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

Returning `map: null` from a transform that changes string lengths breaks
the Rollup sourcemap chain. This causes Rollup/Vite to emit noisy
'Can't resolve original location of error' warnings for every file that
passes through this transform — in practice, every module that contains
'__webpack_require__', which includes all 'use client' component files
processed by the RSC pipeline.

Replace the manual string replacements + `map: null` with MagicString,
which generates a correct sourcemap that reflects the actual character
offset changes (`__webpack_require__` → `__vite_rsc_require__` is not
a same-length replacement). A single regex pass handles both the
`.u` variant and bare `__webpack_require__` without overlapping overwrites.

Fixes: 'Can't resolve original location of error' build warnings

Author: nianyi778

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 }),
+            }
           }
         },
       },