Turbopack builds: Remove canary-gate and add experimental warning

This:
- Removes the Turbopack build canary gate
- Adds a warning that Turbopack build support is experimental.
- Links to the discussion page for feedback

Test Plan: Installed and used in an app. Verified the link is valid.

Author: wbinnssmith

packages/next/src/build/index.ts

@@ -3709,6 +3709,10 @@ export default async function build(
     // Ensure we wait for lockfile patching if present
     await lockfilePatchPromise.cur
 
+    if (isTurbopack) {
+      warnAboutTurbopackBuilds(loadedConfig)
+    }
+
     // Ensure all traces are flushed before finishing the command
     await flushAllTraces()
     teardownTraceSubscriber()
@@ -3725,3 +3729,25 @@ export default async function build(
     }
   }
 }
+
+function warnAboutTurbopackBuilds(config?: NextConfigComplete) {
+  let warningStr =
+    `Support for Turbopack builds is experimental. ` +
+    bold(
+      `We don't recommend deploying mission-critical applications to production.`
+    )
+  warningStr +=
+    '\n\n- It is expected that your bundle size might be different from `next build` with webpack. This will be improved as we work towards stability.'
+
+  if (!config?.experimental.turbo?.unstablePersistentCaching) {
+    warningStr +=
+      '\n- This build is without disk caching; subsequent builds will become faster when disk caching becomes available.'
+  }
+
+  warningStr +=
+    '\n- When comparing output to webpack builds, make sure to first clear the Next.js cache by deleting the `.next` directory.'
+  warningStr +=
+    '\n\nProvide feedback for Turbopack builds at https://github.com/vercel/next.js/discussions/77721'
+
+  Log.warn(warningStr)
+}

packages/next/src/build/turbopack-build/impl.ts

@@ -19,19 +19,12 @@ import loadConfig from '../../server/config'
 import { hasCustomExportOutput } from '../../export/utils'
 import { Telemetry } from '../../telemetry/storage'
 import { setGlobal } from '../../trace'
-import { isStableBuild, CanaryOnlyError } from '../../shared/lib/canary-only'
 
 export async function turbopackBuild(): Promise<{
   duration: number
   buildTraceContext: undefined
   shutdownPromise: Promise<void>
 }> {
-  if (isStableBuild()) {
-    throw new CanaryOnlyError(
-      'Turbopack builds are only available in canary builds of Next.js.'
-    )
-  }
-
   await validateTurboNextConfig({
     dir: NextBuildContext.dir!,
     isDev: false,