test(plugin-rsc): add unit tests for webpack-require sourcemap fix

Extract patchWebpackRequire() into a standalone exported function so it
can be tested directly. Add 6 test cases covering:

- bare __webpack_require__ → __vite_rsc_require__ replacement
- __webpack_require__.u → ({}).u replacement
- both patterns in the same source
- no-op when __webpack_require__ is absent
- returned sourcemap has non-empty mappings (key regression test)
- sourcemap source field is set correctly for offset-changing replacements

Author: nianyi778

packages/plugin-rsc/src/core/plugin.test.ts

@@ -0,0 +1,69 @@
+import { describe, expect, test } from 'vitest'
+import { patchWebpackRequire } from './plugin'
+
+describe('patchWebpackRequire', () => {
+  test('replaces bare __webpack_require__ with __vite_rsc_require__', () => {
+    const code = 'var x = __webpack_require__(42);'
+    const result = patchWebpackRequire(code, 'test.js')
+    expect(result).toBeDefined()
+    expect(result!.code).toBe('var x = __vite_rsc_require__(42);')
+  })
+
+  test('replaces __webpack_require__.u with ({}).u', () => {
+    const code = 'var chunkUrl = __webpack_require__.u;'
+    const result = patchWebpackRequire(code, 'test.js')
+    expect(result).toBeDefined()
+    expect(result!.code).toBe('var chunkUrl = ({}).u;')
+  })
+
+  test('handles both patterns in the same source', () => {
+    const code = [
+      'var u = __webpack_require__.u;',
+      'var x = __webpack_require__(1);',
+      'var y = __webpack_require__(2);',
+    ].join('\n')
+    const result = patchWebpackRequire(code, 'test.js')
+    expect(result).toBeDefined()
+    expect(result!.code).toBe(
+      [
+        'var u = ({}).u;',
+        'var x = __vite_rsc_require__(1);',
+        'var y = __vite_rsc_require__(2);',
+      ].join('\n'),
+    )
+  })
+
+  test('returns undefined when no __webpack_require__ present', () => {
+    const code = 'var x = 1;'
+    const result = patchWebpackRequire(code, 'test.js')
+    expect(result).toBeUndefined()
+  })
+
+  test('returns a valid sourcemap with non-empty mappings', () => {
+    // This is the key regression test: the old code returned `map: null`
+    // which broke the Rollup sourcemap chain, causing
+    // "Can't resolve original location of error" warnings.
+    const code = 'var x = __webpack_require__(42);'
+    const result = patchWebpackRequire(code, 'test.js')
+    expect(result).toBeDefined()
+    expect(result!.map).toBeDefined()
+    expect(result!.map).not.toBeNull()
+    // mappings must be a non-empty string (not "" which is an empty map)
+    expect(result!.map.mappings).toBeTruthy()
+    expect(typeof result!.map.mappings).toBe('string')
+    expect(result!.map.mappings.length).toBeGreaterThan(0)
+  })
+
+  test('sourcemap maps positions correctly for offset-changing replacement', () => {
+    // __webpack_require__ (18 chars) → __vite_rsc_require__ (20 chars)
+    // This non-same-length replacement is what broke sourcemaps before.
+    const code = 'call(__webpack_require__)'
+    const result = patchWebpackRequire(code, 'test.js')
+    expect(result).toBeDefined()
+    expect(result!.code).toBe('call(__vite_rsc_require__)')
+
+    // Verify source is set correctly
+    const map = result!.map
+    expect(map.sources).toContain('test.js')
+  })
+})

packages/plugin-rsc/src/core/plugin.ts

@@ -1,49 +1,57 @@
 import MagicString from 'magic-string'
 import type { Plugin } from 'vite'
 
+/**
+ * Replace `__webpack_require__` references with safe alternatives.
+ *
+ * - `__webpack_require__.u` → `({}).u`  (avoid import side-effect access)
+ * - `__webpack_require__`   → `__vite_rsc_require__`
+ *
+ * Returns `{ code, map }` with a proper sourcemap so the Rollup sourcemap
+ * chain stays intact, or `undefined` if no replacements were made.
+ */
+export function patchWebpackRequire(
+  code: string,
+  id: string,
+): { code: string; map: ReturnType<MagicString['generateMap']> } | undefined {
+  if (!code.includes('__webpack_require__')) return
+
+  const s = new MagicString(code)
+
+  // Match `__webpack_require__.u` first (longer pattern), then bare
+  // `__webpack_require__`, in a single left-to-right pass to avoid
+  // overlapping overwrites into MagicString.
+  const re = /__webpack_require__(?:\.u)?/g
+  let match: RegExpExecArray | null
+  while ((match = re.exec(code)) !== null) {
+    const { index } = match
+    if (match[0] === '__webpack_require__.u') {
+      // avoid accessing `__webpack_require__` on import side effect
+      // https://github.com/facebook/react/blob/a9bbe34622885ef5667d33236d580fe7321c0d8b/packages/react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpackBrowser.js#L16-L17
+      s.overwrite(index, index + match[0].length, '({}).u')
+    } else {
+      // the existance of `__webpack_require__` global can break some packages
+      // https://github.com/TooTallNate/node-bindings/blob/c8033dcfc04c34397384e23f7399a30e6c13830d/bindings.js#L90-L94
+      s.overwrite(index, index + match[0].length, '__vite_rsc_require__')
+    }
+  }
+
+  if (s.hasChanged()) {
+    return {
+      code: s.toString(),
+      map: s.generateMap({ hires: true, source: id }),
+    }
+  }
+}
+
 export default function vitePluginRscCore(): Plugin[] {
   return [
     {
       name: 'rsc:patch-react-server-dom-webpack',
       transform: {
         filter: { code: '__webpack_require__' },
         handler(code, id, _options) {
-          if (!code.includes('__webpack_require__')) return
-
-          // Use MagicString to perform replacements with a proper sourcemap,
-          // so the Rollup sourcemap chain stays intact and doesn't emit
-          // 'Can't resolve original location of error' warnings for every
-          // file processed by this transform (e.g. all "use client" modules).
-          const s = new MagicString(code)
-
-          // Match `__webpack_require__.u` first (longer pattern), then bare
-          // `__webpack_require__`, in a single left-to-right pass to avoid
-          // overlapping overwrites into MagicString.
-          const re = /__webpack_require__(?:\.u)?/g
-          let match: RegExpExecArray | null
-          while ((match = re.exec(code)) !== null) {
-            const { index } = match
-            if (match[0] === '__webpack_require__.u') {
-              // avoid accessing `__webpack_require__` on import side effect
-              // https://github.com/facebook/react/blob/a9bbe34622885ef5667d33236d580fe7321c0d8b/packages/react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpackBrowser.js#L16-L17
-              s.overwrite(index, index + match[0].length, '({}).u')
-            } else {
-              // the existance of `__webpack_require__` global can break some packages
-              // https://github.com/TooTallNate/node-bindings/blob/c8033dcfc04c34397384e23f7399a30e6c13830d/bindings.js#L90-L94
-              s.overwrite(
-                index,
-                index + match[0].length,
-                '__vite_rsc_require__',
-              )
-            }
-          }
-
-          if (s.hasChanged()) {
-            return {
-              code: s.toString(),
-              map: s.generateMap({ hires: true, source: id }),
-            }
-          }
+          return patchWebpackRequire(code, id)
         },
       },
     },