Improve automatic prerendering warning (vercel#7759)

errors/opt-out-automatic-prerendering.md

@@ -0,0 +1,35 @@
+# Opt-out of Automatic Prerendering
+
+#### Why This Warning Occurred
+
+You are using `getInitialProps` in your [Custom `<App>`](https://github.com/zeit/next.js#custom-app).
+
+This causes **all pages** to be executed on the server -- disabling [Automatic Prerendering](https://github.com/zeit/next.js#automatic-prerendering).
+
+#### Possible Ways to Fix It
+
+Be sure you meant to use `getInitialProps` in `pages/_app`!
+There are some valid use cases for this, but it is often better to handle `getInitialProps` on a _per-page_ basis.
+
+If you copied the [Custom `<App>`](https://github.com/zeit/next.js#custom-app) example, you may be able to remove your `getInitialProps`.
+
+The following `getInitialProps` does nothing and may be removed:
+
+```js
+class MyApp extends App {
+  // Remove me, I do nothing!
+  static async getInitialProps({ Component, ctx }) {
+    let pageProps = {}
+
+    if (Component.getInitialProps) {
+      pageProps = await Component.getInitialProps(ctx)
+    }
+
+    return { pageProps }
+  }
+
+  render() {
+    // ...
+  }
+}
+```

packages/next/build/index.ts

@@ -329,7 +329,13 @@ export default async function build(dir: string, conf = null): Promise<void> {
 
         if (customAppGetInitialProps) {
           console.warn(
-            'Opting out of automatic exporting due to custom `getInitialProps` in `pages/_app`\n'
+            chalk.bold.yellow(`Warning: `) +
+              chalk.yellow(
+                `You have opted-out of Automatic Prerendering due to \`getInitialProps\` in \`pages/_app\`.`
+              )
+          )
+          console.warn(
+            'Read more: https://err.sh/next.js/opt-out-automatic-prerendering\n'
           )
         }
       }