add new bundler detection

packages/nextjs/src/config/util.ts

@@ -65,3 +65,68 @@ export function supportsProductionCompileHook(version: string): boolean {
 
   return false;
 }
+
+/**
+ * Checks if the current Next.js version uses Turbopack as the default bundler.
+ * Starting from Next.js 15.6.0-canary.38, turbopack became the default for `next build`.
+ *
+ * @param version - Next.js version string to check.
+ * @returns true if the version uses Turbopack by default
+ */
+export function isTurbopackDefaultForVersion(version: string): boolean {
+  if (!version) {
+    return false;
+  }
+
+  const { major, minor, prerelease } = parseSemver(version);
+
+  if (major === undefined || minor === undefined) {
+    return false;
+  }
+
+  // Next.js 16+ uses turbopack by default
+  if (major >= 16) {
+    return true;
+  }
+
+  // For Next.js 15, only canary versions 15.6.0-canary.38+ use turbopack by default
+  // Stable 15.x releases still use webpack by default
+  if (major === 15 && minor >= 6 && prerelease && prerelease.startsWith('canary.')) {
+    if (minor >= 7) {
+      return true;
+    }
+    const canaryNumber = parseInt(prerelease.split('.')[1] || '0', 10);
+    if (canaryNumber >= 38) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+/**
+ * Determines which bundler is actually being used based on environment variables,
+ * CLI flags, and Next.js version.
+ *
+ * @param nextJsVersion - The Next.js version string
+ * @returns 'turbopack', 'webpack', or undefined if it cannot be determined
+ */
+export function detectActiveBundler(nextJsVersion: string | undefined): 'turbopack' | 'webpack' | undefined {
+  if (process.env.TURBOPACK || process.argv.includes('--turbo')) {
+    return 'turbopack';
+  }
+
+  // Explicit opt-in to webpack via --webpack flag
+  if (process.argv.includes('--webpack')) {
+    return 'webpack';
+  }
+
+  // Fallback to version-based default behavior
+  if (nextJsVersion) {
+    const turbopackIsDefault = isTurbopackDefaultForVersion(nextJsVersion);
+    return turbopackIsDefault ? 'turbopack' : 'webpack';
+  }
+
+  // Unlikely but at this point, we just assume webpack for older behavior
+  return 'webpack';
+}

packages/nextjs/test/config/util.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it } from 'vitest';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
 import * as util from '../../src/config/util';
 
 describe('util', () => {
@@ -96,4 +96,181 @@ describe('util', () => {
       });
     });
   });
+
+  describe('isTurbopackDefaultForVersion', () => {
+    describe('returns true for versions where turbopack is default', () => {
+      it.each([
+        // Next.js 16+ stable versions
+        ['16.0.0', 'Next.js 16.0.0 stable'],
+        ['16.0.1', 'Next.js 16.0.1 stable'],
+        ['16.1.0', 'Next.js 16.1.0 stable'],
+        ['16.2.5', 'Next.js 16.2.5 stable'],
+
+        // Next.js 16+ pre-release versions
+        ['16.0.0-rc.1', 'Next.js 16.0.0-rc.1'],
+        ['16.0.0-canary.1', 'Next.js 16.0.0-canary.1'],
+        ['16.1.0-beta.2', 'Next.js 16.1.0-beta.2'],
+
+        // Next.js 17+
+        ['17.0.0', 'Next.js 17.0.0'],
+        ['18.0.0', 'Next.js 18.0.0'],
+        ['20.0.0', 'Next.js 20.0.0'],
+
+        // Next.js 15.6.0-canary.38+ (boundary case)
+        ['15.6.0-canary.38', 'Next.js 15.6.0-canary.38 (exact threshold)'],
+        ['15.6.0-canary.39', 'Next.js 15.6.0-canary.39'],
+        ['15.6.0-canary.40', 'Next.js 15.6.0-canary.40'],
+        ['15.6.0-canary.100', 'Next.js 15.6.0-canary.100'],
+
+        // Next.js 15.7+ canary versions
+        ['15.7.0-canary.1', 'Next.js 15.7.0-canary.1'],
+        ['15.7.0-canary.50', 'Next.js 15.7.0-canary.50'],
+        ['15.8.0-canary.1', 'Next.js 15.8.0-canary.1'],
+        ['15.10.0-canary.1', 'Next.js 15.10.0-canary.1'],
+      ])('returns true for %s (%s)', version => {
+        expect(util.isTurbopackDefaultForVersion(version)).toBe(true);
+      });
+    });
+
+    describe('returns false for versions where webpack is still default', () => {
+      it.each([
+        // Next.js 15.6.0-canary.37 and below
+        ['15.6.0-canary.37', 'Next.js 15.6.0-canary.37 (just below threshold)'],
+        ['15.6.0-canary.36', 'Next.js 15.6.0-canary.36'],
+        ['15.6.0-canary.1', 'Next.js 15.6.0-canary.1'],
+        ['15.6.0-canary.0', 'Next.js 15.6.0-canary.0'],
+
+        // Next.js 15.6.x stable releases (NOT canary)
+        ['15.6.0', 'Next.js 15.6.0 stable'],
+        ['15.6.1', 'Next.js 15.6.1 stable'],
+        ['15.6.2', 'Next.js 15.6.2 stable'],
+        ['15.6.10', 'Next.js 15.6.10 stable'],
+
+        // Next.js 15.6.x rc releases (NOT canary)
+        ['15.6.0-rc.1', 'Next.js 15.6.0-rc.1'],
+        ['15.6.0-rc.2', 'Next.js 15.6.0-rc.2'],
+
+        // Next.js 15.7+ stable releases (NOT canary)
+        ['15.7.0', 'Next.js 15.7.0 stable'],
+        ['15.8.0', 'Next.js 15.8.0 stable'],
+        ['15.10.0', 'Next.js 15.10.0 stable'],
+
+        // Next.js 15.5 and below (all versions)
+        ['15.5.0', 'Next.js 15.5.0'],
+        ['15.5.0-canary.100', 'Next.js 15.5.0-canary.100'],
+        ['15.4.1', 'Next.js 15.4.1'],
+        ['15.0.0', 'Next.js 15.0.0'],
+        ['15.0.0-canary.1', 'Next.js 15.0.0-canary.1'],
+
+        // Next.js 14.x and below
+        ['14.2.0', 'Next.js 14.2.0'],
+        ['14.0.0', 'Next.js 14.0.0'],
+        ['14.0.0-canary.50', 'Next.js 14.0.0-canary.50'],
+        ['13.5.0', 'Next.js 13.5.0'],
+        ['13.0.0', 'Next.js 13.0.0'],
+        ['12.0.0', 'Next.js 12.0.0'],
+      ])('returns false for %s (%s)', version => {
+        expect(util.isTurbopackDefaultForVersion(version)).toBe(false);
+      });
+    });
+
+    describe('edge cases', () => {
+      it.each([
+        ['', 'empty string'],
+        ['invalid', 'invalid version string'],
+        ['15', 'missing minor and patch'],
+        ['15.6', 'missing patch'],
+        ['not.a.version', 'completely invalid'],
+        ['15.6.0-alpha.1', 'alpha prerelease (not canary)'],
+        ['15.6.0-beta.1', 'beta prerelease (not canary)'],
+      ])('returns false for %s (%s)', version => {
+        expect(util.isTurbopackDefaultForVersion(version)).toBe(false);
+      });
+    });
+
+    describe('canary number parsing edge cases', () => {
+      it.each([
+        ['15.6.0-canary.', 'canary with no number'],
+        ['15.6.0-canary.abc', 'canary with non-numeric value'],
+        ['15.6.0-canary.38.extra', 'canary with extra segments'],
+      ])('handles malformed canary versions: %s (%s)', version => {
+        // Should not throw, just return appropriate boolean
+        expect(() => util.isTurbopackDefaultForVersion(version)).not.toThrow();
+      });
+
+      it('handles canary.38 exactly (boundary)', () => {
+        expect(util.isTurbopackDefaultForVersion('15.6.0-canary.38')).toBe(true);
+      });
+
+      it('handles canary.37 exactly (boundary)', () => {
+        expect(util.isTurbopackDefaultForVersion('15.6.0-canary.37')).toBe(false);
+      });
+    });
+  });
+
+  describe('detectActiveBundler', () => {
+    const originalArgv = process.argv;
+    const originalEnv = process.env;
+
+    beforeEach(() => {
+      process.argv = [...originalArgv];
+      process.env = { ...originalEnv };
+      delete process.env.TURBOPACK;
+    });
+
+    afterEach(() => {
+      process.argv = originalArgv;
+      process.env = originalEnv;
+    });
+
+    it('returns turbopack when TURBOPACK env var is set', () => {
+      process.env.TURBOPACK = '1';
+      expect(util.detectActiveBundler('15.5.0')).toBe('turbopack');
+    });
+
+    it('returns webpack when --webpack flag is present', () => {
+      process.argv.push('--webpack');
+      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');
+    });
+
+    it('returns turbopack for Next.js 15.6.0-canary.38+', () => {
+      expect(util.detectActiveBundler('15.6.0-canary.38')).toBe('turbopack');
+      expect(util.detectActiveBundler('15.6.0-canary.50')).toBe('turbopack');
+    });
+
+    it('returns webpack for Next.js 15.6.0 stable', () => {
+      expect(util.detectActiveBundler('15.6.0')).toBe('webpack');
+    });
+
+    it('returns webpack for Next.js 15.5.x and below', () => {
+      expect(util.detectActiveBundler('15.5.0')).toBe('webpack');
+      expect(util.detectActiveBundler('15.0.0')).toBe('webpack');
+      expect(util.detectActiveBundler('14.2.0')).toBe('webpack');
+    });
+
+    it('returns webpack when version is undefined', () => {
+      expect(util.detectActiveBundler(undefined)).toBe('webpack');
+    });
+
+    it('prioritizes TURBOPACK env var over version detection', () => {
+      process.env.TURBOPACK = '1';
+      expect(util.detectActiveBundler('14.0.0')).toBe('turbopack');
+    });
+
+    it('prioritizes --webpack flag over version detection', () => {
+      process.argv.push('--webpack');
+      expect(util.detectActiveBundler('16.0.0')).toBe('webpack');
+    });
+
+    it('prioritizes TURBOPACK env var over --webpack flag', () => {
+      process.env.TURBOPACK = '1';
+      process.argv.push('--webpack');
+      expect(util.detectActiveBundler('15.5.0')).toBe('turbopack');
+    });
+  });
 });