[costajohnt]

Closes #10880

optimizeLocales.turbopack() declares each loader's options as {locales: readonly string[]}. Next's TurbopackLoaderOptions is Record<string, JSONValue>, and JSONValue[] is a mutable array, so spreading the returned rules into a typed NextConfig fails with TS2322 even though the rule works at runtime. This changes the declared loader option to string[], which is the change suggested in the issue. The Options input type stays readonly string[], so callers can still pass as const arrays. Only the shape handed to Next changes.

I checked it against next@16.4.2 and TypeScript 6.0.3 (strict) with a next.config.ts covering a single config, a conditioned list, a readonly as const locales array, and a path/content RegExp condition. With the current declaration all four report TS2322 (The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'JSONValue[]'). With this change tsc --noEmit passes.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at AGENTS.md.

Type-only change to a .d.ts, so no unit test, docs or a11y change applies. The plugin directory is excluded from yarn check-types, so the verification is the standalone check below. Drafted with Claude Code; I reviewed the change and the verification myself.

📝 Test Instructions:

1. In a scratch project, install next@16.4.2 and typescript@6, and copy this package's LocalesPlugin.d.ts and package.json into node_modules/@react-aria/optimize-locales-plugin/.
2. Add the next.config.ts from optimize-locales-plugin: turbopack() rules are refused by Next 16.4's NextConfig type (readonly locales) #10880:

   import optimizeLocales from "@react-aria/optimize-locales-plugin";
   import type { NextConfig } from "next";
   
   const nextConfig: NextConfig = {
     turbopack: {
       rules: {
         ...optimizeLocales.turbopack({ locales: ["de-DE"] }).rules,
       },
     },
   };
   
   export default nextConfig;

3. Run npx tsc --noEmit --strict. It reports TS2322 on main and passes with this branch.

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