fix(next-mdx): forward plugin load errors to webpack callback

packages/next-mdx/mdx-js-loader.js

@@ -58,21 +58,22 @@ module.exports = function nextMdxLoader(...args) {
   const callback = this.async().bind(this)
   const loaderContext = this
 
-  getOptions(options, this.context).then((userProvidedMdxOptions) => {
-    const proxy = new Proxy(loaderContext, {
-      get(target, prop, receiver) {
-        if (prop === 'getOptions') {
-          return () => userProvidedMdxOptions
-        }
-
-        if (prop === 'async') {
-          return () => callback
-        }
-
-        return Reflect.get(target, prop, receiver)
-      },
+  getOptions(options, this.context)
+    .then((userProvidedMdxOptions) => {
+      const proxy = new Proxy(loaderContext, {
+        get(target, prop, receiver) {
+          if (prop === 'getOptions') {
+            return () => userProvidedMdxOptions
+          }
+          if (prop === 'async') {
+            return () => callback
+          }
+          return Reflect.get(target, prop, receiver)
+        },
+      })
+      mdxLoader.call(proxy, ...args)
+    })
+    .catch((error) => {
+      callback(error)
     })
-
-    mdxLoader.call(proxy, ...args)
-  })
 }

test/e2e/app-dir/mdx/mdx.test.ts

@@ -1,5 +1,54 @@
 import { nextTestSetup } from 'e2e-utils'
 
+describe('mdx without-mdx-rs - invalid plugin error handling', () => {
+  const { next, isNextStart } = nextTestSetup({
+    files: __dirname,
+    dependencies: {
+      '@next/mdx': 'canary',
+      '@mdx-js/loader': '^2.2.1',
+      '@mdx-js/react': '^2.2.1',
+    },
+    env: {
+      WITH_MDX_RS: 'false',
+    },
+    skipStart: true,
+  })
+
+  if (!isNextStart) {
+    it('should skip in dev mode', () => {})
+    return
+  }
+
+  beforeAll(async () => {
+    await next.patchFile(
+      'next.config.ts',
+      `
+import nextMDX from '@next/mdx'
+const withMDX = nextMDX({
+  options: {
+    remarkPlugins: ['non-existent-remark-plugin'],
+  },
+})
+const nextConfig = {
+  pageExtensions: ['js', 'jsx', 'ts', 'tsx', 'mdx'],
+}
+export default withMDX(nextConfig)
+`
+    )
+    try {
+      await next.start()
+    } catch {}
+  })
+
+  it('should report a build error when a plugin path fails to resolve', () => {
+    // Without the .catch() fix, the webpack callback was never called and
+    // the build would hang or throw an UnhandledPromiseRejection.
+    // With the fix, the error is forwarded to webpack and appears in cliOutput.
+    expect(next.cliOutput).toContain('non-existent-remark-plugin')
+    expect(next.cliOutput).not.toContain('UnhandledPromiseRejection')
+  })
+})
+
 for (const type of ['with-mdx-rs', 'without-mdx-rs']) {
   describe(`mdx ${type}`, () => {
     const { next } = nextTestSetup({