fix(cx-api): improve compatibility messages for cli <=> app

packages/@aws-cdk/cx-api/lib/cloud-assembly.ts

@@ -143,6 +143,11 @@ export class CloudAssemblyBuilder {
     const manifestFilePath = path.join(this.outdir, MANIFEST_FILE);
     fs.writeFileSync(manifestFilePath, JSON.stringify(manifest, undefined, 2));
 
+    // "backwards compatibility": in order for the old CLI to tell the user they
+    // need a new version, we'll emit the legacy manifest with only "version".
+    // this will result in an error "CDK Toolkit >= 0.33.0 is required in order to interact with this program."
+    fs.writeFileSync(path.join(this.outdir, 'cdk.out'), JSON.stringify({ version: CLOUD_ASSEMBLY_VERSION }));
+
     return new CloudAssembly(this.outdir);
   }
 }

packages/@aws-cdk/cx-api/lib/versioning.ts

@@ -27,12 +27,13 @@ export function verifyManifestVersion(manifetVersion: string) {
 
   // if framework > cli, we require a newer cli version
   if (semver.gt(frameworkVersion, toolkitVersion)) {
-    throw new Error(`CLI >= ${frameworkVersion} is required to interact with this app`);
+    throw new Error(`CDK CLI >= ${frameworkVersion} is required to interact with this app`);
   }
 
   // if framework < cli, we require a newer framework version
   if (semver.lt(frameworkVersion, toolkitVersion)) {
-    throw new Error(`App used framework v${frameworkVersion} but it must be >= v${CLOUD_ASSEMBLY_VERSION} in order to interact with this CLI`);
+    throw new Error(
+      `CDK CLI can only be used with apps created by CDK >= ${CLOUD_ASSEMBLY_VERSION}`);
   }
 }
 

packages/@aws-cdk/cx-api/test/cloud-assembly.test.ts

@@ -95,6 +95,6 @@ test('fails for invalid dependencies', () => {
 
 test('verifyManifestVersion', () => {
   verifyManifestVersion('0.33.0');
-  expect(() => verifyManifestVersion('0.31.0')).toThrow('App used framework v0.31.0 but it must be >= v0.33.0 in order to interact with this CLI');
-  expect(() => verifyManifestVersion('0.34.0')).toThrow('CLI >= 0.34.0 is required to interact with this app');
+  expect(() => verifyManifestVersion('0.31.0')).toThrow('CDK CLI can only be used with apps created by CDK >= 0.33.0');
+  expect(() => verifyManifestVersion('0.34.0')).toThrow('CDK CLI >= 0.34.0 is required to interact with this app');
 });