Turbopack build: Replace process.env.TURBOPACK usage

packages/next/next-runtime.webpack-config.js

@@ -229,6 +229,7 @@ module.exports = ({ dev, turbo, bundleType, experimental, ...rest }) => {
           experimental ? true : false
         ),
         'process.env.NEXT_RUNTIME': JSON.stringify('nodejs'),
+        'process.turbopack': JSON.stringify(turbo),
         'process.env.TURBOPACK': JSON.stringify(turbo),
       }),
       !!process.env.ANALYZE &&

packages/next/src/build/index.ts

@@ -2274,7 +2274,6 @@ export default async function build(
 
                 // @ts-expect-error internal field TODO: fix this, should use a separate mechanism to pass the info.
                 isExperimentalCompile: isCompileMode,
-                isTurbopackBuild: isTurbopack,
               },
             },
             appDir: dir,

packages/next/src/cli/next-start.ts

@@ -33,16 +33,6 @@ const nextStart = async (options: NextStartOptions, directory?: string) => {
     printAndExit(getReservedPortExplanation(port), 1)
   }
 
-  const isTurbopack = Boolean(
-    options.turbo ||
-      options.turbopack ||
-      // TODO: Used for Testing in Next.js CI. Rename to something better like `NEXT_TEST_TURBOPACK`.
-      process.env.TURBOPACK
-  )
-  if (isTurbopack) {
-    process.env.TURBOPACK = '1'
-  }
-
   await startServer({
     dir,
     isDev: false,

packages/next/src/server/next-server.ts

@@ -192,20 +192,6 @@ export default class NextNodeServer extends BaseServer<
     this.isDev = isDev
     this.sriEnabled = Boolean(options.conf.experimental?.sri?.algorithm)
 
-    // @ts-expect-error internal field not publicly exposed
-    const isTurbopackBuild = this.nextConfig.experimental?.isTurbopackBuild
-    if (!isDev && typeof isTurbopackBuild !== 'undefined') {
-      if (process.env.TURBOPACK && !isTurbopackBuild) {
-        throw new Error(
-          `Invariant: --turbopack is set but the build used Webpack`
-        )
-      } else if (!process.env.TURBOPACK && isTurbopackBuild) {
-        throw new Error(
-          `Invariant: --turbopack is not set but the build used Turbopack. Add --turbopack to "next start".`
-        )
-      }
-    }
-
     /**
      * This sets environment variable to be used at the time of SSR by head.tsx.
      * Using this from process.env allows targeting SSR by calling

packages/next/src/server/next.ts

@@ -240,9 +240,6 @@ export class NextServer implements NextWrapperServer {
         // @ts-expect-error internal field
         config.experimental.isExperimentalCompile =
           serializedConfig.experimental.isExperimentalCompile
-        // @ts-expect-error internal field
-        config.experimental.isTurbopackBuild =
-          serializedConfig.experimental.isTurbopackBuild
       } catch (_) {
         // if distDir is customized we don't know until we
         // load the config so fallback to loading the config