URL.init(string:) handling of unix file paths has changed in Swift 6

[rnro]

Initializing a URL from a unix file path in Swift 6 on Linux now returns a non-nil value (an empty String). In earlier Swift versions and on macOS it returns nil.

On Linux Swift 5.10:

  1> import Foundation
  2> let u = Foundation.URL(string: "unix:///tmp/file")
  [snip]
  3> print(u!.host)
nil

On Linux Swift 6.0.1:

  1> import Foundation
  2> let u = Foundation.URL(string: "unix:///tmp/file")
  [snip]
  3> print(u!.host)
Optional("")

[jrflat]

This is expected after updating URL to RFC 3986, which defines:

hier-part     = "//" authority path-abempty
authority     = [ userinfo "@" ] host [ ":" port ]

If a URL starts with // or contains // after a scheme, this implies that authority exists, which implies that host exists (but may be empty). This helps distinguish a URL with an authority and an empty host, from one where the host does not exist (such as in the URL string relative/path, where .host is nil). I see this was worked-around in AHC, but I'll keep an eye out for any more bincompat reports.