fix(nextjs): handle webpack detection when using rspack

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);

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');
+    });
   });
 });