We just hit an interesting side effect of this.
Using a 5 snapshot, you can't build something depending on swift-protobuf because it falls back to the Package.swift which I guess it assumes is only Swift 3. (there's no comment marker, so I guess I was hoping it would assume something more current)
So until RFC: Tools version with version-specific manifests lands, it won't use the Package@Swift-4.2.swift as a fall back, and instead Swift 5 decides it can't build swift-protobuf at all.
I guess to support folks wanting to use a snapshot we have to make a new release with a Package@swift-5.swift or with a change to the Package.swift. But the existing releases just won't work?