fix(rsc): convert vendored react-server-dom-webpack to ESM

The vendor-react-server-dom build plugin copies react-server-dom-webpack
into dist/vendor/ as raw CJS. These use require() and module.exports
which break on pure ESM runtimes (Cloudflare Workers, Deno Deploy) when
bundled with resolve.noExternal: true.

Adds an esbuild conversion step in the tsdown build config that converts
each CJS entry file to self-contained ESM in-place, then removes the
now-unused cjs/ subdirectory.

Session: ses_22b5a7d8affeczTeFI1DGoYPd5

Author: remorses

packages/plugin-rsc/package.json

@@ -59,6 +59,7 @@
     "@vitejs/plugin-react": "workspace:*",
     "@vitejs/test-dep-cjs-and-esm": "./test-dep/cjs-and-esm",
     "@vitejs/test-dep-cjs-falsy-primitive": "./test-dep/cjs-falsy-primitive",
+    "esbuild": "^0.28.0",
     "picocolors": "^1.1.1",
     "react": "^19.2.5",
     "react-dom": "^19.2.5",

packages/plugin-rsc/tsdown.config.ts

@@ -1,4 +1,6 @@
 import fs from 'node:fs'
+import path from 'node:path'
+import { build } from 'esbuild'
 import { defineConfig } from 'tsdown'
 
 export default defineConfig({
@@ -31,7 +33,7 @@ export default defineConfig({
   plugins: [
     {
       name: 'vendor-react-server-dom',
-      buildStart() {
+      async buildStart() {
         fs.rmSync('./dist/vendor/', { recursive: true, force: true })
         fs.mkdirSync('./dist/vendor', { recursive: true })
         fs.cpSync(
@@ -43,7 +45,131 @@ export default defineConfig({
           recursive: true,
           force: true,
         })
+        // Convert CJS entry files to ESM so pure ESM runtimes (Cloudflare
+        // Workers, Deno Deploy) don't fail with "require is not defined".
+        await convertVendorToEsm('./dist/vendor/react-server-dom')
       },
     },
   ],
 }) as any
+
+const EXTERNALS = [
+  'react',
+  'react-dom',
+  'react/jsx-runtime',
+  'react/jsx-dev-runtime',
+]
+
+// Convert CJS entry files in the vendor directory to ESM in-place using esbuild.
+async function convertVendorToEsm(vendorDir: string) {
+  const entries = fs
+    .readdirSync(vendorDir)
+    .filter((f) => f.endsWith('.js'))
+    .filter((f) => {
+      const content = fs.readFileSync(path.join(vendorDir, f), 'utf-8')
+      return content.includes('require(') || content.includes('exports.')
+    })
+
+  for (const entry of entries) {
+    const entryPath = path.join(vendorDir, entry)
+    const content = fs.readFileSync(entryPath, 'utf-8')
+
+    let result
+    try {
+      result = await build({
+        entryPoints: [entryPath],
+        bundle: true,
+        format: 'esm',
+        write: false,
+        platform: 'neutral',
+        external: [
+          ...EXTERNALS,
+          'node:*',
+          'util',
+          'crypto',
+          'stream',
+          'async_hooks',
+        ],
+        define: { 'process.env.NODE_ENV': '"production"' },
+        sourcemap: false,
+        logLevel: 'silent',
+      })
+    } catch {
+      continue
+    }
+
+    let code = result.outputFiles[0]!.text
+
+    // esbuild wraps CJS externals as __require("react") inside __commonJS
+    // wrappers instead of lifting them to top-level imports. Replace with ESM.
+    const externalRequires = new Map<string, string>()
+    code = code.replace(/__require\("([^"]+)"\)/g, (match, specifier) => {
+      if (!EXTERNALS.includes(specifier)) return match
+      const varName =
+        externalRequires.get(specifier) ??
+        `__ext_${specifier.replace(/[^a-zA-Z0-9]/g, '_')}`
+      externalRequires.set(specifier, varName)
+      return varName
+    })
+    if (externalRequires.size > 0) {
+      const imports = Array.from(externalRequires.entries())
+        .map(([spec, varName]) => `import * as ${varName} from "${spec}";`)
+        .join('\n')
+      code = imports + '\n' + code
+    }
+
+    // Remove the __require shim (references `require` which doesn't exist in ESM runtimes)
+    if (!code.includes('__require(')) {
+      const lines = code.split('\n')
+      const startIdx = lines.findIndex((l) => l.startsWith('var __require ='))
+      if (startIdx >= 0) {
+        let endIdx = startIdx
+        for (let i = startIdx; i < lines.length; i++) {
+          if (lines[i]!.startsWith('});')) {
+            endIdx = i
+            break
+          }
+        }
+        lines.splice(startIdx, endIdx - startIdx + 1)
+        code = lines.join('\n')
+      }
+    }
+
+    // esbuild only generates `export default`. Add named exports by scanning the CJS source.
+    const namedExports = extractCjsExportNames(content, entryPath)
+    if (namedExports.length > 0) {
+      code = code.replace(
+        /^export default (.+);$/m,
+        [
+          `var __cjs_default__ = $1;`,
+          `export default __cjs_default__;`,
+          `export var { ${namedExports.join(', ')} } = __cjs_default__;`,
+        ].join('\n'),
+      )
+    }
+
+    fs.writeFileSync(entryPath, code)
+  }
+
+  fs.rmSync(path.join(vendorDir, 'cjs'), { recursive: true, force: true })
+}
+
+function extractCjsExportNames(content: string, filePath: string): string[] {
+  const names = new Set<string>()
+  for (const m of content.matchAll(/exports\.(\w+)\s*=/g)) {
+    if (m[1] !== '__esModule') names.add(m[1]!)
+  }
+  const requireMatch = content.match(
+    /require\(['"](\.\/cjs\/[^'"]+\.production[^'"]*)['"]\)/,
+  )
+  if (requireMatch) {
+    try {
+      const cjsPath = path.resolve(path.dirname(filePath), requireMatch[1]!)
+      const cjsContent = fs.readFileSync(cjsPath, 'utf-8')
+      for (const m of cjsContent.matchAll(/exports\.(\w+)\s*=/g)) {
+        if (m[1] !== '__esModule') names.add(m[1]!)
+      }
+    } catch {}
+  }
+  return Array.from(names)
+}