feat: add experimental.swcEnvOptions for SWC preset-env configuration (#92272)
### What?
Add `experimental.swcEnvOptions` to expose SWC's preset-env `env`
configuration options — including `mode`, `coreJs`, `include`,
`exclude`, `skip`, `shippedProposals`, `forceAllTransforms`, `debug`,
and `loose`.
### Why?
Currently Next.js only passes `env.targets` (derived from browserslist)
to SWC for **syntax downleveling**, but does not expose the polyfill
injection capabilities that SWC already supports. This means:
- Users who need automatic core-js polyfills (e.g.
`Array.prototype.at()`, `Promise.withResolvers()`, `Set` methods) have
no built-in way to get them.
- The only workarounds are importing `core-js` globally (which bloats
bundles significantly) or ejecting to Babel with `useBuiltIns: 'usage'`
(which sacrifices SWC's performance benefits).
- In the Babel era, Next.js supported this via `@babel/preset-env`'s
`useBuiltIns` (PR #10574). That capability was lost when Next.js
migrated to SWC.
### How?
A new `experimental.swcEnvOptions` config is added. Its properties are
spread into the `env` block that Next.js passes to SWC for client-side
compilation, alongside the existing browserslist-derived `targets`.
Server-side compilation is unaffected (always targets `node`).
The option surface mirrors [SWC's preset-env
docs](https://swc.rs/docs/configuration/supported-browsers) 1:1, keeping
it familiar and forward-compatible.
```js
// next.config.js
module.exports = {
experimental: {
swcEnvOptions: {
mode: 'usage',
coreJs: '3.38',
},
},
}
```
#### Changes:
config-shared.ts — type definition with JSDoc
config-schema.ts — zod validation
next-swc-loader.ts → swc/options.ts — plumb config into the SWC env
block
Unit tests (6 cases) + e2e test (dev & production)
Related issues #66562, #63104, #74978
Related discussion #46724
---------
Co-authored-by: Benjamin Woodruff <benjamin.woodruff@vercel.com>
next.js
2bf38b01 - feat: add experimental.swcEnvOptions for SWC preset-env configuration (#92272)
