Add "auto" mode to sourcekit-lsp backgroundIndexing setting

package.json

@@ -475,9 +475,14 @@
             "order": 3
           },
           "swift.sourcekit-lsp.backgroundIndexing": {
-            "type": "boolean",
-            "default": false,
-            "markdownDescription": "**Experimental**: Enable or disable background indexing. This option has no effect in Swift versions prior to 6.0.",
+            "type": "string",
+            "enum": [
+              "on",
+              "off",
+              "auto"
+            ],
+            "default": "auto",
+            "markdownDescription": "Turns background indexing `on` or `off`. `auto` will enable background indexing if the Swift version is >= 6.1. This option has no effect in Swift versions prior to 6.0.",
             "order": 4
           },
           "swift.sourcekit-lsp.trace.server": {

src/configuration.ts

@@ -242,10 +242,17 @@ const configuration = {
             .get<boolean>("backgroundCompilation", false);
     },
     /** background indexing */
-    get backgroundIndexing(): boolean {
-        return vscode.workspace
+    get backgroundIndexing(): "on" | "off" | "auto" {
+        const value = vscode.workspace
             .getConfiguration("swift.sourcekit-lsp")
-            .get("backgroundIndexing", false);
+            .get("backgroundIndexing", "auto");
+
+        // Legacy versions of this setting were a boolean, convert to the new string version.
+        if (typeof value === "boolean") {
+            return value ? "on" : "off";
+        } else {
+            return value;
+        }
     },
     /** focus on problems view whenever there is a build error */
     get actionAfterBuildError(): ActionAfterBuildError {

src/sourcekit-lsp/LanguageClientManager.ts

@@ -129,6 +129,8 @@ export class LanguageClientManager {
     // that are not at the root of their workspace
     public subFolderWorkspaces: vscode.Uri[];
     private namedOutputChannels: Map<string, LSPOutputChannel> = new Map();
+    private swiftVersion: Version;
+
     /** Get the current state of the underlying LanguageClient */
     public get state(): langclient.State {
         if (!this.languageClient) {
@@ -142,9 +144,8 @@ export class LanguageClientManager {
             LanguageClientManager.indexingLogName,
             new LSPOutputChannel(LanguageClientManager.indexingLogName, false, true)
         );
-        this.singleServerSupport = workspaceContext.swiftVersion.isGreaterThanOrEqual(
-            new Version(5, 7, 0)
-        );
+        this.swiftVersion = workspaceContext.swiftVersion;
+        this.singleServerSupport = this.swiftVersion.isGreaterThanOrEqual(new Version(5, 7, 0));
         this.subscriptions = [];
         this.subFolderWorkspaces = [];
         if (this.singleServerSupport) {
@@ -613,10 +614,18 @@ export class LanguageClientManager {
             },
         };
 
-        if (configuration.backgroundIndexing) {
+        // Swift 6.0.0 and later supports background indexing.
+        // In 6.0.0 it is experimental so only "true" enables it.
+        // In 6.1.0 it is no longer experimental, and so "auto" or "true" enables it.
+        if (
+            this.swiftVersion.isGreaterThanOrEqual(new Version(6, 0, 0)) &&
+            (configuration.backgroundIndexing === "on" ||
+                (configuration.backgroundIndexing === "auto" &&
+                    this.swiftVersion.isGreaterThanOrEqual(new Version(6, 1, 0))))
+        ) {
             options = {
                 ...options,
-                backgroundIndexing: configuration.backgroundIndexing,
+                backgroundIndexing: true,
                 backgroundPreparationMode: "enabled",
             };
         }

test/unit-tests/sourcekit-lsp/LanguageClientManager.test.ts

@@ -65,6 +65,14 @@ suite("LanguageClientManager Suite", () => {
     let createFilesEmitter: AsyncEventEmitter<vscode.FileCreateEvent>;
     let deleteFilesEmitter: AsyncEventEmitter<vscode.FileDeleteEvent>;
 
+    const doesNotHave = (prop: any) =>
+        match(function (actual) {
+            if (typeof actual === "object") {
+                return !(prop in actual);
+            }
+            return actual[prop] === undefined;
+        }, "doesNotHave");
+
     setup(async () => {
         // Mock pieces of the VSCode API
         mockedVSCodeWindow.activeTextEditor = undefined;
@@ -153,7 +161,7 @@ suite("LanguageClientManager Suite", () => {
         // LSP configuration defaults
         mockedConfig.path = "";
         mockedConfig.buildArguments = [];
-        mockedConfig.backgroundIndexing = false;
+        mockedConfig.backgroundIndexing = "off";
         mockedConfig.swiftEnvironmentVariables = {};
         mockedLspConfig.supportCFamily = "cpptools-inactive";
         mockedLspConfig.disable = false;
@@ -177,6 +185,50 @@ suite("LanguageClientManager Suite", () => {
         expect(languageClientMock.start).to.have.been.calledOnce;
     });
 
+    test("chooses the correct backgroundIndexing value is auto, swift version if 6.0.0", async () => {
+        mockedWorkspace.swiftVersion = new Version(6, 0, 0);
+        mockedConfig.backgroundIndexing = "auto";
+        new LanguageClientManager(instance(mockedWorkspace));
+        await waitForReturnedPromises(languageClientMock.start);
+
+        expect(mockedLangClientModule.LanguageClient).to.have.been.calledOnceWith(
+            match.string,
+            match.string,
+            match.object,
+            match.hasNested("initializationOptions", doesNotHave("backgroundIndexing"))
+        );
+    });
+
+    test("chooses the correct backgroundIndexing value is auto, swift version if 6.1.0", async () => {
+        mockedWorkspace.swiftVersion = new Version(6, 1, 0);
+        mockedConfig.backgroundIndexing = "auto";
+
+        new LanguageClientManager(instance(mockedWorkspace));
+        await waitForReturnedPromises(languageClientMock.start);
+
+        expect(mockedLangClientModule.LanguageClient).to.have.been.calledOnceWith(
+            match.string,
+            match.string,
+            match.object,
+            match.hasNested("initializationOptions.backgroundIndexing", match.truthy)
+        );
+    });
+
+    test("chooses the correct backgroundIndexing value is true, swift version if 6.0.0", async () => {
+        mockedWorkspace.swiftVersion = new Version(6, 0, 0);
+        mockedConfig.backgroundIndexing = "on";
+
+        new LanguageClientManager(instance(mockedWorkspace));
+        await waitForReturnedPromises(languageClientMock.start);
+
+        expect(mockedLangClientModule.LanguageClient).to.have.been.calledOnceWith(
+            match.string,
+            match.string,
+            match.object,
+            match.hasNested("initializationOptions.backgroundIndexing", match.truthy)
+        );
+    });
+
     test("notifies SourceKit-LSP of WorkspaceFolder changes", async () => {
         const folder1 = mockObject<FolderContext>({
             isRootFolder: false,