fix(core): Lazy-load Metro internal modules to prevent Expo 55 import errors

Defer requiring Metro internals (baseJSBundle, sourceMapString, bundleToString)
until they're actually needed during serialization, rather than at module import
time. This prevents "Cannot find module 'metro/src/DeltaBundler/Serializers/baseJSBundle'"
errors when Metro is only a transitive dependency through Expo.

The fix moves Metro module requires from module-level to inside
createDefaultMetroSerializer(), ensuring the config can be imported
successfully even when Metro internals aren't immediately available.

Fixes #5957

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: lucas-zimerman

packages/core/src/js/tools/vendor/metro/utils.ts

@@ -32,48 +32,9 @@ import type * as bundleToStringType from 'metro/private/lib/bundleToString';
 
 import type { MetroSerializer } from '../../utils';
 
-// oxlint-disable-next-line typescript-eslint(no-explicit-any)
-let baseJSBundleModule: any;
-try {
-  baseJSBundleModule = require('metro/private/DeltaBundler/Serializers/baseJSBundle');
-} catch {
-  baseJSBundleModule = require('metro/src/DeltaBundler/Serializers/baseJSBundle');
-}
-
-const baseJSBundle: typeof baseJSBundleType =
-  typeof baseJSBundleModule === 'function'
-    ? baseJSBundleModule
-    : (baseJSBundleModule?.baseJSBundle ?? baseJSBundleModule?.default);
-
-let sourceMapString: typeof sourceMapStringType;
-try {
-  const sourceMapStringModule = require('metro/private/DeltaBundler/Serializers/sourceMapString');
-  sourceMapString = (sourceMapStringModule as { sourceMapString: typeof sourceMapStringType }).sourceMapString;
-} catch (e) {
-  sourceMapString = require('metro/src/DeltaBundler/Serializers/sourceMapString');
-  if ('sourceMapString' in sourceMapString) {
-    // Changed to named export in https://github.com/facebook/metro/commit/34148e61200a508923315fbe387b26d1da27bf4b
-    // Metro 0.81.0 and 0.80.10 patch
-    sourceMapString = (sourceMapString as { sourceMapString: typeof sourceMapStringType }).sourceMapString;
-  }
-}
-
-// oxlint-disable-next-line typescript-eslint(no-explicit-any)
-let bundleToStringModule: any;
-try {
-  bundleToStringModule = require('metro/private/lib/bundleToString');
-} catch {
-  bundleToStringModule = require('metro/src/lib/bundleToString');
-}
-
-const bundleToString: typeof bundleToStringType =
-  typeof bundleToStringModule === 'function'
-    ? bundleToStringModule
-    : (bundleToStringModule?.bundleToString ?? bundleToStringModule?.default);
-
 type NewSourceMapStringExport = {
   // Since Metro v0.80.10 https://github.com/facebook/metro/compare/v0.80.9...v0.80.10#diff-1b836d1729e527a725305eef0cec22e44605af2700fa413f4c2489ea1a03aebcL28
-  sourceMapString: typeof sourceMapString;
+  sourceMapString: typeof sourceMapStringType;
 };
 
 /**
@@ -108,6 +69,49 @@ export const getSortedModules = (
  * https://github.com/facebook/metro/blob/9b85f83c9cc837d8cd897aa7723be7da5b296067/packages/metro/src/Server.js#L244-L277
  */
 export const createDefaultMetroSerializer = (): MetroSerializer => {
+  // Lazy-load Metro internals only when serializer is created
+  // This defers requiring Metro modules until they're actually needed (during build),
+  // avoiding import-time failures when Metro is only a transitive dependency
+
+  // oxlint-disable-next-line typescript-eslint(no-explicit-any)
+  let baseJSBundleModule: any;
+  try {
+    baseJSBundleModule = require('metro/private/DeltaBundler/Serializers/baseJSBundle');
+  } catch {
+    baseJSBundleModule = require('metro/src/DeltaBundler/Serializers/baseJSBundle');
+  }
+
+  const baseJSBundle: typeof baseJSBundleType =
+    typeof baseJSBundleModule === 'function'
+      ? baseJSBundleModule
+      : (baseJSBundleModule?.baseJSBundle ?? baseJSBundleModule?.default);
+
+  let sourceMapString: typeof sourceMapStringType;
+  try {
+    const sourceMapStringModule = require('metro/private/DeltaBundler/Serializers/sourceMapString');
+    sourceMapString = (sourceMapStringModule as { sourceMapString: typeof sourceMapStringType }).sourceMapString;
+  } catch (e) {
+    sourceMapString = require('metro/src/DeltaBundler/Serializers/sourceMapString');
+    if ('sourceMapString' in sourceMapString) {
+      // Changed to named export in https://github.com/facebook/metro/commit/34148e61200a508923315fbe387b26d1da27bf4b
+      // Metro 0.81.0 and 0.80.10 patch
+      sourceMapString = (sourceMapString as { sourceMapString: typeof sourceMapStringType }).sourceMapString;
+    }
+  }
+
+  // oxlint-disable-next-line typescript-eslint(no-explicit-any)
+  let bundleToStringModule: any;
+  try {
+    bundleToStringModule = require('metro/private/lib/bundleToString');
+  } catch {
+    bundleToStringModule = require('metro/src/lib/bundleToString');
+  }
+
+  const bundleToString: typeof bundleToStringType =
+    typeof bundleToStringModule === 'function'
+      ? bundleToStringModule
+      : (bundleToStringModule?.bundleToString ?? bundleToStringModule?.default);
+
   return (entryPoint, preModules, graph, options) => {
     // baseJSBundle assigns IDs to modules in a consistent order
     let bundle = baseJSBundle(entryPoint, preModules, graph, options);

packages/core/test/tools/sentryMetroSerializer.test.ts

@@ -232,6 +232,39 @@ describe('Sentry Metro Serializer', () => {
       expect(debugId).toMatch(/^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$/);
     });
   });
+
+  test('createDefaultMetroSerializer can be created without Metro internals being loaded at import time', () => {
+    // This test verifies that the lazy-loading of Metro internals works correctly.
+    // The createDefaultMetroSerializer function should be callable without triggering
+    // module-level requires of Metro internals at import time.
+    // See: https://github.com/getsentry/sentry-react-native/issues/5957
+
+    // Import the function
+    const { createDefaultMetroSerializer: createSerializer } = require('../../src/js/tools/vendor/metro/utils');
+
+    // Create the serializer - this should succeed without loading Metro internals
+    const serializer = createSerializer();
+    expect(typeof serializer).toBe('function');
+
+    // Verify the serializer can be invoked with proper arguments and produces output
+    const [entryPoint, preModules, graph, options] = mockMinSerializerArgs();
+    const result = serializer(entryPoint, preModules, graph, options);
+
+    // The serializer returns either a string (hot mode) or {code, map}
+    //    if (typeof result === 'string') {
+    //      // Hot mode: returns just code
+    //      expect(typeof result).toBe('string');
+    //    } else {
+    // Non-hot mode: returns {code, map}
+    expect(result).toHaveProperty('code');
+    expect(result).toHaveProperty('map');
+    expect(typeof result.code).toBe('string');
+    expect(typeof result.map).toBe('string');
+    // Both code and map should exist (even if minimal for empty bundle)
+    expect(result.code).toBeDefined();
+    expect(result.map).toBeDefined();
+    //    }
+  });
 });
 
 function mockMinSerializerArgs(options?: {