From dddc983dd22bcc4cd7a822e59ce2157849a7a5ba Mon Sep 17 00:00:00 2001
From: Alex Hoppen <ahoppen@apple.com>
Date: Wed, 3 Sep 2025 08:39:59 +0200
Subject: [PATCH 1/4] Make BuildServerManager.toolchain only take a target, no
 document
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The document wasn’t used in here.
---
 .../BuildServerIntegration/BuildServerManager.swift   |  8 ++------
 .../UpdateIndexStoreTaskDescription.swift             |  2 +-
 Sources/SourceKitLSP/SourceKitLSPServer.swift         | 11 ++++++-----
 3 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/Sources/BuildServerIntegration/BuildServerManager.swift b/Sources/BuildServerIntegration/BuildServerManager.swift
index 44a03768..6a8e007b 100644
--- a/Sources/BuildServerIntegration/BuildServerManager.swift
+++ b/Sources/BuildServerIntegration/BuildServerManager.swift
@@ -757,16 +757,12 @@ package actor BuildServerManager: QueueBasedMessageHandler {
 
   // MARK: Build server queries
 
-  /// Returns the toolchain that should be used to process the given document.
+  /// Returns the toolchain that should be used to process the given target.
   package func toolchain(
-    for uri: DocumentURI,
-    in target: BuildTargetIdentifier?,
+    for target: BuildTargetIdentifier,
     language: Language
   ) async -> Toolchain? {
     let toolchainPath = await orLog("Getting toolchain from build targets") { () -> URL? in
-      guard let target else {
-        return nil
-      }
       let targets = try await self.buildTargets()
       guard let target = targets[target]?.target else {
         logger.error("Failed to find target \(target.forLogging) to determine toolchain")
diff --git a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
index c43bdafe..971909a5 100644
--- a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
+++ b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
@@ -280,7 +280,7 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
       logger.error("Not indexing \(file.forLogging) because it has fallback compiler arguments")
       return
     }
-    guard let toolchain = await buildServerManager.toolchain(for: file.mainFile, in: target, language: language) else {
+    guard let toolchain = await buildServerManager.toolchain(for: target, language: language) else {
       logger.error(
         "Not updating index store for \(file.forLogging) because no toolchain could be determined for the document"
       )
diff --git a/Sources/SourceKitLSP/SourceKitLSPServer.swift b/Sources/SourceKitLSP/SourceKitLSPServer.swift
index b06eed91..298b80e7 100644
--- a/Sources/SourceKitLSP/SourceKitLSPServer.swift
+++ b/Sources/SourceKitLSP/SourceKitLSPServer.swift
@@ -567,11 +567,12 @@ package actor SourceKitLSPServer {
       return existingLanguageServices
     }
 
-    let toolchain = await workspace.buildServerManager.toolchain(
-      for: uri,
-      in: workspace.buildServerManager.canonicalTarget(for: uri),
-      language: language
-    )
+    let toolchain: Toolchain? =
+      if let target = await workspace.buildServerManager.canonicalTarget(for: uri) {
+        await workspace.buildServerManager.toolchain(for: target, language: language)
+      } else {
+        nil
+      }
     guard let toolchain else {
       logger.error("Failed to determine toolchain for \(uri)")
       return []

From 690fd9288f9e20a11a061d12c975e21083431b92 Mon Sep 17 00:00:00 2001
From: Alex Hoppen <ahoppen@apple.com>
Date: Wed, 3 Sep 2025 08:45:04 +0200
Subject: [PATCH 2/4] Extract target out of the `FileIndexInfo` used by
 `UpdateIndexStoreTaskDescription`

This way we can guarantee that all files passed to `UpdateIndexStoreTaskDescription` belong to the same target, which will simplify multi-file indexing.
---
 .../SemanticIndex/SemanticIndexManager.swift  | 41 ++++++++++++++++---
 .../UpdateIndexStoreTaskDescription.swift     | 29 +++++++------
 .../ExpectedIndexTaskTracker.swift            |  2 +-
 3 files changed, 50 insertions(+), 22 deletions(-)

diff --git a/Sources/SemanticIndex/SemanticIndexManager.swift b/Sources/SemanticIndex/SemanticIndexManager.swift
index c1138660..4bb116d6 100644
--- a/Sources/SemanticIndex/SemanticIndexManager.swift
+++ b/Sources/SemanticIndex/SemanticIndexManager.swift
@@ -91,6 +91,13 @@ private struct InProgressIndexStore {
   var fileModificationDate: Date?
 }
 
+/// The information that's needed to index a file within a given target.
+package struct FileIndexInfo: Sendable, Hashable {
+  package let file: FileToIndex
+  package let target: BuildTargetIdentifier
+  package let outputPath: OutputPath
+}
+
 /// Status of document indexing / target preparation in `inProgressIndexAndPreparationTasks`.
 package enum IndexTaskStatus: Comparable {
   case scheduled
@@ -722,16 +729,18 @@ package final actor SemanticIndexManager {
     }
   }
 
-  /// Update the index store for the given files, assuming that their targets have already been prepared.
+  /// Update the index store for the given files, assuming that their targets has already been prepared.
   private func updateIndexStore(
-    for filesAndTargets: [FileIndexInfo],
+    for fileAndOutputPaths: [FileAndOutputPath],
+    target: BuildTargetIdentifier,
     indexFilesWithUpToDateUnit: Bool,
     preparationTaskID: UUID,
     priority: TaskPriority?
   ) async {
     let taskDescription = AnyIndexTaskDescription(
       UpdateIndexStoreTaskDescription(
-        filesToIndex: filesAndTargets,
+        filesToIndex: fileAndOutputPaths,
+        target: target,
         buildServerManager: self.buildServerManager,
         index: index,
         indexStoreUpToDateTracker: indexStoreUpToDateTracker,
@@ -747,7 +756,12 @@ package final actor SemanticIndexManager {
         self.indexProgressStatusDidChange()
         return
       }
-      for fileAndTarget in filesAndTargets {
+      for fileAndOutputPath in fileAndOutputPaths {
+        let fileAndTarget = FileIndexInfo(
+          file: fileAndOutputPath.file,
+          target: target,
+          outputPath: fileAndOutputPath.outputPath
+        )
         switch self.inProgressIndexTasks[fileAndTarget]?.state {
         case .updatingIndexStore(let registeredTask, _):
           if registeredTask == OpaqueQueuedIndexTask(task) {
@@ -763,7 +777,12 @@ package final actor SemanticIndexManager {
       }
       self.indexProgressStatusDidChange()
     }
-    for fileAndTarget in filesAndTargets {
+    for fileAndOutputPath in fileAndOutputPaths {
+      let fileAndTarget = FileIndexInfo(
+        file: fileAndOutputPath.file,
+        target: target,
+        outputPath: fileAndOutputPath.outputPath
+      )
       switch inProgressIndexTasks[fileAndTarget]?.state {
       case .waitingForPreparation(preparationTaskID, let indexTask), .preparing(preparationTaskID, let indexTask):
         inProgressIndexTasks[fileAndTarget]?.state = .updatingIndexStore(
@@ -938,8 +957,18 @@ package final actor SemanticIndexManager {
             // (https://github.com/swiftlang/sourcekit-lsp/issues/1268)
             for fileBatch in filesByTarget[target]!.partition(intoBatchesOfSize: 1) {
               taskGroup.addTask {
+                let fileAndOutputPaths: [FileAndOutputPath] = fileBatch.compactMap {
+                  guard $0.target == target else {
+                    logger.fault(
+                      "FileIndexInfo refers to different target than should be indexed \($0.target.forLogging) vs \(target.forLogging)"
+                    )
+                    return nil
+                  }
+                  return FileAndOutputPath(file: $0.file, outputPath: $0.outputPath)
+                }
                 await self.updateIndexStore(
-                  for: fileBatch,
+                  for: fileAndOutputPaths,
+                  target: target,
                   indexFilesWithUpToDateUnit: indexFilesWithUpToDateUnit,
                   preparationTaskID: preparationTaskID,
                   priority: priority
diff --git a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
index 971909a5..62c6eda1 100644
--- a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
+++ b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
@@ -79,11 +79,13 @@ package enum FileToIndex: CustomLogStringConvertible, Hashable {
   }
 }
 
-/// The information that's needed to index a file within a given target.
-package struct FileIndexInfo: Sendable, Hashable {
+/// A source file to index and the output path that should be used for indexing.
+package struct FileAndOutputPath: Sendable, Hashable {
   package let file: FileToIndex
-  package let target: BuildTargetIdentifier
   package let outputPath: OutputPath
+
+  fileprivate var mainFile: DocumentURI { file.mainFile }
+  fileprivate var sourceFile: DocumentURI { file.sourceFile }
 }
 
 /// Describes a task to index a set of source files.
@@ -94,7 +96,10 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
   package let id = updateIndexStoreIDForLogging.fetchAndIncrement()
 
   /// The files that should be indexed.
-  package let filesToIndex: [FileIndexInfo]
+  package let filesToIndex: [FileAndOutputPath]
+
+  /// The target in whose context the files should be indexed.
+  package let target: BuildTargetIdentifier
 
   /// The build server manager that is used to get the toolchain and build settings for the files to index.
   private let buildServerManager: BuildServerManager
@@ -143,7 +148,8 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
   }
 
   init(
-    filesToIndex: [FileIndexInfo],
+    filesToIndex: [FileAndOutputPath],
+    target: BuildTargetIdentifier,
     buildServerManager: BuildServerManager,
     index: UncheckedIndex,
     indexStoreUpToDateTracker: UpToDateTracker<DocumentURI, BuildTargetIdentifier>,
@@ -156,6 +162,7 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
     hooks: IndexHooks
   ) {
     self.filesToIndex = filesToIndex
+    self.target = target
     self.buildServerManager = buildServerManager
     self.index = index
     self.indexStoreUpToDateTracker = indexStoreUpToDateTracker
@@ -185,11 +192,7 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
       // TODO: Once swiftc supports it, we should group files by target and index files within the same target together
       // in one swiftc invocation. (https://github.com/swiftlang/sourcekit-lsp/issues/1268)
       for fileIndexInfo in filesToIndex {
-        await updateIndexStore(
-          forSingleFile: fileIndexInfo.file,
-          in: fileIndexInfo.target,
-          outputPath: fileIndexInfo.outputPath
-        )
+        await updateIndexStore(forSingleFile: fileIndexInfo.file, outputPath: fileIndexInfo.outputPath)
       }
       // If we know the output paths, make sure that we load their units into indexstore-db. We would eventually also
       // pick the units up through file watching but that would leave a short time period in which we think that
@@ -230,11 +233,7 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
     }
   }
 
-  private func updateIndexStore(
-    forSingleFile file: FileToIndex,
-    in target: BuildTargetIdentifier,
-    outputPath: OutputPath
-  ) async {
+  private func updateIndexStore(forSingleFile file: FileToIndex, outputPath: OutputPath) async {
     guard await !indexStoreUpToDateTracker.isUpToDate(file.sourceFile, target) else {
       // If we know that the file is up-to-date without having ot hit the index, do that because it's fastest.
       return
diff --git a/Tests/SourceKitLSPTests/ExpectedIndexTaskTracker.swift b/Tests/SourceKitLSPTests/ExpectedIndexTaskTracker.swift
index bfc77c24..2e283534 100644
--- a/Tests/SourceKitLSPTests/ExpectedIndexTaskTracker.swift
+++ b/Tests/SourceKitLSPTests/ExpectedIndexTaskTracker.swift
@@ -264,7 +264,7 @@ actor ExpectedIndexTaskTracker {
   }
 }
 
-fileprivate extension FileIndexInfo {
+fileprivate extension FileAndOutputPath {
   var sourceFileName: String? {
     return self.file.sourceFile.fileURL?.lastPathComponent
   }

From 5c4f1ca93c4969c96bc0c39846eddf847d00675c Mon Sep 17 00:00:00 2001
From: Alex Hoppen <ahoppen@apple.com>
Date: Wed, 3 Sep 2025 08:58:01 +0200
Subject: [PATCH 3/4] Make `UpdateIndexStoreTaskDescription.updateIndexStore` 
 handle mulitple files

This pushes multi-file indexing one level closer to the actual compiler invocation.
---
 .../UpdateIndexStoreTaskDescription.swift     | 170 +++++++++---------
 1 file changed, 89 insertions(+), 81 deletions(-)

diff --git a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
index 62c6eda1..09ccf9f5 100644
--- a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
+++ b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
@@ -189,11 +189,7 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
         "Starting updating index store with priority \(Task.currentPriority.rawValue, privacy: .public): \(filesToIndexDescription)"
       )
       let filesToIndex = filesToIndex.sorted(by: { $0.file.sourceFile.stringValue < $1.file.sourceFile.stringValue })
-      // TODO: Once swiftc supports it, we should group files by target and index files within the same target together
-      // in one swiftc invocation. (https://github.com/swiftlang/sourcekit-lsp/issues/1268)
-      for fileIndexInfo in filesToIndex {
-        await updateIndexStore(forSingleFile: fileIndexInfo.file, outputPath: fileIndexInfo.outputPath)
-      }
+      await updateIndexStore(forFiles: filesToIndex)
       // If we know the output paths, make sure that we load their units into indexstore-db. We would eventually also
       // pick the units up through file watching but that would leave a short time period in which we think that
       // indexing has finished (because the index process has terminated) but when the new symbols aren't present in
@@ -233,88 +229,100 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
     }
   }
 
-  private func updateIndexStore(forSingleFile file: FileToIndex, outputPath: OutputPath) async {
-    guard await !indexStoreUpToDateTracker.isUpToDate(file.sourceFile, target) else {
+  private func updateIndexStore(forFiles fileInfos: [FileAndOutputPath]) async {
+    let fileInfos = await fileInfos.asyncFilter { fileInfo in
       // If we know that the file is up-to-date without having ot hit the index, do that because it's fastest.
-      return
-    }
-    guard
-      indexFilesWithUpToDateUnit
-        || !index.checked(for: .modifiedFiles).hasUpToDateUnit(
-          for: file.sourceFile,
-          mainFile: file.mainFile,
-          outputPath: outputPath
-        )
-    else {
-      logger.debug("Not indexing \(file.forLogging) because index has an up-to-date unit")
-      // We consider a file's index up-to-date if we have any up-to-date unit. Changing build settings does not
-      // invalidate the up-to-date status of the index.
-      return
-    }
-    if file.mainFile != file.sourceFile {
-      logger.log("Updating index store of \(file.forLogging) using main file \(file.mainFile.forLogging)")
-    }
-    guard let language = await buildServerManager.defaultLanguage(for: file.mainFile, in: target) else {
-      logger.error("Not indexing \(file.forLogging) because its language could not be determined")
-      return
-    }
-    let buildSettings = await buildServerManager.buildSettings(
-      for: file.mainFile,
-      in: target,
-      language: language,
-      fallbackAfterTimeout: false
-    )
-    guard let buildSettings else {
-      logger.error("Not indexing \(file.forLogging) because it has no compiler arguments")
-      return
-    }
-    if buildSettings.isFallback {
-      // Fallback build settings don’t even have an indexstore path set, so they can't generate index data that we would
-      // pick up. Also, indexing with fallback args has some other problems:
-      // - If it did generate a unit file, we would consider the file’s index up-to-date even if the compiler arguments
-      //   change, which means that we wouldn't get any up-to-date-index even when we have build settings for the file.
-      // - It's unlikely that the index from a single file with fallback arguments will be very useful as it can't tie
-      //   into the rest of the project.
-      // So, don't index the file.
-      logger.error("Not indexing \(file.forLogging) because it has fallback compiler arguments")
-      return
-    }
-    guard let toolchain = await buildServerManager.toolchain(for: target, language: language) else {
-      logger.error(
-        "Not updating index store for \(file.forLogging) because no toolchain could be determined for the document"
+      if await indexStoreUpToDateTracker.isUpToDate(fileInfo.file.sourceFile, target) {
+        return false
+      }
+      if indexFilesWithUpToDateUnit {
+        return true
+      }
+      let hasUpToDateUnit = index.checked(for: .modifiedFiles).hasUpToDateUnit(
+        for: fileInfo.sourceFile,
+        mainFile: fileInfo.mainFile,
+        outputPath: fileInfo.outputPath
       )
+      if !hasUpToDateUnit {
+        logger.debug("Not indexing \(fileInfo.file.forLogging) because index has an up-to-date unit")
+        // We consider a file's index up-to-date if we have any up-to-date unit. Changing build settings does not
+        // invalidate the up-to-date status of the index.
+      }
+      return !hasUpToDateUnit
+    }
+    if fileInfos.isEmpty {
       return
     }
-    let startDate = Date()
-    switch language.semanticKind {
-    case .swift:
-      do {
-        try await updateIndexStore(
-          forSwiftFile: file.mainFile,
-          buildSettings: buildSettings,
-          toolchain: toolchain
-        )
-      } catch {
-        logger.error("Updating index store for \(file.forLogging) failed: \(error.forLogging)")
-        BuildSettingsLogger.log(settings: buildSettings, for: file.mainFile)
-      }
-    case .clang:
-      do {
-        try await updateIndexStore(
-          forClangFile: file.mainFile,
-          buildSettings: buildSettings,
-          toolchain: toolchain
-        )
-      } catch {
-        logger.error("Updating index store for \(file) failed: \(error.forLogging)")
-        BuildSettingsLogger.log(settings: buildSettings, for: file.mainFile)
-      }
-    case nil:
-      logger.error(
-        "Not updating index store for \(file) because it is a language that is not supported by background indexing"
+    for fileInfo in fileInfos where fileInfo.mainFile != fileInfo.sourceFile {
+      logger.log(
+        "Updating index store of \(fileInfo.file.forLogging) using main file \(fileInfo.mainFile.forLogging)"
       )
     }
-    await indexStoreUpToDateTracker.markUpToDate([(file.sourceFile, target)], updateOperationStartDate: startDate)
+
+    for fileInfo in fileInfos {
+      guard let language = await buildServerManager.defaultLanguage(for: fileInfo.mainFile, in: target) else {
+        logger.error("Not indexing \(fileInfo.file.forLogging) because its language could not be determined")
+        continue
+      }
+      let buildSettings = await buildServerManager.buildSettings(
+        for: fileInfo.mainFile,
+        in: target,
+        language: language,
+        fallbackAfterTimeout: false
+      )
+      guard let buildSettings else {
+        logger.error("Not indexing \(fileInfo.file.forLogging) because it has no compiler arguments")
+        continue
+      }
+      if buildSettings.isFallback {
+        // Fallback build settings don’t even have an indexstore path set, so they can't generate index data that we would
+        // pick up. Also, indexing with fallback args has some other problems:∂
+        // - If it did generate a unit file, we would consider the file’s index up-to-date even if the compiler arguments
+        //   change, which means that we wouldn't get any up-to-date-index even when we have build settings for the file.
+        // - It's unlikely that the index from a single file with fallback arguments will be very useful as it can't tie
+        //   into the rest of the project.
+        // So, don't index the file.
+        logger.error("Not indexing \(fileInfo.file.forLogging) because it has fallback compiler arguments")
+        continue
+      }
+
+      guard let toolchain = await buildServerManager.toolchain(for: target, language: buildSettings.language) else {
+        logger.fault(
+          "Unable to determine toolchain to index \(buildSettings.language.description, privacy: .public) files in \(target.forLogging)"
+        )
+        continue
+      }
+      let startDate = Date()
+      switch buildSettings.language.semanticKind {
+      case .swift:
+        do {
+          try await updateIndexStore(
+            forSwiftFile: fileInfo.mainFile,
+            buildSettings: buildSettings,
+            toolchain: toolchain
+          )
+        } catch {
+          logger.error("Updating index store for \(fileInfo.mainFile) failed: \(error.forLogging)")
+          BuildSettingsLogger.log(settings: buildSettings, for: fileInfo.mainFile)
+        }
+      case .clang:
+        do {
+          try await updateIndexStore(
+            forClangFile: fileInfo.mainFile,
+            buildSettings: buildSettings,
+            toolchain: toolchain
+          )
+        } catch {
+          logger.error("Updating index store for \(fileInfo.mainFile.forLogging) failed: \(error.forLogging)")
+          BuildSettingsLogger.log(settings: buildSettings, for: fileInfo.mainFile)
+        }
+      case nil:
+        logger.error(
+          "Not updating index store for \(fileInfo.mainFile.forLogging) because it is a language that is not supported by background indexing"
+        )
+      }
+      await indexStoreUpToDateTracker.markUpToDate([(fileInfo.sourceFile, target)], updateOperationStartDate: startDate)
+    }
   }
 
   /// If `args` does not contain an `-index-store-path` argument, add it, pointing to the build server's index store

From 78608933ba42a3afabec23de4656d585f4f545b3 Mon Sep 17 00:00:00 2001
From: Alex Hoppen <ahoppen@apple.com>
Date: Wed, 3 Sep 2025 09:09:37 +0200
Subject: [PATCH 4/4] Guarantee that all files in a
 `UpdateIndexStoreTaskDescription` have the same language
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

It doesn’t make sense to try and index files with different langauges in a single compiler invocation.
---
 .../BuildServerManager.swift                  |  8 ++-
 .../SemanticIndex/SemanticIndexManager.swift  | 67 +++++++++++--------
 .../UpdateIndexStoreTaskDescription.swift     |  9 +--
 Sources/SourceKitLSP/SourceKitLSPServer.swift | 10 ++-
 4 files changed, 56 insertions(+), 38 deletions(-)

diff --git a/Sources/BuildServerIntegration/BuildServerManager.swift b/Sources/BuildServerIntegration/BuildServerManager.swift
index 6a8e007b..4180f746 100644
--- a/Sources/BuildServerIntegration/BuildServerManager.swift
+++ b/Sources/BuildServerIntegration/BuildServerManager.swift
@@ -758,11 +758,17 @@ package actor BuildServerManager: QueueBasedMessageHandler {
   // MARK: Build server queries
 
   /// Returns the toolchain that should be used to process the given target.
+  ///
+  /// If `target` is `nil` or the build server does not explicitly specify a toolchain for this target, the preferred
+  /// toolchain for the given language is returned.
   package func toolchain(
-    for target: BuildTargetIdentifier,
+    for target: BuildTargetIdentifier?,
     language: Language
   ) async -> Toolchain? {
     let toolchainPath = await orLog("Getting toolchain from build targets") { () -> URL? in
+      guard let target else {
+        return nil
+      }
       let targets = try await self.buildTargets()
       guard let target = targets[target]?.target else {
         logger.error("Failed to find target \(target.forLogging) to determine toolchain")
diff --git a/Sources/SemanticIndex/SemanticIndexManager.swift b/Sources/SemanticIndex/SemanticIndexManager.swift
index 4bb116d6..be73209e 100644
--- a/Sources/SemanticIndex/SemanticIndexManager.swift
+++ b/Sources/SemanticIndex/SemanticIndexManager.swift
@@ -94,6 +94,7 @@ private struct InProgressIndexStore {
 /// The information that's needed to index a file within a given target.
 package struct FileIndexInfo: Sendable, Hashable {
   package let file: FileToIndex
+  package let language: Language
   package let target: BuildTargetIdentifier
   package let outputPath: OutputPath
 }
@@ -507,10 +508,13 @@ package final actor SemanticIndexManager {
             continue
           }
           // If this is a source file, just index it.
+          guard let language = await buildServerManager.defaultLanguage(for: uri, in: target) else {
+            continue
+          }
           didFindTargetToIndex = true
           filesToReIndex.append(
             (
-              FileIndexInfo(file: .indexableFile(uri), target: target, outputPath: outputPath),
+              FileIndexInfo(file: .indexableFile(uri), language: language, target: target, outputPath: outputPath),
               modifiedFilesIndex.modificationDate(of: uri)
             )
           )
@@ -550,10 +554,14 @@ package final actor SemanticIndexManager {
       {
         continue
       }
+      guard let language = await buildServerManager.defaultLanguage(for: uri, in: targetAndOutputPath.key) else {
+        continue
+      }
       filesToReIndex.append(
         (
           FileIndexInfo(
             file: .headerFile(header: uri, mainFile: mainFile),
+            language: language,
             target: targetAndOutputPath.key,
             outputPath: outputPath
           ),
@@ -733,6 +741,7 @@ package final actor SemanticIndexManager {
   private func updateIndexStore(
     for fileAndOutputPaths: [FileAndOutputPath],
     target: BuildTargetIdentifier,
+    language: Language,
     indexFilesWithUpToDateUnit: Bool,
     preparationTaskID: UUID,
     priority: TaskPriority?
@@ -741,6 +750,7 @@ package final actor SemanticIndexManager {
       UpdateIndexStoreTaskDescription(
         filesToIndex: fileAndOutputPaths,
         target: target,
+        language: language,
         buildServerManager: self.buildServerManager,
         index: index,
         indexStoreUpToDateTracker: indexStoreUpToDateTracker,
@@ -759,6 +769,7 @@ package final actor SemanticIndexManager {
       for fileAndOutputPath in fileAndOutputPaths {
         let fileAndTarget = FileIndexInfo(
           file: fileAndOutputPath.file,
+          language: language,
           target: target,
           outputPath: fileAndOutputPath.outputPath
         )
@@ -780,6 +791,7 @@ package final actor SemanticIndexManager {
     for fileAndOutputPath in fileAndOutputPaths {
       let fileAndTarget = FileIndexInfo(
         file: fileAndOutputPath.file,
+        language: language,
         target: target,
         outputPath: fileAndOutputPath.outputPath
       )
@@ -873,13 +885,7 @@ package final actor SemanticIndexManager {
     var newIndexTasks = 0
 
     for (fileIndexInfo, fileModificationDate) in filesToIndex {
-      guard
-        let language = await buildServerManager.defaultLanguage(
-          for: fileIndexInfo.file.mainFile,
-          in: fileIndexInfo.target
-        ),
-        UpdateIndexStoreTaskDescription.canIndex(language: language)
-      else {
+      guard UpdateIndexStoreTaskDescription.canIndex(language: fileIndexInfo.language) else {
         continue
       }
 
@@ -952,27 +958,34 @@ package final actor SemanticIndexManager {
         // And after preparation is done, index the files in the targets.
         await withTaskGroup(of: Void.self) { taskGroup in
           for target in targetsBatch {
-            // TODO: Once swiftc supports indexing of multiple files in a single invocation, increase the batch size to
-            // allow it to share AST builds between multiple files within a target.
-            // (https://github.com/swiftlang/sourcekit-lsp/issues/1268)
-            for fileBatch in filesByTarget[target]!.partition(intoBatchesOfSize: 1) {
-              taskGroup.addTask {
-                let fileAndOutputPaths: [FileAndOutputPath] = fileBatch.compactMap {
-                  guard $0.target == target else {
-                    logger.fault(
-                      "FileIndexInfo refers to different target than should be indexed \($0.target.forLogging) vs \(target.forLogging)"
-                    )
-                    return nil
+            var filesByLanguage: [Language: [FileIndexInfo]] = [:]
+            for fileInfo in filesByTarget[target]! {
+              filesByLanguage[fileInfo.language, default: []].append(fileInfo)
+            }
+            for (language, fileInfos) in filesByLanguage {
+              // TODO: Once swiftc supports indexing of multiple files in a single invocation, increase the batch size to
+              // allow it to share AST builds between multiple files within a target.
+              // (https://github.com/swiftlang/sourcekit-lsp/issues/1268)
+              for fileBatch in fileInfos.partition(intoBatchesOfSize: 1) {
+                taskGroup.addTask {
+                  let fileAndOutputPaths: [FileAndOutputPath] = fileBatch.compactMap {
+                    guard $0.target == target else {
+                      logger.fault(
+                        "FileIndexInfo refers to different target than should be indexed \($0.target.forLogging) vs \(target.forLogging)"
+                      )
+                      return nil
+                    }
+                    return FileAndOutputPath(file: $0.file, outputPath: $0.outputPath)
                   }
-                  return FileAndOutputPath(file: $0.file, outputPath: $0.outputPath)
+                  await self.updateIndexStore(
+                    for: fileAndOutputPaths,
+                    target: target,
+                    language: language,
+                    indexFilesWithUpToDateUnit: indexFilesWithUpToDateUnit,
+                    preparationTaskID: preparationTaskID,
+                    priority: priority
+                  )
                 }
-                await self.updateIndexStore(
-                  for: fileAndOutputPaths,
-                  target: target,
-                  indexFilesWithUpToDateUnit: indexFilesWithUpToDateUnit,
-                  preparationTaskID: preparationTaskID,
-                  priority: priority
-                )
               }
             }
           }
diff --git a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
index 09ccf9f5..ba289aa1 100644
--- a/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
+++ b/Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift
@@ -101,6 +101,9 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
   /// The target in whose context the files should be indexed.
   package let target: BuildTargetIdentifier
 
+  /// The common language of all main files in `filesToIndex`.
+  package let language: Language
+
   /// The build server manager that is used to get the toolchain and build settings for the files to index.
   private let buildServerManager: BuildServerManager
 
@@ -150,6 +153,7 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
   init(
     filesToIndex: [FileAndOutputPath],
     target: BuildTargetIdentifier,
+    language: Language,
     buildServerManager: BuildServerManager,
     index: UncheckedIndex,
     indexStoreUpToDateTracker: UpToDateTracker<DocumentURI, BuildTargetIdentifier>,
@@ -163,6 +167,7 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
   ) {
     self.filesToIndex = filesToIndex
     self.target = target
+    self.language = language
     self.buildServerManager = buildServerManager
     self.index = index
     self.indexStoreUpToDateTracker = indexStoreUpToDateTracker
@@ -260,10 +265,6 @@ package struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
     }
 
     for fileInfo in fileInfos {
-      guard let language = await buildServerManager.defaultLanguage(for: fileInfo.mainFile, in: target) else {
-        logger.error("Not indexing \(fileInfo.file.forLogging) because its language could not be determined")
-        continue
-      }
       let buildSettings = await buildServerManager.buildSettings(
         for: fileInfo.mainFile,
         in: target,
diff --git a/Sources/SourceKitLSP/SourceKitLSPServer.swift b/Sources/SourceKitLSP/SourceKitLSPServer.swift
index 298b80e7..b3d331c3 100644
--- a/Sources/SourceKitLSP/SourceKitLSPServer.swift
+++ b/Sources/SourceKitLSP/SourceKitLSPServer.swift
@@ -567,12 +567,10 @@ package actor SourceKitLSPServer {
       return existingLanguageServices
     }
 
-    let toolchain: Toolchain? =
-      if let target = await workspace.buildServerManager.canonicalTarget(for: uri) {
-        await workspace.buildServerManager.toolchain(for: target, language: language)
-      } else {
-        nil
-      }
+    let toolchain = await workspace.buildServerManager.toolchain(
+      for: await workspace.buildServerManager.canonicalTarget(for: uri),
+      language: language
+    )
     guard let toolchain else {
       logger.error("Failed to determine toolchain for \(uri)")
       return []