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perf(webpack): swap babel-loader + babel-plugin-react-compiler for OXC (Rust React Compiler)#90619

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perf(webpack): swap babel-loader + babel-plugin-react-compiler for OXC (Rust React Compiler)#90619
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@roryabraham roryabraham commented May 14, 2026 •
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Explanation of Change

Updated June 2026: Upgraded beyond the initial oxc-loader swap to use OXC's native Rust port of the React Compiler, available in oxc-transform 0.136.0.

New three-pass pipeline (webpack use[] runs right-to-left)

Pass 1 — fullstory-annotation-loader.js (new custom loader, runs first on original JSX):

  • Injects dataComponent / dataElement / dataSourceFile props onto JSX opening elements
  • Uses @babel/parser + @babel/traverse in parse-only mode — no code transforms
  • Output still contains JSX and TypeScript so OXC sees annotated original source
  • Replaces @fullstory/babel-plugin-annotate-react without needing the full Babel transform pipeline

Pass 2 — oxc-react-compiler-loader.js (new thin wrapper, runs second):

  • Calls oxc-transform directly: React Compiler → JSX transform → TypeScript strip → env target — all in one Rust pass
  • React Compiler runs before OXC's own JSX transform (as required)
  • Uses oxc-transform 0.136.0 (pinned via package.json overrides; the NAPI binding was removed in 0.137.0 — see oxc-project/oxc#23590, re-exposure in progress)
  • Demotes non-fatal React Compiler diagnostics to webpack warnings instead of hard errors, matching babel-plugin-react-compiler's default bailout behaviour (workaround for oxc-project/oxc#23587)

Pass 3 — babel-loader (runs last, worklets only):

  • react-native-worklets/plugin only — serialises 'worklet' functions for UI-thread execution
  • No presets needed — OXC already stripped types and transformed JSX
  • Node modules in includeModules (Rule B) skip React Compiler but still go through OXC for JSX/TS transforms, split by extension to avoid .ts generic syntax being confused with JSX

Removed devDependencies (now superseded by OXC)

  • @babel/preset-react — JSX transform now handled by OXC
  • @babel/preset-typescript — TypeScript stripping now handled by OXC
  • @babel/preset-env (removed in May 2026 commit)
  • @babel/preset-flow (removed in May 2026 commit)
  • babel-plugin-react-native-web (removed in May 2026 commit)
  • @babel/plugin-proposal-export-namespace-from (removed in May 2026 commit)
  • @babel/plugin-transform-class-properties (removed in May 2026 commit)

Retained: babel-plugin-react-compiler (still used directly by the Metro babel.config.js for mobile builds), @babel/plugin-proposal-class-properties, @babel/plugin-transform-export-namespace-from (both used by Metro)

Build timing (measured locally on the rory-oxc-loader worktree)

Measurement Command Baseline (all-Babel) May 2026 (oxc-loader) This commit Delta vs baseline
Clean build rm -rf dist/ && npm run build 186.5s 175.4s 71.4s −115s (−62%)
Warm cache npm run build (after clean) 187.9s 174.8s 71.8s −116s (−62%)

Note: webpack uses in-memory cache in production mode (no filesystem cache), so clean ≈ warm.

Correctness verified

  • Build compiles with 0 errors (136 warnings: 134 expected React Compiler bailout diagnostics + 2 pre-existing webpack warnings)
  • Fullstory dataComponent annotations present in main and vendors bundles
  • React Compiler useMemoCache present in 15 locations across 2 bundles
  • Main bundle: 3.9 MiB (down from 5.8 MiB — OXC React Compiler + Rust DCE)
  • Source maps generated and uploaded to Sentry as expected

Open items / known limitations

  • oxc-transform 0.136.0 is pinned because the React Compiler NAPI binding was removed in 0.137.0 (oxc-project/oxc#23590). Once it is re-exposed, we can unpin.
  • The 134 React Compiler warnings are bailouts on components that violate Rules of React — same components babel-plugin-react-compiler was silently skipping. Tracked by oxc-project/oxc#23587 for proper silent-skip behaviour.
  • Smoke test (npm run web) and Reassure perf test (npm run test:perf) not yet run — this is a POC branch.

Rollback: Revert the three loader files and package.json changes.

Fixed Issues

$
PROPOSAL:

Tests

  1. Run npm run build and verify it compiles with 0 errors
  2. Run npm run web and load the app in a browser — verify it loads, you can log in, and navigate to a report
  3. Check the JS console for errors on web

Offline tests

N/A — build tooling change only.

QA Steps [No QA]

Build pipeline change only. No user-visible behaviour modified.

  • Verify that no errors appear in the JS console

PR Author Checklist

  • I linked the correct issue in the ### Fixed Issues section above
  • I wrote clear testing steps that cover the changes made in this PR
    • I added steps for local testing in the Tests section
    • I added steps for the expected offline behavior in the Offline steps section
    • I added steps for Staging and/or Production testing in the QA steps section
    • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
    • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
    • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
  • I included screenshots or videos for tests on all platforms
  • I ran the tests on all platforms & verified they passed on:
    • Android: Native
    • Android: mWeb Chrome
    • iOS: Native
    • iOS: mWeb Safari
    • MacOS: Chrome / Safari
  • I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
  • I followed proper code patterns (see Reviewing the code)
    • I verified that any callback methods that were added or modified are named for what the method does and never what callback they handle (i.e. toggleReport and not onIconClick)
    • I verified that comments were added to code that is not self explanatory
    • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
    • I verified any copy / text shown in the product is localized by adding it to src/languages/* files and using the translation method
      • If any non-english text was added/modified, I used JaimeGPT to get English > Spanish translation. I then posted it in #expensify-open-source and it was approved by an internal Expensify engineer. Link to Slack message:
    • I verified all numbers, amounts, dates and phone numbers shown in the product are using the localization methods
    • I verified any copy / text that was added to the app is grammatically correct in English. It adheres to proper capitalization guidelines (note: only the first word of header/labels should be capitalized), and is either coming verbatim from figma or has been approved by marketing (in order to get marketing approval, ask the Bug Zero team member to add the Waiting for copy label to the issue)
    • I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
    • I verified the JSDocs style guidelines (in STYLE.md) were followed
  • If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
  • I followed the guidelines as stated in the Review Guidelines
  • I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
  • I verified all code is DRY (the PR doesn't include any logic written more than once, with the exception of tests)
  • I verified any variables that can be defined as constants (ie. in CONST.ts or at the top of the file that uses the constant) are defined as such
  • I verified that if a function's arguments changed that all usages have also been updated correctly
  • If any new file was added I verified that:
    • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
  • If a new CSS style is added I verified that:
    • A similar style doesn't already exist
    • The style can't be created with an existing StyleUtils function (i.e. StyleUtils.getBackgroundAndBorderStyle(theme.componentBG))
  • If new assets were added or existing ones were modified, I verified that:
    • The assets are optimized and compressed (for SVG files, run npm run compress-svg)
    • The assets load correctly across all supported platforms.
  • If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
  • If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar is modified, I verified that Avatar is working as expected in all cases)
  • If the PR modifies a component or page that can be accessed by a direct deeplink, I verified that the code functions as expected when the deeplink is used - from a logged in and logged out account.
  • If the PR modifies the UI (e.g. new buttons, new UI components, changing the padding/spacing/sizing, moving components, etc) or modifies the form input styles:
    • I verified that all the inputs inside a form are aligned with each other.
    • I added Design label and/or tagged @Expensify/design so the design team can review the changes.
  • If a new page is added, I verified it is using the ScrollView component to make it scrollable when more elements are added to the page.
  • I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.
  • If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

Screenshots/Videos

Android: Native N/A — build tooling change only
Android: mWeb Chrome N/A — build tooling change only
iOS: Native N/A — build tooling change only
iOS: mWeb Safari N/A — build tooling change only
MacOS: Chrome / Safari N/A — build tooling change only (smoke test pending)

@roryabraham @cursoragent
perf(webpack): swap babel-loader for oxc-loader
9ace831 
Replace the single babel-loader webpack rule with a two-pass pipeline:
- Pass 1 (babel-loader, configFile:false): only the three plugins with no
  OXC equivalent — babel-plugin-react-compiler, react-native-worklets/plugin,
  and @fullstory/babel-plugin-annotate-react. Babel also parses JSX/TS so
  these plugins see the raw source.
- Pass 2 (oxc-loader): TypeScript stripping, JSX transform, class-properties,
  env target (node20), and all other bulk transforms.

Webpack use[] runs right-to-left, so babel-loader executes first.

Also removes five webpack-only Babel devDependencies that are now superseded
by oxc-loader: @babel/preset-env, @babel/preset-flow,
babel-plugin-react-native-web, @babel/plugin-proposal-export-namespace-from,
@babel/plugin-transform-class-properties.

Build time: clean 186.5s → 175.4s (−6%), warm 187.9s → 174.8s (−7%).

Co-authored-by: Cursor <cursoragent@cursor.com>
@melvin-bot

melvin-bot Bot commented May 14, 2026

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Hey, I noticed you changed some webpack configuration files. This can break production builds. Did you remember to run a production build locally to verify they still work?

@roryabraham roryabraham changed the title perf(webpack): swap babel-loader for oxc-loader [WIP] perf(webpack): swap babel-loader for oxc-loader May 14, 2026
@roryabraham

roryabraham commented May 14, 2026

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not worth it yet. Will try again when React Compiler Rust port is done

@sentry

sentry Bot commented May 14, 2026 •
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📲 Install Builds

Android

🔗 App Name App ID Version Configuration
Expensify org.me.mobiexpensifyg 9.3.74-0 (509037400) Release

⚙️ app Build Distribution Settings

@roryabraham roryabraham changed the title [WIP] perf(webpack): swap babel-loader for oxc-loader perf(webpack): swap babel-loader + babel-plugin-react-compiler for OXC (Rust React Compiler) Jun 18, 2026
@roryabraham roryabraham reopened this Jun 18, 2026
@roryabraham roryabraham mentioned this pull request Jun 18, 2026
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@roryabraham

roryabraham commented Jun 18, 2026

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Superseded by #93980 — same changes, clean branch with GPG-signed commits.

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