fix(plugin-rsc): use MagicString to preserve sourcemap chain in webpack-require transform

The rsc:patch-react-server-dom-webpack transform replaces
__webpack_require__ (18 chars) with __vite_rsc_require__ (20 chars)
— a non-same-length substitution — and previously returned map: null,
which broke the Rollup sourcemap chain.

Replace the manual replaceAll + map: null with MagicString, which
generates a correct hires sourcemap reflecting the actual offset changes.

Added e2e test that builds the starter example with --sourcemap and
verifies no sourcemap warnings are emitted for this transform.

Author: nianyi778

packages/plugin-rsc/e2e/sourcemap.test.ts

@@ -0,0 +1,44 @@
+import fs from 'node:fs'
+import path from 'node:path'
+import { expect, test } from '@playwright/test'
+import { x } from 'tinyexec'
+
+test.describe('sourcemap', () => {
+  const root = 'examples/starter'
+
+  test('build --sourcemap produces valid sourcemaps without rsc:patch-react-server-dom-webpack warnings', async () => {
+    // Clean previous build
+    fs.rmSync(path.join(root, 'dist'), { recursive: true, force: true })
+
+    const result = await x('pnpm', ['build', '--sourcemap'], {
+      nodeOptions: { cwd: root },
+      throwOnError: true,
+    })
+    expect(result.exitCode).toBe(0)
+
+    // The rsc:patch-react-server-dom-webpack plugin replaces
+    // __webpack_require__ with __vite_rsc_require__ (different lengths).
+    // With the MagicString fix, this transform preserves the sourcemap
+    // chain and must not appear in any "Sourcemap" warnings.
+    const output = result.stdout + result.stderr
+    expect(output).not.toContain(
+      '[plugin rsc:patch-react-server-dom-webpack] Sourcemap is likely to be incorrect',
+    )
+
+    // Verify the rsc build output has a valid sourcemap with non-empty mappings.
+    // The rsc bundle contains the vendored react-server-dom-webpack code
+    // that goes through the __webpack_require__ transform.
+    const rscDir = path.join(root, 'dist/rsc')
+    const mapFiles = fs.readdirSync(rscDir).filter((f) => f.endsWith('.js.map'))
+    expect(mapFiles.length).toBeGreaterThan(0)
+
+    for (const mapFile of mapFiles) {
+      const map = JSON.parse(
+        fs.readFileSync(path.join(rscDir, mapFile), 'utf-8'),
+      )
+      // Sourcemap must have non-empty mappings
+      expect(map.mappings).toBeTruthy()
+      expect(map.mappings.length).toBeGreaterThan(0)
+    }
+  })
+})

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

@@ -1,3 +1,4 @@
+import MagicString from 'magic-string'
 import type { Plugin } from 'vite'
 
 export default function vitePluginRscCore(): Plugin[] {
@@ -6,25 +7,42 @@ export default function vitePluginRscCore(): Plugin[] {
       name: 'rsc:patch-react-server-dom-webpack',
       transform: {
         filter: { code: '__webpack_require__' },
-        handler(originalCode, _id, _options) {
-          let code = originalCode
-          if (code.includes('__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
-            code = code.replaceAll('__webpack_require__.u', '({}).u')
-          }
+        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)
 
-          // the existance of `__webpack_require__` global can break some packages
-          // https://github.com/TooTallNate/node-bindings/blob/c8033dcfc04c34397384e23f7399a30e6c13830d/bindings.js#L90-L94
-          if (code.includes('__webpack_require__')) {
-            code = code.replaceAll(
-              '__webpack_require__',
-              '__vite_rsc_require__',
-            )
+          // 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 (code !== originalCode) {
-            return { code, map: null }
+          if (s.hasChanged()) {
+            return {
+              code: s.toString(),
+              map: s.generateMap({ hires: true, source: id }),
+            }
           }
         },
       },