Show message if background indexing is enabled but the workspace doesn’t support background indexing

Sources/SKCore/BuildServerBuildSystem.swift

@@ -259,6 +259,8 @@ private func readReponseDataKey(data: LSPAny?, key: String) -> String? {
 }
 
 extension BuildServerBuildSystem: BuildSystem {
+  public nonisolated var supportsPreparation: Bool { false }
+
   /// The build settings for the given file.
   ///
   /// Returns `nil` if no build settings have been received from the build

Sources/SKCore/BuildSystem.swift

@@ -121,6 +121,10 @@ public protocol BuildSystem: AnyObject, Sendable {
   ///   context.
   func setDelegate(_ delegate: BuildSystemDelegate?) async
 
+  /// Whether the build system is capable of preparing a target for indexing, ie. if the `prepare` methods has been
+  /// implemented.
+  var supportsPreparation: Bool { get }
+
   /// Retrieve build settings for the given document with the given source
   /// language.
   ///

Sources/SKCore/BuildSystemManager.swift

@@ -68,6 +68,10 @@ public actor BuildSystemManager {
     }
   }
 
+  public var supportsPreparation: Bool {
+    return buildSystem?.supportsPreparation ?? false
+  }
+
   /// Create a BuildSystemManager that wraps the given build system. The new
   /// manager will modify the delegate of the underlying build system.
   public init(

Sources/SKCore/CompilationDatabaseBuildSystem.swift

@@ -93,6 +93,8 @@ public actor CompilationDatabaseBuildSystem {
 }
 
 extension CompilationDatabaseBuildSystem: BuildSystem {
+  public nonisolated var supportsPreparation: Bool { false }
+
   public var indexDatabasePath: AbsolutePath? {
     indexStorePath?.parentDirectory.appending(component: "IndexDatabase")
   }

Sources/SKSwiftPMWorkspace/SwiftPMBuildSystem.swift

@@ -342,6 +342,7 @@ extension SwiftPMBuildSystem {
 }
 
 extension SwiftPMBuildSystem: SKCore.BuildSystem {
+  public nonisolated var supportsPreparation: Bool { true }
 
   public var buildPath: TSCAbsolutePath {
     return TSCAbsolutePath(buildParameters.buildPath)

Sources/SourceKitLSP/SourceKitLSPServer.swift

@@ -99,6 +99,12 @@ public actor SourceKitLSPServer {
   /// Initialization can be awaited using `waitUntilInitialized`.
   private var initialized: Bool = false
 
+  /// Set to `true` after the user has opened a project that doesn't support background indexing while having background
+  /// indexing enabled.
+  ///
+  /// This ensures that we only inform the user about background indexing not being supported for these projects once.
+  private var didSendBackgroundIndexingNotSupportedNotification = false
+
   var options: Options
 
   let toolchainRegistry: ToolchainRegistry
@@ -916,7 +922,7 @@ extension SourceKitLSPServer {
       "Created workspace at \(workspaceFolder.uri.forLogging) as \(type(of: buildSystem)) with project root \(projectRoot ?? "<nil>")"
     )
 
-    return try? await Workspace(
+    let workspace = try? await Workspace(
       documentManager: self.documentManager,
       rootUri: workspaceFolder.uri,
       capabilityRegistry: capabilityRegistry,
@@ -935,6 +941,21 @@ extension SourceKitLSPServer {
         self?.indexProgressManager.indexProgressStatusDidChange()
       }
     )
+    if let workspace, options.indexOptions.enableBackgroundIndexing, workspace.semanticIndexManager == nil,
+      !self.didSendBackgroundIndexingNotSupportedNotification
+    {
+      self.sendNotificationToClient(
+        ShowMessageNotification(
+          type: .info,
+          message: """
+            Background indexing is currently only supported for SwiftPM projects. \
+            For all other project types, please run a build to update the index.
+            """
+        )
+      )
+      self.didSendBackgroundIndexingNotSupportedNotification = true
+    }
+    return workspace
   }
 
   func initialize(_ req: InitializeRequest) async throws -> InitializeResult {

Sources/SourceKitLSP/Swift/SwiftLanguageService.swift

@@ -405,7 +405,7 @@ extension SwiftLanguageService {
     if buildSettings == nil || buildSettings!.isFallback, let fileUrl = note.textDocument.uri.fileURL {
       // Do not show this notification for non-file URIs to make sure we don't see this notificaiton for newly created
       // files (which get opened as with a `untitled:Unitled-1` URI by VS Code.
-      await sourceKitLSPServer?.sendNotificationToClient(
+      sourceKitLSPServer?.sendNotificationToClient(
         ShowMessageNotification(
           type: .warning,
           message: """

Sources/SourceKitLSP/Workspace.swift

@@ -108,7 +108,10 @@ public final class Workspace: Sendable {
       mainFilesProvider: uncheckedIndex,
       toolchainRegistry: toolchainRegistry
     )
-    if let uncheckedIndex, options.indexOptions.enableBackgroundIndexing {
+    if options.indexOptions.enableBackgroundIndexing,
+      let uncheckedIndex,
+      await buildSystemManager.supportsPreparation
+    {
       self.semanticIndexManager = SemanticIndexManager(
         index: uncheckedIndex,
         buildSystemManager: buildSystemManager,

Tests/SKCoreTests/BuildSystemManagerTests.swift

@@ -455,6 +455,8 @@ class ManualBuildSystem: BuildSystem {
     self.delegate = delegate
   }
 
+  public nonisolated var supportsPreparation: Bool { false }
+
   func buildSettings(for uri: DocumentURI, in buildTarget: ConfiguredTarget, language: Language) -> FileBuildSettings? {
     return map[uri]
   }

Tests/SourceKitLSPTests/BackgroundIndexingTests.swift

@@ -862,4 +862,15 @@ final class BackgroundIndexingTests: XCTestCase {
       "No file should exist at \(nestedIndexBuildURL)"
     )
   }
+
+  func testShowMessageWhenOpeningAProjectThatDoesntSupportBackgroundIndexing() async throws {
+    let project = try await MultiFileTestProject(
+      files: [
+        "compile_commands.json": ""
+      ],
+      enableBackgroundIndexing: true
+    )
+    let message = try await project.testClient.nextNotification(ofType: ShowMessageNotification.self)
+    XCTAssert(message.message.contains("Background indexing"), "Received unexpected message: \(message.message)")
+  }
 }

Tests/SourceKitLSPTests/BuildSystemTests.swift

@@ -43,6 +43,8 @@ actor TestBuildSystem: BuildSystem {
     buildSettingsByFile[uri] = buildSettings
   }
 
+  public nonisolated var supportsPreparation: Bool { false }
+
   func buildSettings(
     for document: DocumentURI,
     in buildTarget: ConfiguredTarget,