Revert "Revert "Revert "Revert "Add a --webpack flag and default --turbopack to true (#84216)"""" (#84394)

Reverts #84389

Attempt number 3, #84374 fixed propagation of bundler environment variables to vercel cli operations, to ensure the test configuration is respected. 

[Deployment Tests Run 1](https://github.com/vercel/next.js/actions/runs/18146684793/job/51649631635).  A fair number of failures.

[Deployment Tests Run 2](https://github.com/vercel/next.js/actions/runs/18154875270). After #84395.  These still have a set of failures, but i confirmed that the deployment builds are running `webpack`.  

The tests that are failing are related to 'prefetches' (test/e2e/app-dir/segment-cache/prefetch-runtime/prefetch-runtime.test.ts and test/e2e/app-dir/segment-cache/prefetch-layout-sharing/prefetch-layout-sharing.test.ts) and generally the error is a timeout.  Recent deployment runs for canary releases are also failing, but on different tests.


Trying again after #[84419](#84419).  [Deployment Tests Run 3](https://github.com/vercel/next.js/actions/runs/18173375427).  Failures appeared flaky, rerunning failures...

Author: lukesandberg

.github/workflows/build_and_test.yml

@@ -502,6 +502,7 @@ jobs:
 
         export NEXT_TEST_MODE=start
         export NEXT_TEST_WASM=true
+        export IS_WEBPACK_TEST=1
         node run-tests.js \
           test/production/pages-dir/production/test/index.test.ts \
           test/e2e/streaming-ssr/index.test.ts
@@ -711,7 +712,7 @@ jobs:
 
     secrets: inherit
 
-  test-dev:
+  test-dev: # TODO: rename to include webpack
     name: test dev
     needs: ['optimize-ci', 'changes', 'build-native', 'build-next']
     if: ${{ needs.optimize-ci.outputs.skip == 'false' && needs.changes.outputs.docs-only == 'false' }}
@@ -728,6 +729,7 @@ jobs:
     uses: ./.github/workflows/build_reusable.yml
     with:
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         export NEXT_TEST_MODE=dev
         export NEXT_TEST_REACT_VERSION="${{ matrix.react }}"
 
@@ -752,8 +754,10 @@ jobs:
 
     uses: ./.github/workflows/build_reusable.yml
     with:
+      # Should this be using turbopack? a variation?
       afterBuild: |
         export NEXT_TEST_MODE=dev
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           test/e2e/app-dir/app/index.test.ts \
@@ -780,6 +784,7 @@ jobs:
     with:
       nodeVersion: 20.9.0
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         node run-tests.js \
           --concurrency 4 \
           test/production/pages-dir/production/test/index.test.ts \
@@ -808,6 +813,7 @@ jobs:
     with:
       afterBuild: |
         export NEXT_TEST_MODE=start
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js --type production \
           test/e2e/app-dir/app/index.test.ts \
@@ -836,6 +842,7 @@ jobs:
     uses: ./.github/workflows/build_reusable.yml
     with:
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         export NEXT_TEST_MODE=start
         export NEXT_TEST_REACT_VERSION="${{ matrix.react }}"
 
@@ -875,6 +882,7 @@ jobs:
     with:
       nodeVersion: 20.9.0
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         export NEXT_TEST_REACT_VERSION="${{ matrix.react }}"
 
         node run-tests.js \
@@ -896,6 +904,7 @@ jobs:
 
         # these all run without concurrency because they're heavier
         export TEST_CONCURRENCY=1
+        export IS_WEBPACK_TEST=1
 
         BROWSER_NAME=firefox node run-tests.js \
           test/production/pages-dir/production/test/index.test.ts
@@ -923,6 +932,7 @@ jobs:
       afterBuild: |
         export __NEXT_EXPERIMENTAL_PPR=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/ppr-tests-manifest.json"
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -945,6 +955,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_PPR=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/ppr-tests-manifest.json"
         export NEXT_TEST_MODE=dev
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -968,6 +979,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_PPR=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/ppr-tests-manifest.json"
         export NEXT_TEST_MODE=start
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -990,6 +1002,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_PPR=true # for compatibility with the existing tests
         export __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -1014,6 +1027,7 @@ jobs:
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
         export NEXT_TEST_MODE=dev
         export __NEXT_EXPERIMENTAL_ISOLATED_DEV_BUILD=true
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -1038,6 +1052,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
         export NEXT_TEST_MODE=start
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \

.github/workflows/pull_request_stats.yml

@@ -60,3 +60,7 @@ jobs:
 
       - uses: ./.github/actions/next-stats-action
         if: ${{ steps.docs-change.outputs.DOCS_CHANGE == 'nope' }}
+        env:
+          # This uses the webpack bundle analyzer and for consistent results we need to use webpack.
+          # Once there is an equivalent analyzer for turbopack, we can remove this.
+          IS_WEBPACK_TEST: 1

.github/workflows/test_e2e_deploy_release.yml

@@ -78,7 +78,15 @@ jobs:
       matrix:
         group: [1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8, 8/8]
     with:
-      afterBuild: npm i -g vercel@latest && NEXT_E2E_TEST_TIMEOUT=240000 NEXT_TEST_MODE=deploy NEXT_EXTERNAL_TESTS_FILTERS="test/deploy-tests-manifest.json" NEXT_TEST_VERSION="${{ github.event.inputs.nextVersion || 'canary' }}" VERCEL_CLI_VERSION="${{ github.event.inputs.vercelCliVersion || 'vercel@latest' }}" node run-tests.js --timings -g ${{ matrix.group }} -c 2 --type e2e
+      afterBuild: |
+        npm i -g vercel@latest && \
+        NEXT_E2E_TEST_TIMEOUT=240000 \
+        NEXT_TEST_MODE=deploy \
+        IS_WEBPACK_TEST=1 \
+        NEXT_EXTERNAL_TESTS_FILTERS="test/deploy-tests-manifest.json" \
+        NEXT_TEST_VERSION="${{ github.event.inputs.nextVersion || 'canary' }}" \
+        VERCEL_CLI_VERSION="${{ github.event.inputs.vercelCliVersion || 'vercel@latest' }}" \
+        node run-tests.js --timings -g ${{ matrix.group }} -c 2 --type e2e
       skipNativeBuild: 'yes'
       skipNativeInstall: 'no'
       stepName: 'test-deploy-${{ matrix.group }}'

bench/heavy-npm-deps/package.json

@@ -4,9 +4,9 @@
   "private": true,
   "scripts": {
     "dev-turbopack": "next dev --turbopack",
-    "dev-webpack": "next dev",
+    "dev-webpack": "next dev --webpack",
     "build-turbopack": "next build --turbopack",
-    "build-webpack": "next build",
+    "build-webpack": "next build --webpack",
     "start-turbopack": "next start",
     "start-webpack": "next start",
     "build-application": "next build",

bench/module-cost/package.json

@@ -3,10 +3,10 @@
   "scripts": {
     "prepare-bench": "node scripts/prepare-bench.mjs",
     "benchmark": "node scripts/benchmark-runner.mjs",
-    "dev-webpack": "next dev",
-    "dev-turbopack": "next dev --turbo",
-    "build-webpack": "next build",
-    "build-turbopack": "next build --turbo",
+    "dev-webpack": "next dev --webpack",
+    "dev-turbopack": "next dev --turbopack",
+    "build-webpack": "next build --webpack",
+    "build-turbopack": "next build --turbopack",
     "start": "next start"
   },
   "devDependencies": {

package.json

@@ -17,29 +17,49 @@
     "ci:publish": "tsx ./scripts/release/publish-npm.ts",
     "test-types": "tsc",
     "test-unit": "jest test/unit/ packages/next/ packages/font",
-    "test-dev": "cross-env NEXT_TEST_MODE=dev pnpm testheadless",
-    "test-dev-experimental": "cross-env NEXT_TEST_MODE=dev pnpm run with-experimental pnpm testheadless",
-    "test-dev-rspack": "pnpm run with-rspack pnpm run test-dev",
-    "test-dev-experimental-rspack": "pnpm run with-rspack pnpm run test-dev-experimental",
-    "test-dev-turbo": "cross-env NEXT_TEST_MODE=dev IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm testheadless",
-    "test-dev-experimental-turbo": "cross-env NEXT_TEST_MODE=dev IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm run with-experimental pnpm testheadless",
-    "test-start": "cross-env NEXT_TEST_MODE=start pnpm testheadless",
-    "test-start-experimental": "cross-env NEXT_TEST_MODE=start pnpm run with-experimental pnpm testheadless",
-    "test-start-rspack": "pnpm run with-rspack pnpm run test-start",
-    "test-start-experimental-rspack": "pnpm run with-rspack pnpm run test-start-experimental",
-    "test-start-turbo": "cross-env NEXT_TEST_MODE=start IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testheadless",
-    "test-start-experimental-turbo": "cross-env NEXT_TEST_MODE=start IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm run with-experimental pnpm testheadless",
-    "test-deploy": "cross-env NEXT_TEST_MODE=deploy pnpm testheadless",
-    "testonly-dev": "cross-env NEXT_TEST_MODE=dev pnpm testonly",
-    "testonly-dev-rspack": "pnpm run with-rspack pnpm run testonly-dev",
-    "testonly-dev-turbo": "cross-env NEXT_TEST_MODE=dev IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm testonly",
-    "testonly-start": "cross-env NEXT_TEST_MODE=start pnpm testonly",
-    "testonly-start-rspack": "pnpm run with-rspack pnpm run testonly-start",
-    "testonly-start-turbo": "cross-env NEXT_TEST_MODE=start IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testonly",
-    "testonly-deploy": "cross-env NEXT_TEST_MODE=deploy pnpm testonly",
-    "test": "pnpm testheadless",
-    "test-rspack": "pnpm run with-rspack pnpm run test",
-    "test-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 TURBOPACK_BUILD=1 pnpm testheadless",
+    "test-dev-inner": "cross-env NEXT_TEST_MODE=dev pnpm testheadless",
+    "test-dev": "pnpm run test-dev-webpack",
+    "test-dev-webpack": "pnpm run with-webpack pnpm test-dev-inner",
+    "test-dev-experimental-inner": "pnpm run with-experimental pnpm test-dev-inner",
+    "test-dev-experimental": "pnpm run test-dev-experimental-webpack",
+    "test-dev-experimental-webpack": "pnpm run with-webpack pnpm test-dev-experimental-inner",
+    "test-dev-rspack": "pnpm run with-rspack pnpm run test-dev-inner",
+    "test-dev-experimental-rspack": "pnpm run with-rspack pnpm run test-dev-experimental-inner",
+    "test-dev-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm test-dev-inner",
+    "test-dev-experimental-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm run with-experimental pnpm test-dev-inner",
+    "test-start-inner": "cross-env NEXT_TEST_MODE=start pnpm testheadless",
+    "test-start": "pnpm run test-start-webpack",
+    "test-start-webpack": "pnpm run with-webpack pnpm run test-start-inner",
+    "test-start-experimental-inner": "pnpm run with-experimental pnpm test-start-inner",
+    "test-start-experimental": "pnpm run test-start-experimental-webpack",
+    "test-start-experimental-webpack": "pnpm run with-webpack pnpm run test-start-experimental-inner",
+    "test-start-rspack": "pnpm run with-rspack pnpm run test-start-inner",
+    "test-start-experimental-rspack": "pnpm run with-rspack pnpm run test-start-experimental-inner",
+    "test-start-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm test-start-inner",
+    "test-start-experimental-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm run with-experimental pnpm test-start-experimental-inner",
+    "test-deploy-inner": "cross-env NEXT_TEST_MODE=deploy pnpm testheadless",
+    "test-deploy": "pnpm run test-deploy-webpack",
+    "test-deploy-webpack": "pnpm run with-webpack pnpm test-deploy-inner",
+    "test-deploy-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm test-deploy-inner",
+    "testonly-dev-inner": "cross-env NEXT_TEST_MODE=dev pnpm testonly",
+    "testonly-dev": "pnpm run testonly-dev-webpack",
+    "testonly-dev-webpack": "pnpm run with-webpack pnpm run testonly-dev-inner",
+    "testonly-dev-rspack": "pnpm run with-rspack pnpm run testonly-dev-inner",
+    "testonly-dev-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm testonly-dev-inner",
+    "testonly-start-inner": "cross-env NEXT_TEST_MODE=start pnpm testonly",
+    "testonly-start": "pnpm run testonly-start-webpack",
+    "testonly-start-webpack": "pnpm run with-webpack pnpm run testonly-start-inner",
+    "testonly-start-rspack": "pnpm run with-rspack pnpm run testonly-start-inner",
+    "testonly-start-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testonly-start-inner",
+    "testonly-deploy-inner": "cross-env NEXT_TEST_MODE=deploy pnpm testonly",
+    "testonly-deploy": "pnpm run testonly-deploy-webpack",
+    "testonly-deploy-webpack": "pnpm run with-webpack pnpm run testonly-deploy-inner",
+    "testonly-deploy-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testonly-deploy-inner",
+    "test-inner": "pnpm testheadless",
+    "test": "pnpm test-webpack",
+    "test-webpack": "pnpm run with-webpack pnpm run test-inner",
+    "test-rspack": "pnpm run with-rspack pnpm run test-inner",
+    "test-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 TURBOPACK_BUILD=1 pnpm test-inner",
     "testonly": "jest --runInBand",
     "testheadless": "cross-env HEADLESS=true pnpm testonly",
     "genstats": "cross-env LOCAL_STATS=true node .github/actions/next-stats-action/src/index.js",
@@ -89,6 +109,7 @@
     "build-storybook": "turbo run build-storybook",
     "test-storybook": "turbo run test-storybook",
     "with-rspack": "cross-env NEXT_RSPACK=1 NEXT_TEST_USE_RSPACK=1",
+    "with-webpack": "cross-env IS_WEBPACK_TEST=1",
     "with-experimental": "cross-env __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true __NEXT_EXPERIMENTAL_PPR=true"
   },
   "devDependencies": {

packages/next-rspack/index.js

@@ -1,5 +1,15 @@
 module.exports = function withRspack(config) {
   process.env.NEXT_RSPACK = 'true'
   process.env.RSPACK_CONFIG_VALIDATE = 'loose-silent'
+  if (process.env.TURBOPACK === 'auto') {
+    // If next has defaulted to turbopack, override it.
+    delete process.env.TURBOPACK
+  } else {
+    console.error(
+      `Cannot call withRspack and pass the ${process.env.TURBOPACK ? '--turbopack' : '--webpack'} flag.`
+    )
+    console.error('Please configure only one bundler.')
+    process.exit(1)
+  }
   return config
 }

packages/next/src/bin/next.ts

@@ -133,8 +133,9 @@ program
   .option('--no-mangling', 'Disables mangling.')
   .option('--profile', 'Enables production profiling for React.')
   .option('--experimental-app-only', 'Builds only App Router routes.')
-  .option('--turbo', 'Starts development mode using Turbopack.')
-  .option('--turbopack', 'Starts development mode using Turbopack.')
+  .option('--turbo', 'Builds using Turbopack.')
+  .option('--turbopack', 'Builds using Turbopack.')
+  .option('--webpack', 'Builds using webpack.')
   .addOption(
     new Option(
       '--experimental-build-mode [mode]',
@@ -173,6 +174,7 @@ program
   )
   .option('--turbo', 'Starts development mode using Turbopack.')
   .option('--turbopack', 'Starts development mode using Turbopack.')
+  .option('--webpack', 'Starts development mode using webpack.')
   .addOption(
     new Option(
       '-p, --port <port>',