in Sources/System/FilePath/FilePathWindows.swift [339:351]
func parseUNC(deviceSigil: SystemChar?) -> _ParsedWindowsRoot {
let serverRange = lexer.eatComponent()
guard lexer.eatBackslash() else {
fatalError("expected normalized root to contain backslash")
}
let shareRange = lexer.eatComponent()
let rootEnd = lexer.current
_ = lexer.eatBackslash()
return .unc(
deviceSigil: deviceSigil,
server: serverRange, share: shareRange,
endingAt: rootEnd, relativeBegin: lexer.current)
}