Make .index-build a subdirectory of .build

This means that users of SourceKit-LSP’s background indexing don’t need to add another directory to their `.gitignore`.

Author: ahoppen

.devcontainer/default/devcontainer.json

@@ -9,9 +9,7 @@
 
   "mounts": [
     // Use a named volume for the build products for optimal performance (https://code.visualstudio.com/remote/advancedcontainers/improve-performance#_use-a-targeted-named-volume)
-    "source=${localWorkspaceFolderBasename}-build,target=${containerWorkspaceFolder}/.build,type=volume",
-    // Do the same for experimental background indexing
-    "source=${localWorkspaceFolderBasename}-index-build,target=${containerWorkspaceFolder}/.index-build,type=volume"
+    "source=${localWorkspaceFolderBasename}-build,target=${containerWorkspaceFolder}/.build,type=volume"
   ],
   "customizations": {
     "vscode": {

Documentation/Enable Experimental Background Indexing.md

@@ -34,4 +34,4 @@ By default SourceKit-LSP does not update its global index in the background or b
 
 If you hit any issues that are not mentioned above, please [file a GitHub issue](https://github.com/swiftlang/sourcekit-lsp/issues/new/choose) and attach the following information, if possible:
 - A diagnostic bundle generated by running `path/to/sourcekit-lsp diagnose`.
-- Your project including the `.index-build` folder, if possible.
+- Your project including the `.build` folder, if possible.

Sources/BuildSystemIntegration/SwiftPMBuildSystem.swift

@@ -205,7 +205,7 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
 
   package let connectionToSourceKitLSP: any Connection
 
-  /// Whether the `SwiftPMBuildSystem` is pointed at a `.index-build` directory that's independent of the
+  /// Whether the `SwiftPMBuildSystem` is pointed at a `.build/index-build` directory that's independent of the
   /// user's build.
   private var isForIndexBuild: Bool { options.backgroundIndexingOrDefault }
 
@@ -320,7 +320,7 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
       fileSystem: localFileSystem
     )
     if options.backgroundIndexingOrDefault {
-      location.scratchDirectory = AbsolutePath(projectRoot.appending(component: ".index-build"))
+      location.scratchDirectory = AbsolutePath(projectRoot.appending(components: ".build", "index-build"))
     } else if let scratchDirectory = options.swiftPMOrDefault.scratchPath,
       let scratchDirectoryPath = try? AbsolutePath(validating: scratchDirectory)
     {

Tests/SourceKitLSPTests/BackgroundIndexingTests.swift

@@ -889,7 +889,8 @@ final class BackgroundIndexingTests: XCTestCase {
     let nestedIndexBuildURL = try XCTUnwrap(
       project.uri(for: "OtherLib.swift").fileURL?
         .deletingLastPathComponent()
-        .appendingPathComponent(".index-build")
+        .appendingPathComponent(".build")
+        .appendingPathComponent("index-build")
     )
     XCTAssertFalse(
       FileManager.default.fileExists(at: nestedIndexBuildURL),
@@ -1258,7 +1259,7 @@ final class BackgroundIndexingTests: XCTestCase {
     // Simulate a package update which goes as follows:
     //  - The user runs `swift package update`
     //  - This updates `Package.resolved`, which we watch
-    //  - We reload the package, which updates `Dependency.swift` in `.index-build/checkouts`, which we also watch.
+    //  - We reload the package, which updates `Dependency.swift` in `.build/index-build/checkouts`, which we also watch.
     try await Process.run(
       arguments: [
         unwrap(ToolchainRegistry.forTesting.default?.swift?.pathString),