SourceKitLSP: properly handle paths on Windows

This corrects the path identification to include Windows paths properly.
Although this is not entirely ideal due to the platform specific code,
but avoids crashing the SourceKit LSP server if it encounters a space in
the path.

Author: compnerd

Sources/SourceKitLSP/Swift/SwiftLanguageServer.swift

@@ -18,6 +18,9 @@ import SKCore
 import SKSupport
 import SourceKitD
 import TSCBasic
+#if os(Windows)
+import WinSDK
+#endif
 
 fileprivate extension Range {
   /// Checks if this range overlaps with the other range, counting an overlap with an empty range as a valid overlap.
@@ -1130,7 +1133,24 @@ extension SwiftLanguageServer: SKDNotificationHandler {
 
       self.queue.async {
         let uri: DocumentURI
-        if name.starts(with: "/") {
+
+        // Paths are expected to be absolute; on Windows, this means that the
+        // path is either drive letter prefixed (and thus `PathGetDriveNumberW`
+        // will provide the driver number OR it is a UNC path and `PathIsUNCW`
+        // will return `true`.  On Unix platforms, the path will start with `/`
+        // which takes care of both a regular absolute path and a POSIX
+        // alternate root path.
+
+        // TODO: this is not completely portable, e.g. MacOS 9 HFS paths are
+        // unhandled.
+#if os(Windows)
+        let isPath: Bool = !name.withCString(encodedAs: UTF16.self) {
+          PathIsUNCW($0) || (0...25) ~= PathGetDriveNumberW($0)
+        }
+#else
+        let isPath: Bool = name.starts(with: "/")
+#endif
+        if isPath {
           // If sourcekitd returns us a path, translate it back into a URL
           uri = DocumentURI(URL(fileURLWithPath: name))
         } else {