Expose ReactNativeFeatureFlags through react-private-interface

[giaBaoJS]

Summary:

Fixes #57933.

@react-native/virtualized-lists is published separately and imported ReactNativeFeatureFlags through react-native/src/private/featureflags/ReactNativeFeatureFlags, which is not listed in react-native's "exports". Metro therefore warned and fell back to file-based resolution whenever an app rendered a virtualized list.

Thanks @huntie for the patch and the direction — this PR now applies it instead of the original approach:

• ReactNativeFeatureFlags is exposed on the existing private package boundary, react-native/react-private-interface (both the runtime getter and the .js.flow re-export);
• VirtualizedList.js and VirtualizeUtils.js import it from there.

No new src/private/* subpath is exported, and the feature-flag singleton is unchanged.

Per your review, the scripts/monorepo-tests/__tests__/check-packages-test.js and scripts/shared/monorepoUtils.js changes have been dropped — the PR is now just the patch above. Happy to look at enabling @react-native/no-deep-imports on virtualized-lists as a follow-up if that's wanted.

VirtualizeUtils.js is included alongside VirtualizedList.js because it carried the same runtime deep import. The remaining occurrences are out of scope: the four in @react-native/jest-preset are all import type and are erased before resolution, and the one in VirtualizeUtils-test.js is not shipped (virtualized-lists excludes **/__tests__/** from files).

Changelog:

[GENERAL] [FIXED] - Fix the Metro package-exports warning caused by @react-native/virtualized-lists importing an unexported React Native subpath.

Test Plan:

No new test is added. The existing virtualized-lists suites already cover this route, because VirtualizeUtils/VirtualizedList read the flags at runtime through the new boundary.

Counterfactual — dropping only the ReactNativeFeatureFlags getter and its import typeof from react-private-interface.js, keeping the two virtualized-lists imports:

TypeError: Cannot read properties of undefined (reading 'fixVirtualizeListCollapseWindowSize')

      182 |     let lastWillAddMore;
      183 |
    > 184 |     if (ReactNativeFeatureFlags.fixVirtualizeListCollapseWindowSize()) {
          |                                ^

      at computeWindowedRenderLimits (packages/virtualized-lists/Lists/VirtualizeUtils.js:184:32)
      at Object.<anonymous> (packages/virtualized-lists/Lists/__tests__/VirtualizeUtils-test.js:261:47)

Test Suites: 2 failed, 6 passed, 8 total
Tests:       20 failed, 1 skipped, 151 passed, 172 total

Restoring the getter makes it green again.

$ yarn jest packages/virtualized-lists scripts/monorepo-tests packages/react-native/Libraries/ReactPrivate --runInBand
Test Suites: 9 passed, 9 total
Tests:       1 skipped, 176 passed, 177 total
Snapshots:   69 passed, 69 total

$ yarn flow-check
Found 0 errors

$ yarn lint
$ eslint --max-warnings 0 .
Done in 10.23s.

[huntie]

Great spot! We'll need to solve this slightly differently however, I think by adding this API to react-private-interface.

import {ReactNativeFeatureFlags} from 'react-native/react-private-interface';

Patch here: https://gist.github.com/huntie/0523e2dd114dacf904d28211ad6b67be. Can you apply/retitle this PR to match? :)

[huntie]

Can we drop or separate out the changes from this file?

At minimum, we've reimplemented ESLint and should instead reuse it — should be a case of enabling the react-native/no-deep-imports rule directly on the virtualized-lists codebase. (But also, we're probably fine without adding new checks.)

[giaBaoJS]

Thanks for the patch @huntie — applied and retitled. ReactNativeFeatureFlags now comes from react-native/react-private-interface, and I've dropped the check-packages-test.js/monorepoUtils.js changes entirely as you suggested. Happy to do the @react-native/no-deep-imports enablement on virtualized-lists as a separate PR if you'd like it.

I also routed VirtualizeUtils.js through the same boundary since it had the identical deep import. yarn flow-check, yarn lint and the virtualized-lists suites are green.

[huntie]

Thanks for the edits :)

[huntie]

@giaBaoJS The failing build_js_types job is likely due to @microsoft/api-extractor not reading package.json "exports" — I've drafted #58075 to fix this, which will need to land first (once finished, eta Monday). Sorry for the blocker!

[giaBaoJS]

No problem at all — thanks for digging into the root cause and drafting #58075 rather than asking me to work around it. I'll leave this as-is and rebase once that lands; ping me if you want the @react-native/no-deep-imports enablement for virtualized-lists as a follow-up.