fix: allow to enable lazy compilation via Rspack config

e2e/cases/lazy-compilation/basic/index.test.ts

@@ -10,6 +10,50 @@ rspackOnlyTest(
 
     const rsbuild = await dev({
       cwd: __dirname,
+      rsbuildConfig: {
+        dev: {
+          lazyCompilation: true,
+        },
+      },
+    });
+
+    await gotoPage(page, rsbuild, 'page1');
+    await expect(page.locator('#test')).toHaveText('Page 1');
+    await expectPoll(() =>
+      rsbuild.logs.some((log) => log.includes('building src/page1/index.js')),
+    ).toBeTruthy();
+    expect(
+      rsbuild.logs.some((log) => log.includes('building src/page2/index.js')),
+    ).toBeFalsy();
+
+    await gotoPage(page, rsbuild, 'page2');
+    await expect(page.locator('#test')).toHaveText('Page 2');
+    await expectPoll(() =>
+      rsbuild.logs.some((log) => log.includes('building src/page2/index.js')),
+    ).toBeTruthy();
+
+    await rsbuild.close();
+  },
+);
+
+rspackOnlyTest(
+  'should allow to configure `tools.rspack.experiments.lazyCompilation`',
+  async ({ page }) => {
+    if (process.platform === 'win32') {
+      test.skip();
+    }
+
+    const rsbuild = await dev({
+      cwd: __dirname,
+      rsbuildConfig: {
+        tools: {
+          rspack: {
+            experiments: {
+              lazyCompilation: true,
+            },
+          },
+        },
+      },
     });
 
     await gotoPage(page, rsbuild, 'page1');

e2e/cases/lazy-compilation/basic/rsbuild.config.ts

@@ -9,7 +9,4 @@ export default defineConfig({
       page2: './src/page2/index.js',
     },
   },
-  dev: {
-    lazyCompilation: true,
-  },
 });

packages/core/src/server/devMiddlewares.ts

@@ -1,7 +1,7 @@
 import { isAbsolute, join } from 'node:path';
 import { rspack } from '@rspack/core';
 import { normalizePublicDirs } from '../defaultConfig';
-import { castArray, pick } from '../helpers';
+import { castArray, isMultiCompiler, pick } from '../helpers';
 import { logger } from '../logger';
 import type {
   DevConfig,
@@ -67,7 +67,6 @@ const applySetupMiddlewares = (
 export type Middlewares = Array<RequestHandler | [string, RequestHandler]>;
 
 const applyDefaultMiddlewares = async ({
-  dev,
   devServerAPI,
   middlewares,
   server,
@@ -125,14 +124,30 @@ const applyDefaultMiddlewares = async ({
   if (
     context.action === 'dev' &&
     context.bundlerType === 'rspack' &&
-    dev.lazyCompilation &&
     compilationManager
   ) {
-    middlewares.push(
-      rspack.experiments.lazyCompilationMiddleware(
-        compilationManager.compiler,
-      ) as RequestHandler,
-    );
+    const { compiler } = compilationManager;
+
+    // We check the compiler options to determine whether lazy compilation is enabled.
+    // Rsbuild users can enable lazy compilation in two ways:
+    // 1. Use Rsbuild's `dev.lazyCompilation` option
+    // 2. Use Rspack's `experiments.lazyCompilation` option
+    const isLazyCompilationEnabled = () => {
+      if (isMultiCompiler(compiler)) {
+        return compiler.compilers.some(
+          (childCompiler) => childCompiler.options.experiments?.lazyCompilation,
+        );
+      }
+      return compiler.options.experiments?.lazyCompilation;
+    };
+
+    if (isLazyCompilationEnabled()) {
+      middlewares.push(
+        rspack.experiments.lazyCompilationMiddleware(
+          compiler,
+        ) as RequestHandler,
+      );
+    }
   }
 
   if (server.base && server.base !== '/') {