fix(aws-cdk-lib): feature flag report contains unnecessary flags (#35227)

The feature flag report generated by the core library includes flags that:

- Are not configurable for CDKv2 anymore
- Whose default value is the same as the recommended value (i.e., they don't need to be configured at all).

At the same time, the `recommended-feature-flags.json` file does NOT contain those flags, because they aren't necessary.

This means that a newly `cdk init`ed application reports that 18 flags are unconfigured by the user, producing unnecessary warnings.

We will address this in 2 parts.

- Don't emit flags that don't apply in v2 in the library.
- Don't warn about flags where the recommended value is the same as the default value in the CLI.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

packages/aws-cdk-lib/core/lib/private/feature-flag-report.ts

@@ -35,6 +35,11 @@ function parseFeatureFlagInfo(flagName: string, info: FlagInfo, root: IConstruct
 export function generateFeatureFlagReport(builder: CloudAssemblyBuilder, root: IConstruct): void {
   const featureFlags: Record<string, FeatureFlag> = {};
   for (const [flagName, flagInfo] of Object.entries(feats.FLAGS)) {
+    // Skip flags that don't apply to the current version line
+    if (feats.CURRENT_VERSION_EXPIRED_FLAGS.includes(flagName)) {
+      continue;
+    }
+
     featureFlags[flagName] = parseFeatureFlagInfo(flagName, flagInfo, root);
   }
 

packages/aws-cdk-lib/core/test/feature-flag-report.test.ts

@@ -13,7 +13,7 @@ describe('generate feature flag report', () => {
     const app = new App({
       context: {
         '@aws-cdk/aws-ec2:bastionHostUseAmazonLinux2023ByDefault': true,
-        '@aws-cdk/core:aspectStabilization': false,
+        '@aws-cdk/core:newStyleStackSynthesis': false,
       },
     });
     const assembly = app.synth();
@@ -25,9 +25,14 @@ describe('generate feature flag report', () => {
         flags: expect.objectContaining({
           '@aws-cdk/aws-ec2:bastionHostUseAmazonLinux2023ByDefault': expect.objectContaining({
             userValue: true,
+            recommendedValue: true,
           }),
-          '@aws-cdk/core:aspectStabilization': expect.objectContaining({
+          '@aws-cdk/core:newStyleStackSynthesis': expect.objectContaining({
             userValue: false,
+            recommendedValue: true,
+            unconfiguredBehavesLike: {
+              v2: true,
+            },
           }),
         }),
       }),
@@ -47,4 +52,15 @@ describe('generate feature flag report', () => {
       }),
     }));
   });
+  test('expired flags are not included in report', () => {
+    const app = new App();
+    const builder = new cxapi.CloudAssemblyBuilder('/tmp/test');
+    const spy = jest.spyOn(builder, 'addArtifact');
+
+    generateFeatureFlagReport(builder, app);
+
+    const flags = (spy.mock.calls[0][1].properties as FeatureFlagReportProperties).flags;
+    // For example
+    expect(Object.keys(flags)).not.toContain('aws-cdk:enableDiffNoFail');
+  });
 });

packages/aws-cdk-lib/cx-api/lib/features.ts

@@ -1696,18 +1696,16 @@ export const CURRENT_VERSION_EXPIRED_FLAGS: string[] = Object.entries(FLAGS)
 /**
  * Flag values that should apply for new projects
  *
- * Add a flag in here (typically with the value `true`), to enable
- * backwards-breaking behavior changes only for new projects.  New projects
- * generated through `cdk init` will include these flags in their generated
- * project config.
+ * This contains flags that satisfy both criteria of:
  *
- * Tests must cover the default (disabled) case and the future (enabled) case.
- *
- * Going forward, this should *NOT* be consumed directly anymore.
+ * - They are configurable for the current major version line.
+ * - The recommended value is different from the unconfigured value (i.e.,
+ *   configuring a flag is useful)
  */
 export const CURRENTLY_RECOMMENDED_FLAGS = Object.fromEntries(
   Object.entries(FLAGS)
-    .filter(([_, flag]) => flag.recommendedValue !== flag.unconfiguredBehavesLike?.[CURRENT_MV] && flag.introducedIn[CURRENT_MV])
+    .filter(([_, flag]) => flag.introducedIn[CURRENT_MV] !== undefined)
+    .filter(([_, flag]) => flag.recommendedValue !== flag.unconfiguredBehavesLike?.[CURRENT_MV])
     .map(([name, flag]) => [name, flag.recommendedValue]),
 );