Add Next.js adapter opt-in behind flag (#14574)

This adds an opt-in flag for new [Vercel deployment
adapter](https://github.com/nextjs/adapter-vercel)

Author: ijjk

.changeset/nervous-dancers-joke.md

@@ -0,0 +1,5 @@
+---
+'@vercel/next': patch
+---
+
+Add Next.js adapter opt-in behind flag

packages/next/build.mjs

@@ -1,6 +1,7 @@
+import { copy } from 'fs-extra';
 import { readFileSync, promises as fsPromises } from 'node:fs';
 import { createRequire } from 'node:module';
-import { join } from 'node:path';
+import { dirname, join } from 'node:path';
 
 import { esbuild } from '../../utils/build.mjs';
 import buildEdgeFunctionTemplate from './scripts/build-edge-function-template.js';
@@ -46,6 +47,14 @@ await copyFile(
   join(process.cwd(), 'dist/mappings.wasm')
 );
 
+await copy(
+  join(
+    dirname(require.resolve('@next-community/adapter-vercel/package.json')),
+    'dist'
+  ),
+  join(process.cwd(), 'dist/adapter')
+);
+
 await esbuild({
   bundle: true,
   external: ['@vercel/build-utils', ...externals],

packages/next/package.json

@@ -26,6 +26,7 @@
     "@vercel/nft": "1.1.1"
   },
   "devDependencies": {
+    "@next-community/adapter-vercel": "0.0.1-beta.1",
     "@types/aws-lambda": "8.10.19",
     "@types/buffer-crc32": "0.2.0",
     "@types/bytes": "3.1.1",

packages/next/src/index.ts

@@ -508,6 +508,17 @@ export const build: BuildV2 = async buildOptions => {
     env.NODE_ENV = 'production';
   }
 
+  if (
+    // integration tests expect outputs object
+    !process.env.NEXT_BUILDER_INTEGRATION &&
+    process.env.NEXT_ENABLE_ADAPTER
+    // TODO: replace above opt-in with Next.js version
+    // semver.gte(nextVersion, '16.1.1-canary.18', { includePrerelease: true })
+  ) {
+    env.NEXT_ADAPTER_PATH = path.join(__dirname, 'adapter/index.js');
+    env.NEXT_ADAPTER_VERCEL_CONFIG = JSON.stringify(config);
+  }
+
   const shouldRunCompileStep =
     Boolean(buildCommand) || Boolean(buildScriptName);
 
@@ -578,7 +589,7 @@ export const build: BuildV2 = async buildOptions => {
 
   if (buildOutputVersion) {
     return {
-      buildOutputPath: path.join(outputDirectory, 'output'),
+      buildOutputPath: path.join(entryPath, outputDirectory, 'output'),
       buildOutputVersion,
     } as BuildResultBuildOutput;
   }