Add unstable parallel metadata selectors - #97449
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## Summary Add `unstable_selectMetadata` and `unstable_selectViewport` layout exports so applications can override the built-in selection at the next parallel-route fork. Each selector receives slot-keyed handles containing a resolution status and postprocessed value. Returning a handle or its resolved result preserves that branch, while returning a rewritten selected value creates a resolved selection for higher forks. Track metadata and viewport selection independently, apply the closest selector above each fork, and carry nested selections upward before resolving a new fork. Continue using the built-in structural heuristic when no selector is defined and preserve eager metadata and viewport generation. Keep branch outcomes separate from head selection. Navigation and regular errors remain attached to their slot outlets, while selector failures replay through every descendant outlet so whichever slot renders reaches its normal boundary. Convention metadata and viewport are resolved against their independently selected branch paths. Export the new selection types, validate selectors as layout-only server APIs in both route type generators and the RSC transform, and add coverage for direct handles, rewritten values, ancestor and nested selectors, independent metadata and viewport choices, navigation status, and selector error replay. ## Verification - `pnpm --filter=next types` - `pnpm exec eslint --config eslint.config.mjs packages/next/src/build/webpack/plugins/next-types-plugin/index.ts packages/next/src/lib/metadata/metadata-parallel.tsx packages/next/src/lib/metadata/metadata-resolution-primitives.ts packages/next/src/lib/metadata/resolve-metadata-parallel.ts packages/next/src/lib/metadata/types/metadata-interface.ts packages/next/src/server/lib/router-utils/typegen.ts packages/next/src/server/typescript/constant.ts packages/next/src/types.ts test/e2e/app-dir/metadata-streaming-parallel-routes` - `cargo fmt --all -- --check` - `cargo test -p next-custom-transforms --test errors react_server_components_errors` - `pnpm test-dev-turbo test/e2e/app-dir/metadata-streaming-parallel-routes/metadata-streaming-parallel-routes.test.ts` - `pnpm test-start-turbo test/e2e/app-dir/metadata-streaming-parallel-routes/metadata-streaming-parallel-routes.test.ts` - `pnpm test-start-webpack test/e2e/app-dir/metadata-streaming-parallel-routes/metadata-streaming-parallel-routes.test.ts` <!-- NEXT_JS_LLM -->
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Summary
Add
unstable_selectMetadataandunstable_selectViewportlayout exports so applications can override the built-in selection at the next parallel-route fork. Each selector receives slot-keyed handles containing a resolution status and postprocessed value. Returning a handle or its resolved result preserves that branch, while returning a rewritten selected value creates a resolved selection for higher forks.Track metadata and viewport selection independently, apply the closest selector above each fork, and carry nested selections upward before resolving a new fork. Continue using the built-in structural heuristic when no selector is defined and preserve eager metadata and viewport generation.
Keep branch outcomes separate from head selection. Navigation and regular errors remain attached to their slot outlets, while selector failures replay through every descendant outlet so whichever slot renders reaches its normal boundary. Convention metadata and viewport are resolved against their independently selected branch paths.
Export the new selection types, validate selectors as layout-only server APIs in both route type generators and the RSC transform, and add coverage for direct handles, rewritten values, ancestor and nested selectors, independent metadata and viewport choices, navigation status, and selector error replay.
Verification
pnpm --filter=next typespnpm exec eslint --config eslint.config.mjs packages/next/src/build/webpack/plugins/next-types-plugin/index.ts packages/next/src/lib/metadata/metadata-parallel.tsx packages/next/src/lib/metadata/metadata-resolution-primitives.ts packages/next/src/lib/metadata/resolve-metadata-parallel.ts packages/next/src/lib/metadata/types/metadata-interface.ts packages/next/src/server/lib/router-utils/typegen.ts packages/next/src/server/typescript/constant.ts packages/next/src/types.ts test/e2e/app-dir/metadata-streaming-parallel-routescargo fmt --all -- --checkcargo test -p next-custom-transforms --test errors react_server_components_errorspnpm test-dev-turbo test/e2e/app-dir/metadata-streaming-parallel-routes/metadata-streaming-parallel-routes.test.tspnpm test-start-turbo test/e2e/app-dir/metadata-streaming-parallel-routes/metadata-streaming-parallel-routes.test.tspnpm test-start-webpack test/e2e/app-dir/metadata-streaming-parallel-routes/metadata-streaming-parallel-routes.test.ts