From 1969d81b32b06bee1d8123e83a1fad5b15b022ff Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kevin=20M=C3=A5rtensson?= <kevinmartensson@gmail.com>
Date: Mon, 20 Oct 2025 11:11:41 +0200
Subject: [PATCH] fix(nextjs): handle webpack detection when using rspack

---
 packages/nextjs/src/config/util.ts       |  5 +++++
 packages/nextjs/test/config/util.test.ts | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/packages/nextjs/src/config/util.ts b/packages/nextjs/src/config/util.ts
index 8d2d7781230b..6fd017f08844 100644
--- a/packages/nextjs/src/config/util.ts
+++ b/packages/nextjs/src/config/util.ts
@@ -163,6 +163,11 @@ export function detectActiveBundler(nextJsVersion: string | undefined): 'turbopa
     return 'webpack';
   }
 
+  // Explicit opt-in to webpack (using rspack) via environment variable
+  if (process.env.NEXT_RSPACK === 'true') {
+    return 'webpack';
+  }
+
   // Fallback to version-based default behavior
   if (nextJsVersion) {
     const turbopackIsDefault = isTurbopackDefaultForVersion(nextJsVersion);
diff --git a/packages/nextjs/test/config/util.test.ts b/packages/nextjs/test/config/util.test.ts
index 55fd13cf5dc4..1e95c98ee9af 100644
--- a/packages/nextjs/test/config/util.test.ts
+++ b/packages/nextjs/test/config/util.test.ts
@@ -332,6 +332,7 @@ describe('util', () => {
       process.argv = [...originalArgv];
       process.env = { ...originalEnv };
       delete process.env.TURBOPACK;
+      delete process.env.NEXT_RSPACK;
     });
 
     afterEach(() => {
@@ -349,6 +350,11 @@ describe('util', () => {
       expect(util.detectActiveBundler('16.0.0')).toBe('webpack');
     });
 
+    it('returns webpack when NEXT_RSPACK env var is set', () => {
+      process.env.NEXT_RSPACK = 'true';
+      expect(util.detectActiveBundler('16.0.0')).toBe('webpack');
+    });
+
     it('returns turbopack for Next.js 16+ by default', () => {
       expect(util.detectActiveBundler('16.0.0')).toBe('turbopack');
       expect(util.detectActiveBundler('17.0.0')).toBe('turbopack');
@@ -388,5 +394,11 @@ describe('util', () => {
       process.argv.push('--webpack');
       expect(util.detectActiveBundler('15.5.0')).toBe('turbopack');
     });
+
+    it('prioritizes TURBOPACK env var over NEXT_RSPACK env var', () => {
+      process.env.TURBOPACK = '1';
+      process.env.NEXT_RSPACK = 'true';
+      expect(util.detectActiveBundler('15.5.0')).toBe('turbopack');
+    });
   });
 });