Auto add --enable-preview vmArgs for java 12 project

src/commands.ts

@@ -27,6 +27,8 @@ export const JAVA_RESOLVE_MAINMETHOD = "vscode.java.resolveMainMethod";
 
 export const JAVA_INFER_LAUNCH_COMMAND_LENGTH = "vscode.java.inferLaunchCommandLength";
 
+export const JAVA_CHECK_PROJECT_SETTINGS = "vscode.java.checkProjectSettings";
+
 export function executeJavaLanguageServerCommand(...rest) {
     // TODO: need to handle error and trace telemetry
     return vscode.commands.executeCommand(JAVA_EXECUTE_WORKSPACE_COMMAND, ...rest);

src/configurationProvider.ts

@@ -186,6 +186,10 @@ export class JavaDebugConfigurationProvider implements vscode.DebugConfiguration
 
                 // Add the default launch options to the config.
                 config.cwd = config.cwd || _.get(folder, "uri.fsPath");
+                // Auto add '--enable-preview' vmArgs if the java project enables COMPILER_PB_ENABLE_PREVIEW_FEATURES flag.
+                if (await lsPlugin.detectPreviewFlag(config.mainClass, config.projectName)) {
+                    config.vmArgs = (config.vmArgs || "") + " --enable-preview";
+                }
             } else if (config.request === "attach") {
                 if (!config.hostName || !config.port) {
                     throw new utility.UserError({

src/languageServerPlugin.ts

@@ -61,3 +61,22 @@ export function validateLaunchConfig(workspaceUri: vscode.Uri, mainClass: string
 export function inferLaunchCommandLength(config: vscode.DebugConfiguration): Promise<number> {
     return <Promise<number>>commands.executeJavaLanguageServerCommand(commands.JAVA_INFER_LAUNCH_COMMAND_LENGTH, JSON.stringify(config));
 }
+
+export function checkProjectSettings(className: string, projectName: string, inheritedOptions: boolean, expectedOptions: {[key: string]: string}):
+Promise<boolean> {
+    return <Promise<boolean>>commands.executeJavaLanguageServerCommand(
+        commands.JAVA_CHECK_PROJECT_SETTINGS, JSON.stringify({
+            className,
+            projectName,
+            inheritedOptions,
+            expectedOptions,
+        }));
+}
+
+const COMPILER_PB_ENABLE_PREVIEW_FEATURES: string = "org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures";
+export async function detectPreviewFlag(className: string, projectName: string): Promise<boolean> {
+    const expectedOptions = {
+        [COMPILER_PB_ENABLE_PREVIEW_FEATURES]: "enabled",
+    };
+    return await checkProjectSettings(className, projectName, true, expectedOptions);
+}