Planned breaking changes for Rspack 2.0

[hardfist]

This topic is used to collect and discuss breaking changes planned for Rspack 2.0.

Please note that we do not have a clear timeline for Rspack 2.0 yet.

Example:

• remove RSPACK_CONFIG_VALIDATE environment variable and enable it by default

[chenjiahan]

Make @module-federation/runtime-tools optional

Currently, @module-federation/runtime-tools is a direct dependency of @rspack/core.

In Rspack 2.0 we can make it an optional peer dependency to reduce dependencies.

Context

Currently, Rspack's built-in @module-federation/runtime-tools is only used when using the MF 1.5 plugin and the implementation option is not set.

• If the application uses MF 2.0, only @module-federation/enhanced -> @module-federation/runtime-tools will be used:

devDependencies:
@module-federation/enhanced 0.14.0
└── @module-federation/runtime-tools 0.14.0
@rspack/core 1.2.8
└── @module-federation/runtime-tools 0.8.4  ---->  never used!

• If the application uses MF 1.0 or does not use MF, Rspack's built-in @module-federation/runtime-tools will never be used.

Benefit

By making @module-federation/runtime-tools optional, we can avoid installing unnecessary dependencies in the application and speed up the npm installation.

6 dependencies can be removed:

[hardfist]

Enable verbatimModuleSyntax: true by default for buitin:swc-loader

Typescript import elision is a footgun for bundler which can eagerly remove module with sideEffect, and TypeScript introduce a new safe option to disable this behavior which should be enabled by default for builtin:swc-loader.

Ref:

• https://swc.rs/docs/migrating-from-tsc#verbatimmodulesyntax-true
• https://www.typescriptlang.org/tsconfig/#verbatimModuleSyntax

[hardfist]

Drop Node 18 support

Node.js 18 and all earlier versions are End-Of-Life since April 30, 2025, see https://nodejs.org/en/blog/announcements/node-18-eol-support

I'm not sure whether we can drop Node 20 support.

Ref:

• https://nodejs.org/en/about/previous-releases

[chenjiahan]

Node 16 is planned to be dropped in Rspack v1.5

[hardfist]

@rspack/core may still support Node.js 20 but @rspack/cli should drepcate Node.js 20 support

[chenjiahan]

Drop support for Node 18 allows Rspack to use more new features of Node 20, such as require(ESM), which means Rspack can be converted from a CJS package to a pure ESM package.

See

• https://socket.dev/blog/require-esm-backported-to-node-js-20
• https://x.com/rspack_dev/status/1948678450057019852

[hardfist]

Remove config.stats.profile and config.profile support

Rspack recommend use tracing for debugging performance issue and consider remove stats.profile has config.profile support

[chenjiahan]

Simplify target configuration

In Rspack 2.0, we can make Rspack's target option affect the default targets of SWC and LightningCSS (which can still be manually overridden), ensuring that users only need to configure target once, rather than setting target in multiple different places.

Ref:

• https://rspack.rs/config/target#target
• https://rspack.rs/guide/features/builtin-swc-loader#syntax-lowering
• https://rspack.rs/guide/features/builtin-lightningcss-loader#targets
• https://rspack.rs/plugins/rspack/lightning-css-minimizer-rspack-plugin#minimizeroptions

Before

export default defineConfig({
  target: ['web', 'es2022'],
  module: {
    rules: [
      {
        test: /\.js$/,
        use: {
          loader: 'builtin:swc-loader',
          options: {
            jsc: {
              target: 'es2022',
            },
          },
        },
      },
      {
        test: /\.css$/,
        use: [
          {
            loader: 'builtin:lightningcss-loader',
            options: {
              targets: 'Chrome >= 93', // -> es2022
            },
          },
        ],
      },
    ],
  },
  plugins: [
    new rspack.LightningCssMinimizerRspackPlugin({
      minimizerOptions: {
        targets: 'Chrome >= 93', // -> es2022
      },
    }),
  ],
});

After

export default defineConfig({
  target: ['web', 'es2022'],
  module: {
    rules: [
      {
        test: /\.js$/,
        use: { loader: 'builtin:swc-loader' },
      },
      {
        test: /\.css$/,
        use: [
          { loader: 'builtin:lightningcss-loader' },
        ],
      },
    ],
  },
  plugins: [new rspack.LightningCssMinimizerRspackPlugin()],
});

[chenjiahan]

Default options of stats.toJson()

Currently, stats.toJson() has all: true enabled by default, which causes significant communication overhead between Rust and JS, affecting the performance of large applications.

In Rspack 2.0, we need to improve the default options of stats.toJson() to avoid this problem.

@SyMind cc

[chenjiahan]

Consider ignoring aliased dynamic require

We're considering skipping aliased dynamic require by default, which would help reduce bundle size for libraries like moment.js and align with other bundlers' behavior, though whether this should be changed remains open for discussion.

var aliasRequire = require
aliasRequire('./locale' + name)

See filipsobol/bundle-size-benchmark#2 for more details.