Foundation: improve relative path computation for iteration on Windows

This partially corrects the relative path computation.  We no longer
fail to create the file system representation if it exceeds `MAX_PATH`
due to the switch to `withUnsafeNTPath`.  We will still fail to properly
create the relative path if the paths are unrelated and if the relative
path exceeds `MAX_PATH`.  However, this still is an improvement over the
status quo.

Author: compnerd

Sources/Foundation/FileManager+Win32.swift

@@ -1075,23 +1075,24 @@ extension FileManager.NSPathDirectoryEnumerator {
     internal func _nextObject() -> Any? {
         guard let url = innerEnumerator.nextObject() as? URL else { return nil }
 
-        var relativePath: [WCHAR] = Array<WCHAR>(repeating: 0, count: Int(MAX_PATH))
-
-        guard baseURL._withUnsafeWideFileSystemRepresentation({ baseUrlFsr in
-            url._withUnsafeWideFileSystemRepresentation { urlFsr in
-                let fromAttrs = GetFileAttributesW(baseUrlFsr)
-                let toAttrs = GetFileAttributesW(urlFsr)
-                guard fromAttrs != INVALID_FILE_ATTRIBUTES, toAttrs != INVALID_FILE_ATTRIBUTES else {
-                    return false
+        let path: String? = try? baseURL.withUnsafeNTPath { pwszBasePath in
+            let dwBaseAttrs = GetFileAttributesW(pwszBasePath)
+            if dwBaseAttrs == INVALID_FILE_ATTRIBUTES { return nil }
+
+            return try? url.withUnsafeNTPath { pwszPath in
+                let dwAttrs = GetFileAttributesW(pwszPath)
+                if dwAttrs == INVALID_FILE_ATTRIBUTES { return nil }
+
+                return withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(MAX_PATH)) {
+                    guard PathRelativePathToW($0.baseAddress, pwszBasePath, dwBaseAttrs, pwszPath, dwAttrs) else { return nil }
+                    // Drop the leading ".\" from the path
+                    return String(decodingCString: $0.baseAddress!.advanced(by: 2), as: UTF16.self)
                 }
-                return PathRelativePathToW(&relativePath, baseUrlFsr, fromAttrs, urlFsr, toAttrs)
             }
-        }) else { return nil }
+        }
 
-        let path = String(decodingCString: &relativePath, as: UTF16.self)
-        // Drop the leading ".\" from the path
-        _currentItemPath = String(path.dropFirst(2))
-        return _currentItemPath
+        _currentItemPath = path ?? _currentItemPath
+        return path
     }
 
 }

Sources/Foundation/URL.swift

@@ -723,14 +723,6 @@ public struct URL : ReferenceConvertible, Equatable {
         return try block(fsRep)
     }
 
-#if os(Windows)
-    internal func _withUnsafeWideFileSystemRepresentation<ResultType>(_ block: (UnsafePointer<UInt16>?) throws -> ResultType) rethrows -> ResultType {
-      let fsr: UnsafePointer<UInt16> = _url._wideFileSystemRepresentation
-      defer { fsr.deallocate() }
-      return try block(fsr)
-    }
-#endif
-
     // MARK: -
     // MARK: Path manipulation