Improve Proxy invalid export error message (#84887)

Improve invalid Proxy/Middleware export error message to:

```
> Build error occurred
Error: The file "./proxy.ts" must export a function, either as a default export or as a named "proxy" export.
This function is what Next.js runs for every request handled by this proxy (previously called middleware).

Why this happens:
- You are migrating from `middleware` to `proxy`, but haven't updated the exported function.
- The file exists but doesn't export a function.
- The export is not a function (e.g., an object or constant).
- There's a syntax error preventing the export from being recognized.

To fix it:
- Ensure this file has either a default or "proxy" function export.

Learn more: https://nextjs.org/docs/messages/middleware-to-proxy
```

Turbopack:
```
Error: Turbopack build failed with 1 errors:
./proxy.ts
Proxy is missing expected function export name
This function is what Next.js runs for every request handled by this proxy (previously called middleware).

Why this happens:
- You are migrating from `middleware` to `proxy`, but haven't updated the exported function.
- The file exists but doesn't export a function.
- The export is not a function (e.g., an object or constant).
- There's a syntax error preventing the export from being recognized.

To fix it:
- Ensure this file has either a default or "proxy" function export.

Learn more: https://nextjs.org/docs/messages/middleware-to-proxy
```

Closes NEXT-4741

Author: devjiwonchoi

Cargo.lock

@@ -139,23 +139,48 @@ impl Issue for MiddlewareMissingExportIssue {
     }
 
     #[turbo_tasks::function]
-    fn title(&self) -> Vc<StyledString> {
-        let file_name = self.file_path.file_name();
-
-        StyledString::Line(vec![
-            StyledString::Text(rcstr!("The ")),
-            StyledString::Code(self.file_type.clone()),
-            StyledString::Text(rcstr!(" file \"")),
-            StyledString::Code(format!("./{}", file_name).into()),
-            StyledString::Text(rcstr!("\" must export a function named ")),
-            StyledString::Code(format!("`{}`", self.function_name).into()),
-            StyledString::Text(rcstr!(" or a default function.")),
-        ])
-        .cell()
+    async fn title(&self) -> Result<Vc<StyledString>> {
+        let title_text = format!(
+            "{} is missing expected function export name",
+            self.file_type
+        );
+
+        Ok(StyledString::Text(title_text.into()).cell())
     }
 
     #[turbo_tasks::function]
-    fn description(&self) -> Vc<OptionStyledString> {
-        Vc::cell(None)
+    async fn description(&self) -> Result<Vc<OptionStyledString>> {
+        let type_description = if self.file_type == "Proxy" {
+            "proxy (previously called middleware)"
+        } else {
+            "middleware"
+        };
+
+        let migration_bullet = if self.file_type == "Proxy" {
+            "- You are migrating from `middleware` to `proxy`, but haven't updated the exported \
+             function.\n"
+        } else {
+            ""
+        };
+
+        // Rest of the message goes in description to avoid formatIssue indentation
+        let description_text = format!(
+            "This function is what Next.js runs for every request handled by this {}.\n\n\
+             Why this happens:\n\
+             {}\
+             - The file exists but doesn't export a function.\n\
+             - The export is not a function (e.g., an object or constant).\n\
+             - There's a syntax error preventing the export from being recognized.\n\n\
+             To fix it:\n\
+             - Ensure this file has either a default or \"{}\" function export.\n\n\
+             Learn more: https://nextjs.org/docs/messages/middleware-to-proxy",
+            type_description,
+            migration_bullet,
+            self.function_name
+        );
+
+        Ok(Vc::cell(Some(
+            StyledString::Text(description_text.into()).resolved_cell(),
+        )))
     }
 }

crates/next-core/src/middleware.rs

@@ -900,5 +900,6 @@
   "899": "Both \"%s\" and \"%s\" files are detected. Please use \"%s\" instead. Learn more: https://nextjs.org/docs/messages/middleware-to-proxy",
   "900": "Both %s file \"./%s\" and %s file \"./%s\" are detected. Please use \"./%s\" only. Learn more: https://nextjs.org/docs/messages/middleware-to-proxy",
   "901": "Invalid \"cacheHandlers\" provided, expected an object e.g. { default: '/my-handler.js' }, received %s",
-  "902": "Invalid handler fields configured for \"cacheHandlers\":\\n%s"
+  "902": "Invalid handler fields configured for \"cacheHandlers\":\\n%s",
+  "903": "The file \"%s\" must export a function, either as a default export or as a named \"%s\" export.\\nThis function is what Next.js runs for every request handled by this %s.\\n\\nWhy this happens:\\n%s- The file exists but doesn't export a function.\\n- The export is not a function (e.g., an object or constant).\\n- There's a syntax error preventing the export from being recognized.\\n\\nTo fix it:\\n- Ensure this file has either a default or \"%s\" function export.\\n\\nLearn more: https://nextjs.org/docs/messages/middleware-to-proxy"
 }

packages/next/errors.json

@@ -2,7 +2,7 @@ import type { NextConfig } from '../../server/config-shared'
 import type { RouteHas } from '../../lib/load-custom-routes'
 
 import { promises as fs } from 'fs'
-import { basename } from 'path'
+import { relative } from 'path'
 import { LRUCache } from '../../server/lib/lru-cache'
 import {
   extractExportedConstValue,
@@ -329,7 +329,7 @@ function validateMiddlewareProxyExports({
     return
   }
 
-  const fileName = isProxy ? 'proxy' : 'middleware'
+  const fileName = isProxy ? PROXY_FILENAME : MIDDLEWARE_FILENAME
 
   // Parse AST to get export info (since checkExports doesn't return middleware/proxy info)
   let hasDefaultExport = false
@@ -396,9 +396,22 @@ function validateMiddlewareProxyExports({
     (isMiddleware && hasMiddlewareExport) ||
     (isProxy && hasProxyExport)
 
+  const relativeFilePath = `./${relative(process.cwd(), pageFilePath)}`
+
   if (!hasValidExport) {
     throw new Error(
-      `The ${fileName === 'proxy' ? 'Proxy' : 'Middleware'} file "./${basename(pageFilePath)}" must export a function named \`${fileName}\` or a default function.`
+      `The file "${relativeFilePath}" must export a function, either as a default export or as a named "${fileName}" export.\n` +
+        `This function is what Next.js runs for every request handled by this ${fileName === 'proxy' ? 'proxy (previously called middleware)' : 'middleware'}.\n\n` +
+        `Why this happens:\n` +
+        (isProxy
+          ? "- You are migrating from `middleware` to `proxy`, but haven't updated the exported function.\n"
+          : '') +
+        `- The file exists but doesn't export a function.\n` +
+        `- The export is not a function (e.g., an object or constant).\n` +
+        `- There's a syntax error preventing the export from being recognized.\n\n` +
+        `To fix it:\n` +
+        `- Ensure this file has either a default or "${fileName}" function export.\n\n` +
+        `Learn more: https://nextjs.org/docs/messages/middleware-to-proxy`
     )
   }
 }

packages/next/src/build/analysis/get-page-static-info.ts

@@ -19,8 +19,25 @@ const isProxy = page === '/proxy' || page === '/src/proxy'
 const handler = (isProxy ? mod.proxy : mod.middleware) || mod.default
 
 if (typeof handler !== 'function') {
+  const fileName = isProxy ? 'proxy' : 'middleware'
+  // Webpack starts the path with "." as relative, but Turbopack does not.
+  const resolvedRelativeFilePath = relativeFilePath.startsWith('.')
+    ? relativeFilePath
+    : `./${relativeFilePath}`
+
   throw new Error(
-    `The ${isProxy ? 'Proxy' : 'Middleware'} file "${relativeFilePath.startsWith('.') ? relativeFilePath : `./${relativeFilePath}`}" must export a function named \`${isProxy ? 'proxy' : 'middleware'}\` or a default function.`
+    `The file "${resolvedRelativeFilePath}" must export a function, either as a default export or as a named "${fileName}" export.\n` +
+      `This function is what Next.js runs for every request handled by this ${fileName === 'proxy' ? 'proxy (previously called middleware)' : 'middleware'}.\n\n` +
+      `Why this happens:\n` +
+      (isProxy
+        ? "- You are migrating from `middleware` to `proxy`, but haven't updated the exported function.\n"
+        : '') +
+      `- The file exists but doesn't export a function.\n` +
+      `- The export is not a function (e.g., an object or constant).\n` +
+      `- There's a syntax error preventing the export from being recognized.\n\n` +
+      `To fix it:\n` +
+      `- Ensure this file has either a default or "${fileName}" function export.\n\n` +
+      `Learn more: https://nextjs.org/docs/messages/middleware-to-proxy`
   )
 }
 

packages/next/src/build/templates/middleware.ts

@@ -2,8 +2,18 @@ import { nextTestSetup } from 'e2e-utils'
 import { join } from 'node:path'
 import { writeFile } from 'node:fs/promises'
 
-const errorMessage =
-  'The Proxy file "./proxy.ts" must export a function named `proxy` or a default function.'
+const errorMessage = `This function is what Next.js runs for every request handled by this proxy (previously called middleware).
+
+Why this happens:
+- You are migrating from \`middleware\` to \`proxy\`, but haven't updated the exported function.
+- The file exists but doesn't export a function.
+- The export is not a function (e.g., an object or constant).
+- There's a syntax error preventing the export from being recognized.
+
+To fix it:
+- Ensure this file has either a default or "proxy" function export.
+
+Learn more: https://nextjs.org/docs/messages/middleware-to-proxy`
 
 describe('proxy-missing-export', () => {
   const { next, isNextDev, skipped } = nextTestSetup({
@@ -22,14 +32,26 @@ describe('proxy-missing-export', () => {
       'export function middleware() {}'
     )
 
+    let cliOutput: string
+
     if (isNextDev) {
       await next.start().catch(() => {})
       // Use .catch() because Turbopack errors during compile and exits before runtime.
       await next.browser('/').catch(() => {})
-      expect(next.cliOutput).toContain(errorMessage)
+      cliOutput = next.cliOutput
+    } else {
+      cliOutput = (await next.build()).cliOutput
+    }
+
+    // TODO: Investigate why in dev-turbo, the error is shown in the browser console, not CLI output.
+    if (process.env.IS_TURBOPACK_TEST && !isNextDev) {
+      expect(cliOutput).toContain(`./proxy.ts
+Proxy is missing expected function export name
+${errorMessage}`)
     } else {
-      const { cliOutput } = await next.build()
-      expect(cliOutput).toContain(errorMessage)
+      expect(cliOutput)
+        .toContain(`The file "./proxy.ts" must export a function, either as a default export or as a named "proxy" export.
+${errorMessage}`)
     }
 
     await next.stop()
@@ -94,16 +116,27 @@ describe('proxy-missing-export', () => {
       'const proxy = () => {}; export { proxy as handler };'
     )
 
+    let cliOutput: string
+
     if (isNextDev) {
       await next.start().catch(() => {})
       // Use .catch() because Turbopack errors during compile and exits before runtime.
       await next.browser('/').catch(() => {})
-      expect(next.cliOutput).toContain(errorMessage)
+      cliOutput = next.cliOutput
     } else {
-      const { cliOutput } = await next.build()
-      expect(cliOutput).toContain(errorMessage)
+      cliOutput = (await next.build()).cliOutput
     }
 
+    // TODO: Investigate why in dev-turbo, the error is shown in the browser console, not CLI output.
+    if (process.env.IS_TURBOPACK_TEST && !isNextDev) {
+      expect(cliOutput).toContain(`./proxy.ts
+Proxy is missing expected function export name
+${errorMessage}`)
+    } else {
+      expect(cliOutput)
+        .toContain(`The file "./proxy.ts" must export a function, either as a default export or as a named "proxy" export.
+${errorMessage}`)
+    }
     await next.stop()
   })
 })