PathCchIsRoot PathCchIsRoot Updated isRoot extension.

Now that swiftlang/swift-foundation#976 and swiftlang/swift-foundation#980 are fixed we can clean this code up a bit by removing the empty path check on Linux and by using the native `PathCchIsRoot` on Windows.

Existing code is retained for toolchains <6.1

Sources/SwiftFormat/Utilities/URL+isRoot.swift

@@ -12,21 +12,43 @@
 
 import Foundation
 
+#if os(Windows)
+import WinSDK
+#endif
+
 extension URL {
+  /// Returns a `Bool` to indicate if the given `URL` leads to the root of a filesystem.
+  /// A non-filesystem type `URL` will always return false.
   @_spi(Testing) public var isRoot: Bool {
+    guard isFileURL else { return false }
+
+    #if os(macOS)
+    return self.path == NSOpenStepRootDirectory()
+    #endif
+
+    #if compiler(>=6.1)
     #if os(Windows)
-    // FIXME: We should call into Windows' native check to check if this path is a root once https://github.com/swiftlang/swift-foundation/issues/976 is fixed.
+    return self.path.withCString(encodedAs: UTF16.self, PathCchIsRoot)
+    #elseif os(Linux)
+    return self.path == "/"
+    #endif
+    #else
+
+    #if os(Windows)
+    // This is needed as the fixes from #844 aren't in the Swift 6.0 toolchain.
     // https://github.com/swiftlang/swift-format/issues/844
     var pathComponents = self.pathComponents
     if pathComponents.first == "/" {
       // Canonicalize `/C:/` to `C:/`.
       pathComponents = Array(pathComponents.dropFirst())
     }
     return pathComponents.count <= 1
-    #else
+    #elseif os(Linux)
     // On Linux, we may end up with an string for the path due to https://github.com/swiftlang/swift-foundation/issues/980
-    // TODO: Remove the check for "" once https://github.com/swiftlang/swift-foundation/issues/980 is fixed.
+    // This is needed as the fixes from #980 aren't in the Swift 6.0 toolchain.
     return self.path == "/" || self.path == ""
     #endif
+
+    #endif
   }
 }