[robhogan]

Fixes absolute source paths appearing in (dev-only) transform output, which could cause fresh and shared-cache builds to differ cosmetically.

Summary

In dev, @react-native/babel-preset adds @babel/plugin-transform-react-jsx-source, which embeds each file's absolute path in a _jsxFileName variable. Nothing has read that value for ordinary JSX since the preset moved to the automatic runtime in react/metro#849 (RN 0.70), because Babel extracts __source and discards it unless development is set. That leaves an unused declaration in every module with JSX.

createElement paths (key-after-spread, per-file @jsxRuntime classic) do still pass __source, but React has ignored it since react/react#28265 (React 19). Source locations come from _debugStack and source maps instead. This removes both plugins, as babel-preset-expo did in expo/expo#45351.

The motivation is that transforms become independent of the project root. Once #58584 lets checkouts share a transform cache, this also stops one checkout being served another's paths.

The only observable difference I know of is that a custom classic factory (/** @jsx h */) no longer receives __source. At worst, that breaks a fragile assumption in dev only in a very niche case.

Changelog

[General][Removed] - Babel preset: Remove @babel/plugin-transform-react-jsx-source and @babel/plugin-transform-react-jsx-self, as React no longer reads their output

Test plan

AI-driven. On main, the RNTester Android dev bundle goes from 270 _jsxFileName declarations to zero, and those were the only occurrences of the checkout path, for a 0.75% smaller bundle.

The rest ran with this change applied to the installed preset in a 0.88.0 template app (React 19.3.0) plus a probe screen, on an Android emulator:

• Bundles: dev bundles built at two project roots are byte-identical except for the sourceMappingURL filename.
• Shared cache: with asyncRequireModulePath set to metro:runtime/modules/asyncRequire, as in Use metro:runtime for asyncRequireModulePath so Metro's transform cache is shareable across checkouts #58584, and a shared FileStore, the second root gets a full cache hit and a byte-identical bundle. Without this change, that bundle carries 71 of the first root's paths.
• Runtime: element shape, inspector hierarchy, component stacks, symbolicated frames, LogBox content and open-in-editor target all match the unpatched preset, over 3 rounds per variant and then 6 interleaved.
• DevTools: driving the backend over CDP, source locations differ only for LogBox internals, and that set varies per run.
• LogBox's hidden frame count varied between runs (10-13) in both variants, with matching app frames. I can't separate the variants at this sample size, but this change doesn't touch that code.

Not tested: iOS, physical devices, or the DevTools frontend UI.

[robhogan]

(CC @hoxyq)

[meta-codesync]

@javache has imported this pull request. If you are a Meta employee, you can view this in D129319560.

[react-native-bot]

This pull request was successfully merged by @robhogan in 305ab93

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