fix(cli): warn when object-form manualChunks is detected

Author: kazupon

packages/cli/src/migration/__tests__/compat.spec.ts

@@ -0,0 +1,50 @@
+import { describe, expect, it } from 'vitest';
+
+import { checkManualChunksCompat } from '../compat.js';
+import { createMigrationReport } from '../report.js';
+
+describe('checkManualChunksCompat', () => {
+  it('should warn when manualChunks is an object', () => {
+    const report = createMigrationReport();
+    checkManualChunksCompat({ manualChunks: { react: ['react', 'react-dom'] } }, report);
+    expect(report.warnings).toHaveLength(1);
+    expect(report.warnings[0]).toContain('Object-form');
+    expect(report.warnings[0]).toContain('codeSplitting');
+  });
+
+  it('should not warn when manualChunks is a function', () => {
+    const report = createMigrationReport();
+    checkManualChunksCompat({ manualChunks: () => undefined }, report);
+    expect(report.warnings).toHaveLength(0);
+  });
+
+  it('should not warn when manualChunks is not set', () => {
+    const report = createMigrationReport();
+    checkManualChunksCompat({}, report);
+    expect(report.warnings).toHaveLength(0);
+  });
+
+  it('should not warn when output is undefined', () => {
+    const report = createMigrationReport();
+    checkManualChunksCompat(undefined, report);
+    expect(report.warnings).toHaveLength(0);
+  });
+
+  it('should handle array of outputs', () => {
+    const report = createMigrationReport();
+    checkManualChunksCompat(
+      [{ manualChunks: () => undefined }, { manualChunks: { vendor: ['lodash'] } }],
+      report,
+    );
+    expect(report.warnings).toHaveLength(1);
+  });
+
+  it('should only add one warning for multiple object-form outputs', () => {
+    const report = createMigrationReport();
+    checkManualChunksCompat(
+      [{ manualChunks: { react: ['react'] } }, { manualChunks: { lodash: ['lodash'] } }],
+      report,
+    );
+    expect(report.warnings).toHaveLength(1);
+  });
+});

packages/cli/src/migration/bin.ts

@@ -539,6 +539,17 @@ function showMigrationSummary(options: {
   }
 }
 
+async function checkRolldownCompatibility(rootDir: string, report: MigrationReport): Promise<void> {
+  try {
+    const { resolveViteConfig } = await import('../resolve-vite-config.js');
+    const { checkManualChunksCompat } = await import('./compat.js');
+    const config = await resolveViteConfig(rootDir);
+    checkManualChunksCompat(config.build?.rollupOptions?.output, report);
+  } catch {
+    // Config resolution may fail — skip compatibility check silently
+  }
+}
+
 async function executeMigrationPlan(
   workspaceInfoOptional: WorkspaceInfoOptional,
   plan: MigrationPlan,
@@ -724,6 +735,11 @@ async function executeMigrationPlan(
     installArgs,
     { silent: true },
   );
+
+  // 12. Check for Rolldown-incompatible config patterns
+  updateMigrationProgress('Checking config compatibility');
+  await checkRolldownCompatibility(workspaceInfo.rootDir, report);
+
   clearMigrationProgress();
   return {
     installDurationMs: initialInstallSummary.durationMs + finalInstallSummary.durationMs,
@@ -823,7 +839,10 @@ async function main() {
       }
     }
 
-    if (didMigrate) {
+    // Check for Rolldown-incompatible config patterns
+    await checkRolldownCompatibility(workspaceInfoOptional.rootDir, report);
+
+    if (didMigrate || report.warnings.length > 0) {
       clearMigrationProgress();
       showMigrationSummary({
         projectRoot: workspaceInfoOptional.rootDir,

packages/cli/src/migration/compat.ts

@@ -0,0 +1,19 @@
+import { addMigrationWarning, type MigrationReport } from './report.js';
+
+/**
+ * Check for Rolldown-incompatible manualChunks config patterns.
+ */
+export function checkManualChunksCompat(output: unknown, report: MigrationReport): void {
+  const outputs = Array.isArray(output) ? output : output ? [output] : [];
+  for (const out of outputs) {
+    if (out.manualChunks != null && typeof out.manualChunks !== 'function') {
+      addMigrationWarning(
+        report,
+        'Object-form `build.rollupOptions.output.manualChunks` is not supported by Rolldown. ' +
+          'Convert it to function form or use `build.rolldownOptions.output.codeSplitting`. ' +
+          'See: https://rolldown.rs/options/output#manualchunks and https://rolldown.rs/in-depth/manual-code-splitting',
+      );
+      break;
+    }
+  }
+}