Remove canary-gate and add experimental warning for alternate bundler

This:
- Removes the Rspack canary gate
- Adds a warning that Rspack support is driven by the community and 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-rspack/README.md

@@ -1,7 +1,7 @@
 # next-rspack (EXPERIMENTAL)
 
 > [!WARNING]
-> This package is currently experimental and actively developed and supported in Next.js’ `canary` branch. To use this, you must be using a published canary build of Next.js.
+> This package is currently experimental, and is a partner effort with the Rspack team. Help improve Next.js and Rspack by providing feedback at https://github.com/vercel/next.js/discussions/77800
 
 This plugin allows you to use [Rspack](https://rspack.dev) in place of webpack with Next.js.
 

packages/next-rspack/package.json

@@ -3,7 +3,7 @@
   "version": "15.3.0-canary.41",
   "repository": {
     "url": "vercel/next.js",
-    "directory": "packages/next-plugin-rspack"
+    "directory": "packages/next-rspack"
   },
   "types": "index.d.ts",
   "dependencies": {

packages/next/src/shared/lib/get-rspack.ts

@@ -1,7 +1,7 @@
-import { CanaryOnlyError, isStableBuild } from './canary-only'
+import { warnOnce } from '../../build/output/log'
 
 export function getRspackCore() {
-  gateCanary()
+  warnRspack()
   try {
     // eslint-disable-next-line import/no-extraneous-dependencies
     return require('@rspack/core')
@@ -17,7 +17,7 @@ export function getRspackCore() {
 }
 
 export function getRspackReactRefresh() {
-  gateCanary()
+  warnRspack()
   try {
     // eslint-disable-next-line import/no-extraneous-dependencies
     const plugin = require('@rspack/plugin-react-refresh')
@@ -37,10 +37,8 @@ export function getRspackReactRefresh() {
   }
 }
 
-function gateCanary() {
-  if (isStableBuild()) {
-    throw new CanaryOnlyError(
-      'Rspack support is only available in Next.js canary.'
-    )
-  }
+function warnRspack() {
+  warnOnce(
+    'Rspack support is currently experimental, and is a partner effort with the Rspack team. Help improve Next.js and Rspack by providing feedback at https://github.com/vercel/next.js/discussions/77800'
+  )
 }