Pass unstable_transformProfile through Babel caller API and fall back to it in @react-native/babel-preset (#54967)

Summary:
Pull Request resolved: #54967

`unstable_transformProfile` is used by Metro to hint at the appropriate set of Babel transforms for the target engine.

To be effective in `react-native/babel-preset`, it must currently be passed in the preset config.

When the user does not use a custom Babel config, `react-native/metro-babel-transformer` detects that and auto-configures the preset with options from Metro - that's fine.

But if the user has their own Babel config, building on top of `react-native/babel-preset`, the preset for the config is fixed - so `unstable_transformProfile` has no effect.

Ideally, `unstable_transformProfile` should be a function of the target engine - it's totally reasonable for one instance of Metro to build for multiple engines - so this should instead be passed programmatically to the engine by Metro for each transform job.

Babel provides a mechanism to do this efficiently, without reinitialising the preset unnecessarily, via the `caller` API.

This passes `unstable_transformProfile` through `caller`.

Changelog: [Internal]

(This is part of using a more optimised transform profiles for Hermes V1, which is still an experimental opt-in. When we stabilise Hermes V1 in RN, we should stabilise this API)

Reviewed By: vzaidman, huntie

Differential Revision: D82625477

fbshipit-source-id: 76a4d73d192c2deb03bed2064856e526de25f2ec

Author: robhogan

packages/react-native-babel-preset/src/configs/main.js

@@ -39,12 +39,21 @@ function isFirstParty(fileName) {
   );
 }
 
+// Called by Babel whenever caller information changes between transform calls
+// for a given config. If the return value changes, Babel re-evaluates
+// getPreset, which is otherwise cached based on `options`. This must be pure,
+// and should be cheap.
+function getTransformProfile(caller) {
+  return caller?.unstable_transformProfile ?? 'default';
+}
+
 // use `this.foo = bar` instead of `this.defineProperty('foo', ...)`
 const loose = true;
 
-const getPreset = (src, options) => {
+const getPreset = (src, options, babel) => {
   const transformProfile =
-    (options && options.unstable_transformProfile) || 'default';
+    options?.unstable_transformProfile ?? babel?.caller(getTransformProfile);
+
   const isHermesStable = transformProfile === 'hermes-stable';
   const isHermesCanary = transformProfile === 'hermes-canary';
   const isHermes = isHermesStable || isHermesCanary;
@@ -252,14 +261,14 @@ const getPreset = (src, options) => {
   };
 };
 
-module.exports = options => {
+module.exports = (options, babel) => {
   if (options.withDevTools == null) {
     const env = process.env.BABEL_ENV || process.env.NODE_ENV;
     if (!env || env === 'development') {
-      return getPreset(null, {...options, dev: true});
+      return getPreset(null, {...options, dev: true}, babel);
     }
   }
-  return getPreset(null, options);
+  return getPreset(null, options, babel);
 };
 
 module.exports.getPreset = getPreset;

packages/react-native-babel-preset/src/index.js

@@ -14,7 +14,7 @@ const {version: packageVersion} = require('../package.json');
 const main = require('./configs/main');
 
 module.exports = function (babel, options) {
-  return main(options);
+  return main(options, babel);
 };
 
 let cacheKey;

packages/react-native-babel-transformer/src/index.js

@@ -193,7 +193,14 @@ const transform /*: BabelTransformer['transform'] */ = ({
       // ES modules require sourceType='module' but OSS may not always want that
       sourceType: 'unambiguous',
       ...buildBabelConfig(filename, options, plugins),
-      caller: {name: 'metro', bundler: 'metro', platform: options.platform},
+      caller: {
+        // Varies Babel's config cache - presets will be re-initialized
+        // if they use caller information.
+        name: 'metro',
+        bundler: 'metro',
+        platform: options.platform,
+        unstable_transformProfile: options.unstable_transformProfile,
+      },
       ast: true,
 
       // NOTE(EvanBacon): We split the parse/transform steps up to accommodate