Cache the set of buildable source files

This way we don’t need to rebuild the set every time we call `filesToIndex(toCover:)`.

Author: ahoppen

Sources/BuildSystemIntegration/BuildSystemManager.swift

@@ -392,6 +392,20 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
     /// `pathComponents` is the result of `key.fileURL?.pathComponents`. We frequently need these path components to
     /// determine if a file is descendent of the directory and computing them from the `DocumentURI` is expensive.
     let directories: [DocumentURI: (pathComponents: [String]?, info: SourceFileInfo)]
+
+    /// Same as `Set(files.filter(\.value.isBuildable).keys)`. Pre-computed because we need thi pretty frequently in
+    /// `SemanticIndexManager.filesToIndex`.
+    let buildableSourceFiles: Set<DocumentURI>
+
+    internal init(
+      files: [DocumentURI: SourceFileInfo],
+      directories: [DocumentURI: (pathComponents: [String]?, info: SourceFileInfo)]
+    ) {
+      self.files = files
+      self.directories = directories
+      self.buildableSourceFiles = Set(files.filter(\.value.isBuildable).keys)
+    }
+
   }
 
   private let cachedSourceFilesAndDirectories = Cache<SourceFilesAndDirectoriesKey, SourceFilesAndDirectories>()
@@ -1153,6 +1167,13 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
     }
   }
 
+  /// Returns all source files in the project that are considered buildable.
+  ///
+  /// - SeeAlso: Comment in `sourceFilesAndDirectories` for a definition of what `buildable` means.
+  package func buildableSourceFiles() async throws -> Set<DocumentURI> {
+    return try await sourceFilesAndDirectories().buildableSourceFiles
+  }
+
   /// Get all files and directories that are known to the build system, ie. that are returned by a `buildTarget/sources`
   /// request for any target in the project.
   ///

Sources/SemanticIndex/SemanticIndexManager.swift

@@ -305,8 +305,7 @@ package final actor SemanticIndexManager {
             filesToIndex
           } else {
             await orLog("Getting files to index") {
-              try await self.buildSystemManager.sourceFiles(includeNonBuildableFiles: false).keys
-                .sorted { $0.stringValue < $1.stringValue }
+              try await self.buildSystemManager.buildableSourceFiles().sorted { $0.stringValue < $1.stringValue }
             } ?? []
           }
         _ = await self.scheduleIndexing(
@@ -426,7 +425,7 @@ package final actor SemanticIndexManager {
     indexFilesWithUpToDateUnits: Bool
   ) async -> [(file: FileToIndex, fileModificationDate: Date?)] {
     let sourceFiles = await orLog("Getting source files in project") {
-      Set(try await buildSystemManager.sourceFiles(includeNonBuildableFiles: false).keys)
+      try await buildSystemManager.buildableSourceFiles()
     }
     guard let sourceFiles else {
       return []
@@ -461,6 +460,7 @@ package final actor SemanticIndexManager {
           .mainFilesContainingFile(uri: uri, crossLanguage: false)
           .sorted(by: { $0.stringValue < $1.stringValue }).first
         guard let mainFile else {
+          logger.log("Not indexing \(uri) because its main file could not be inferred")
           return nil
         }
         if !indexFilesWithUpToDateUnits, modifiedFilesIndex.hasUpToDateUnit(for: uri, mainFile: mainFile) {