feat: add message about unconfigured feature flags when user runs cdk synth (#765)

#### Notify users about the number of unconfigured feature flags in
their application and the `cdk flags` command when they run `cdk synth`.
---
By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license

Author: vivian12300

packages/aws-cdk/lib/cli/cdk-toolkit.ts

@@ -55,6 +55,7 @@ import {
 } from '../commands/migrate';
 import type { CloudAssembly, CloudExecutable, StackSelector } from '../cxapp';
 import { DefaultSelection, environmentsFromDescriptors, globEnvironmentsFromStacks, looksLikeGlob } from '../cxapp';
+import { OBSOLETE_FLAGS } from '../obsolete-flags';
 import {
   deserializeStructure,
   formatErrorMessage,
@@ -195,7 +196,7 @@ export class CdkToolkit {
       emojis: true,
       ioHost: this.ioHost,
       toolkitStackName: this.toolkitStackName,
-      unstableFeatures: ['refactor'],
+      unstableFeatures: ['refactor', 'flags'],
     });
   }
 
@@ -1058,6 +1059,8 @@ export class CdkToolkit {
   ): Promise<any> {
     const stacks = await this.selectStacksForDiff(stackNames, exclusively, autoValidate);
 
+    await displayFlagsMessage(this.toolkit, this.props.cloudExecutable, this.ioHost.asIoHelper());
+
     // if we have a single stack, print it to STDOUT
     if (stacks.stackCount === 1) {
       if (!quiet) {
@@ -2108,6 +2111,16 @@ async function askUserConfirmation(
     }
   });
 }
+export async function displayFlagsMessage(toolkit: InternalToolkit, cloudExecutable: CloudExecutable,
+  ioHelper: IoHelper): Promise<void> {
+  let numUnconfigured = (await toolkit.flags(cloudExecutable))
+    .filter(flag => !OBSOLETE_FLAGS.includes(flag.name))
+    .filter(flag => flag.userValue === undefined).length;
+
+  if (numUnconfigured > 0) {
+    await ioHelper.defaults.info(`You currently have ${numUnconfigured} unconfigured feature flags that may require attention to keep your application up-to-date. Run 'cdk flags' to learn more.`);
+  }
+}
 
 /**
  * Logger for processing stack metadata

packages/aws-cdk/lib/cli/cli.ts

@@ -457,7 +457,6 @@ export async function exec(args: string[], synthesizer?: Synthesizer): Promise<n
         if (!configuration.settings.get(['unstable']).includes('flags')) {
           throw new ToolkitError('Unstable feature use: \'flags\' is unstable. It must be opted in via \'--unstable\', e.g. \'cdk flags --unstable=flags\'');
         }
-
         const toolkit = new Toolkit({
           ioHost,
           toolkitStackName,

packages/aws-cdk/lib/obsolete-flags.ts

@@ -0,0 +1,5 @@
+export const OBSOLETE_FLAGS = [
+  '@aws-cdk/core:enableStackNameDuplicates',
+  '@aws-cdk/aws-s3:grantWriteWithoutAcl',
+  '@aws-cdk/aws-kms:defaultKeyPolicies',
+];

packages/aws-cdk/test/cli/display-flags-message.test.ts

@@ -0,0 +1,83 @@
+import { Toolkit } from '@aws-cdk/toolkit-lib';
+import { asIoHelper } from '../../lib/api-private';
+import { displayFlagsMessage } from '../../lib/cli/cdk-toolkit';
+import { TestIoHost } from '../_helpers/io-host';
+
+describe('displayFlagsMessage', () => {
+  let ioHost: TestIoHost;
+  let ioHelper: any;
+  let mockToolkit: jest.Mocked<Toolkit>;
+  let mockCloudExecutable: any;
+
+  beforeEach(() => {
+    ioHost = new TestIoHost();
+    ioHelper = asIoHelper(ioHost, 'synth');
+    mockCloudExecutable = {};
+
+    mockToolkit = {
+      flags: jest.fn(),
+    } as any;
+
+    jest.spyOn(Toolkit.prototype, 'flags').mockImplementation(mockToolkit.flags);
+  });
+
+  afterEach(() => {
+    jest.restoreAllMocks();
+  });
+
+  test('displays message with correct count of unconfigured flags, filtering out obsolete flags', async () => {
+    const mockFlagsData = [
+      {
+        name: '@aws-cdk/core:testFlag',
+        userValue: undefined,
+        recommendedValue: 'true',
+        explanation: 'Test flag',
+        module: 'aws-cdk-lib',
+      },
+      {
+        name: '@aws-cdk/s3:anotherFlag',
+        userValue: 'false',
+        recommendedValue: 'false',
+        explanation: 'Another test flag',
+        module: 'aws-cdk-lib',
+      },
+      {
+        name: '@aws-cdk/core:enableStackNameDuplicates',
+        userValue: undefined,
+        recommendedValue: 'true',
+        explanation: 'Obsolete flag',
+        module: 'aws-cdk-lib',
+      },
+    ];
+
+    mockToolkit.flags.mockResolvedValue(mockFlagsData);
+
+    await displayFlagsMessage(mockToolkit as any, mockCloudExecutable, ioHelper);
+
+    expect(mockToolkit.flags).toHaveBeenCalledWith(mockCloudExecutable);
+    expect(ioHost.notifySpy).toHaveBeenCalledWith(
+      expect.objectContaining({
+        message: 'You currently have 1 unconfigured feature flags that may require attention to keep your application up-to-date. Run \'cdk flags\' to learn more.',
+        level: 'info',
+      }),
+    );
+  });
+  test('does not display a message when user has no unconfigured flags', async () => {
+    const mockFlagsData = [
+      {
+        name: '@aws-cdk/s3:anotherFlag',
+        userValue: 'false',
+        recommendedValue: 'false',
+        explanation: 'Another test flag',
+        module: 'aws-cdk-lib',
+      },
+    ];
+    mockToolkit.flags.mockResolvedValue(mockFlagsData);
+
+    await displayFlagsMessage(mockToolkit as any, mockCloudExecutable, ioHelper);
+
+    expect(mockToolkit.flags).toHaveBeenCalledWith(mockCloudExecutable);
+    expect(ioHost.notifySpy).not.toHaveBeenCalled();
+  });
+});
+