Don’t index files with fallback build settings

Fallback build settings don’t even have an indexstore path set, so we would never pick up any index data generated from them. 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 basically means that we wouldn’t get any up-to-date-index even when we do get 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.

Author: ahoppen

Sources/SemanticIndex/UpdateIndexStoreTaskDescription.swift

@@ -225,6 +225,17 @@ public struct UpdateIndexStoreTaskDescription: IndexTaskDescription {
       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 buildSystemManager.toolchain(for: file.mainFile, language) else {
       logger.error(
         "Not updating index store for \(file.forLogging) because no toolchain could be determined for the document"