Remove the fallback value from SourceKitServer.handleRequest

It’s not entire clear why we have a fallback value if no document service can serve a request and I checked that VS Code behaves if we return empty results for jump to definition or if we return an error. To simplify the codebase, we should just throw an error here and remove the `fallback` parameter from `handleRequest`.

Resolves #862
rdar://116703662

Author: ahoppen

Sources/SourceKitLSP/SourceKitServer.swift

@@ -395,16 +395,15 @@ public actor SourceKitServer {
 
   private func handleRequest<RequestType: TextDocumentRequest>(
     for request: Request<RequestType>,
-    requestHandler: @escaping (RequestType, Workspace, ToolchainLanguageServer) async throws -> RequestType.Response,
-    fallback: RequestType.Response
+    requestHandler: @escaping (RequestType, Workspace, ToolchainLanguageServer) async throws -> RequestType.Response
   ) async {
     await handleRequest(request) { request in
       let doc = request.textDocument.uri
       guard let workspace = await self.workspaceForDocument(uri: request.textDocument.uri) else {
         throw ResponseError.workspaceNotOpen(request.textDocument.uri)
       }
       guard let languageService = workspace.documentService[doc] else {
-        return fallback
+        throw ResponseError.unknown("No language service for '\(request.textDocument.uri)' found")
       }
       return try await requestHandler(request, workspace, languageService)
     }
@@ -747,55 +746,47 @@ extension SourceKitServer: MessageHandler {
     case let request as Request<TypeHierarchySubtypesRequest>:
       await self.handleRequest(request, handler: self.subtypes)
     case let request as Request<CompletionRequest>:
-      await self.handleRequest(
-        for: request,
-        requestHandler: self.completion,
-        fallback: CompletionList(isIncomplete: false, items: [])
-      )
+      await self.handleRequest(for: request, requestHandler: self.completion)
     case let request as Request<HoverRequest>:
-      await self.handleRequest(for: request, requestHandler: self.hover, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.hover)
     case let request as Request<OpenInterfaceRequest>:
-      await self.handleRequest(for: request, requestHandler: self.openInterface, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.openInterface)
     case let request as Request<DeclarationRequest>:
-      await self.handleRequest(for: request, requestHandler: self.declaration, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.declaration)
     case let request as Request<DefinitionRequest>:
-      await self.handleRequest(for: request, requestHandler: self.definition, fallback: .locations([]))
+      await self.handleRequest(for: request, requestHandler: self.definition)
     case let request as Request<ReferencesRequest>:
-      await self.handleRequest(for: request, requestHandler: self.references, fallback: [])
+      await self.handleRequest(for: request, requestHandler: self.references)
     case let request as Request<ImplementationRequest>:
-      await self.handleRequest(for: request, requestHandler: self.implementation, fallback: .locations([]))
+      await self.handleRequest(for: request, requestHandler: self.implementation)
     case let request as Request<CallHierarchyPrepareRequest>:
-      await self.handleRequest(for: request, requestHandler: self.prepareCallHierarchy, fallback: [])
+      await self.handleRequest(for: request, requestHandler: self.prepareCallHierarchy)
     case let request as Request<TypeHierarchyPrepareRequest>:
-      await self.handleRequest(for: request, requestHandler: self.prepareTypeHierarchy, fallback: [])
+      await self.handleRequest(for: request, requestHandler: self.prepareTypeHierarchy)
     case let request as Request<SymbolInfoRequest>:
-      await self.handleRequest(for: request, requestHandler: self.symbolInfo, fallback: [])
+      await self.handleRequest(for: request, requestHandler: self.symbolInfo)
     case let request as Request<DocumentHighlightRequest>:
-      await self.handleRequest(for: request, requestHandler: self.documentSymbolHighlight, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.documentSymbolHighlight)
     case let request as Request<FoldingRangeRequest>:
-      await self.handleRequest(for: request, requestHandler: self.foldingRange, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.foldingRange)
     case let request as Request<DocumentSymbolRequest>:
-      await self.handleRequest(for: request, requestHandler: self.documentSymbol, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.documentSymbol)
     case let request as Request<DocumentColorRequest>:
-      await self.handleRequest(for: request, requestHandler: self.documentColor, fallback: [])
+      await self.handleRequest(for: request, requestHandler: self.documentColor)
     case let request as Request<DocumentSemanticTokensRequest>:
-      await self.handleRequest(for: request, requestHandler: self.documentSemanticTokens, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.documentSemanticTokens)
     case let request as Request<DocumentSemanticTokensDeltaRequest>:
-      await self.handleRequest(for: request, requestHandler: self.documentSemanticTokensDelta, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.documentSemanticTokensDelta)
     case let request as Request<DocumentSemanticTokensRangeRequest>:
-      await self.handleRequest(for: request, requestHandler: self.documentSemanticTokensRange, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.documentSemanticTokensRange)
     case let request as Request<ColorPresentationRequest>:
-      await self.handleRequest(for: request, requestHandler: self.colorPresentation, fallback: [])
+      await self.handleRequest(for: request, requestHandler: self.colorPresentation)
     case let request as Request<CodeActionRequest>:
-      await self.handleRequest(for: request, requestHandler: self.codeAction, fallback: nil)
+      await self.handleRequest(for: request, requestHandler: self.codeAction)
     case let request as Request<InlayHintRequest>:
-      await self.handleRequest(for: request, requestHandler: self.inlayHint, fallback: [])
+      await self.handleRequest(for: request, requestHandler: self.inlayHint)
     case let request as Request<DocumentDiagnosticsRequest>:
-      await self.handleRequest(
-        for: request,
-        requestHandler: self.documentDiagnostic,
-        fallback: .full(.init(items: []))
-      )
+      await self.handleRequest(for: request, requestHandler: self.documentDiagnostic)
     // IMPORTANT: When adding a new entry to this switch, also add it to the `TaskMetadata` initializer.
     default:
       reply(.failure(ResponseError.methodNotFound(R.method)))