next-rspack

Purpose

next-rspack is the adapter that lets users replace webpack with rspack — a webpack-API-compatible bundler written in Rust. The package is small (a hundred-odd lines plus a re-export of rspack core) because rspack mostly drops in where webpack does. Activated by setting NEXT_RSPACK=1.

Source: packages/next-rspack/.

Directory layout

packages/next-rspack/
├── README.md
├── index.js            # Plugin / loader registry
├── index.d.ts
├── rspack-core.js      # Re-exports @rspack/core under a stable shape
└── package.json

The package's job is to translate Next.js's webpack-flavored config into rspack-compatible config and to register the rspack-flavored variants of Next.js-specific webpack plugins.

Activation

NEXT_RSPACK=1 next dev
NEXT_RSPACK=1 next build

The framework reads process.env.NEXT_RSPACK in packages/next/src/build/index.ts and packages/next/src/server/dev/hot-reloader-rspack.ts to switch the builder. The selection cascade is:

1. --turbopack flag or default → Turbopack
2. NEXT_RSPACK=1 → rspack
3. --webpack flag → webpack

Plugin compatibility

Some Next.js webpack plugins do not work unmodified on rspack. The adapter:

• Translates plugin shapes that depend on webpack-specific internals.
• Provides rspack-flavored versions of define-env-plugin, the route-bundle-stats plugin, and the build manifest plugin.
• Handles asset module differences (rspack and webpack diverge on a few module.rules shapes).

Differences are tracked in the rspack-specific test manifests at test/rspack-build-tests-manifest.json and test/rspack-dev-tests-manifest.json (which together top 1.6 MB of skipped/known-failing test entries — the rspack parity work is ongoing).

CI and releases

rspack support has its own CI lanes and release flow:

• .github/workflows/release-next-rspack.yml — publishes rspack-compatible builds
• .github/workflows/rspack-nextjs-build-integration-tests.yml and rspack-nextjs-dev-integration-tests.yml — test runs
• .github/workflows/rspack-update-tests-manifest.yml — refreshes the rspack tests manifest

The rspack/ directory at the repo root holds patched binding crates that adapt the upstream @rspack/core to Next.js's needs.

Hot reloader

packages/next/src/server/dev/hot-reloader-rspack.ts (8.3 KB) is the rspack flavor of the dev hot reloader. It mirrors the structure of hot-reloader-webpack.ts but uses the rspack compiler API.

Key source files

File	Purpose
packages/next-rspack/index.js	Plugin / loader registry
packages/next-rspack/rspack-core.js	Re-exports @rspack/core
packages/next/src/server/dev/hot-reloader-rspack.ts	Dev hot reloader
packages/next/src/build/webpack-config.ts	Shared config used (with branches) for both bundlers
rspack/	Patched rspack binding crates