant for values that contain multiple ARC/COW things (such as strings and arrays), and is semantically critical with move-only (or "unique ownership", per the ownership manifesto[1]) types. [1]: https://github.com/apple/swift/blob/master/docs/OwnershipManifesto.md https://github.com/apple/swift/blob/master/docs/OwnershipMan...