fix(cli): warn when object-form manualChunks is detected (#1046)

resolve #900

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kazupon

packages/cli/snap-tests-global/migration-merge-vite-config-ts/package.json

@@ -17,6 +17,7 @@
   },
   "devDependencies": {
     "@vitejs/plugin-react": "^4.2.0",
+    "@vitest/browser-playwright": "^4.0.0",
     "oxfmt": "1",
     "oxlint": "1",
     "vite": "^7.0.0",

packages/cli/snap-tests-global/migration-monorepo-pnpm-overrides-dependency-selector/package.json

@@ -5,6 +5,7 @@
     "dev": "vite"
   },
   "devDependencies": {
+    "@vitejs/plugin-react": "catalog:",
     "vite": "catalog:"
   },
   "packageManager": "pnpm@10.20.0+sha512.cf9998222162dd85864d0a8102e7892e7ba4ceadebbf5a31f9c2fce48dfce317a9c53b9f6464d1ef9042cba2e02ae02a9f7c143a2b438cd93c91840f0192b9dd",

packages/cli/snap-tests-global/migration-monorepo-pnpm-overrides-dependency-selector/snap.txt

@@ -26,6 +26,7 @@ export default defineConfig({
     "prepare": "vp config"
   },
   "devDependencies": {
+    "@vitejs/plugin-react": "catalog:",
     "vite": "catalog:",
     "vite-plus": "catalog:"
   },

packages/cli/snap-tests-global/migration-monorepo-pnpm/package.json

@@ -16,6 +16,7 @@
     "testnpm2": "1.0.0"
   },
   "devDependencies": {
+    "@vitejs/plugin-react": "catalog:",
     "oxfmt": "catalog:",
     "oxlint": "catalog:",
     "vite": "catalog:",

packages/cli/snap-tests-global/migration-monorepo-pnpm/snap.txt

@@ -62,6 +62,7 @@ cat: .oxfmtrc.json: No such file or directory
     "testnpm2": "1.0.0"
   },
   "devDependencies": {
+    "@vitejs/plugin-react": "catalog:",
     "vite": "catalog:",
     "vitest": "catalog:",
     "vite-plus": "catalog:"

packages/cli/snap-tests-global/migration-monorepo-yarn4/package.json

@@ -19,6 +19,7 @@
     "testnpm2": "1.0.0"
   },
   "devDependencies": {
+    "@vitejs/plugin-react": "catalog:",
     "oxfmt": "catalog:",
     "oxlint": "catalog:",
     "vite": "catalog:",

packages/cli/snap-tests-global/migration-monorepo-yarn4/snap.txt

@@ -53,6 +53,7 @@ cat: .oxlintrc.json: No such file or directory
     "testnpm2": "1.0.0"
   },
   "devDependencies": {
+    "@vitejs/plugin-react": "catalog:",
     "vite": "catalog:",
     "vitest": "catalog:",
     "vite-plus": "catalog:"

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

@@ -540,6 +540,22 @@ function showMigrationSummary(options: {
   }
 }
 
+async function checkRolldownCompatibility(rootDir: string, report: MigrationReport): Promise<void> {
+  try {
+    const { resolveConfig } = await import('../index.js');
+    const { checkManualChunksCompat } = await import('./compat.js');
+    // Use 'runner' configLoader to avoid Rolldown bundling the config file,
+    // which prints UNRESOLVED_IMPORT warnings that cannot be suppressed via logLevel.
+    const config = await resolveConfig(
+      { root: rootDir, logLevel: 'silent', configLoader: 'runner' },
+      'build',
+    );
+    checkManualChunksCompat(config.build?.rollupOptions?.output, report);
+  } catch {
+    // Config resolution may fail — skip compatibility check silently
+  }
+}
+
 async function executeMigrationPlan(
   workspaceInfoOptional: WorkspaceInfoOptional,
   plan: MigrationPlan,
@@ -637,7 +653,16 @@ async function executeMigrationPlan(
     cancelAndExit('Vite+ cannot automatically migrate this project yet.', 1);
   }
 
-  // 5. ESLint → Oxlint migration (before main rewrite so .oxlintrc.json gets picked up)
+  // 5. Check for Rolldown-incompatible config patterns (root + workspace packages)
+  updateMigrationProgress('Checking config compatibility');
+  await checkRolldownCompatibility(workspaceInfo.rootDir, report);
+  if (workspaceInfo.packages) {
+    for (const pkg of workspaceInfo.packages) {
+      await checkRolldownCompatibility(path.join(workspaceInfo.rootDir, pkg.path), report);
+    }
+  }
+
+  // 6. ESLint → Oxlint migration (before main rewrite so .oxlintrc.json gets picked up)
   if (plan.migrateEslint) {
     updateMigrationProgress('Migrating ESLint');
     const eslintOk = await migrateEslintToOxlint(
@@ -725,6 +750,7 @@ async function executeMigrationPlan(
     installArgs,
     { silent: true },
   );
+
   clearMigrationProgress();
   return {
     installDurationMs: initialInstallSummary.durationMs + finalInstallSummary.durationMs,
@@ -825,7 +851,18 @@ async function main() {
       }
     }
 
-    if (didMigrate) {
+    // Check for Rolldown-incompatible config patterns (root + workspace packages)
+    await checkRolldownCompatibility(workspaceInfoOptional.rootDir, report);
+    if (workspaceInfoOptional.packages) {
+      for (const pkg of workspaceInfoOptional.packages) {
+        await checkRolldownCompatibility(
+          path.join(workspaceInfoOptional.rootDir, pkg.path),
+          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;
+    }
+  }
+}