fix(@angular/build): consolidate initial shared chunks into main with Rolldown

When esbuild bundles an application with lazy-loaded routes, modules that are statically
reachable from the main entry point and also imported by different subsets of lazy routes are
split into separate shared chunks, often forming multi-level import chains from main.js.
Previously, the Rollup-based chunk optimization pass recognized that the main entry point is
always loaded before any dynamic import can execute and merged all modules in its static
closure back into main.js.

After switching chunk optimization to Rolldown, unconfigured Rolldown only merged direct
single-hop shared chunks into main.js while leaving deeper initial shared chunks as separate
files. Configuring Rolldown with preserveEntrySignatures set to allow-extension and a
codeSplitting group targeting the built-in $initial tag restores the consolidation of all
initial shared modules into the main entry chunk while keeping lazy-only shared chunks
separate.

Author: clydin

packages/angular/build/src/builders/application/chunk-optimizer.ts

@@ -300,12 +300,22 @@ export async function optimizeChunks(
       const { rolldown } = await loadRolldown();
       bundle = await rolldown({
         input: mainFile,
+        // Allow Rolldown to extend the main entry chunk's exports when merging shared initial
+        // modules into `main` so that lazy chunks can import those symbols directly from `main`.
+        preserveEntrySignatures: 'allow-extension',
         plugins,
       });
 
       const result = await bundle.generate({
         minify: { mangle: false, compress: false },
         sourcemap,
+        codeSplitting: {
+          // Consolidate all modules statically reachable from the initial entry point (`$initial`)
+          // back into the `main` chunk. This prevents multi-level initial chunk waterfalls caused
+          // by esbuild splitting out modules that are shared across `main` and subsets of lazy
+          // routes, while keeping shared chunks used exclusively by lazy routes separate.
+          groups: [{ name: 'main', tags: ['$initial'] }],
+        },
         chunkFileNames: (chunkInfo) =>
           `${chunkInfo.name.replace(/-[a-zA-Z0-9_-]{8}$/, '')}-[hash].js`,
       });

packages/angular/build/src/builders/application/tests/behavior/chunk-optimization-server_spec.ts

@@ -187,5 +187,115 @@ describeBuilder(buildApplication, APPLICATION_BUILDER_INFO, (harness) => {
           .toBeTrue();
       }
     });
+
+    it('consolidates multi-level initial shared modules into main while preserving lazy-only shared chunks', async () => {
+      await harness.writeFiles({
+        'src/shared-1.ts': `
+          import { shared2 } from './shared-2';
+          export function shared1(): string {
+            return 's1-' + shared2();
+          }
+        `,
+        'src/shared-2.ts': `
+          import { shared3 } from './shared-3';
+          export function shared2(): string {
+            return 's2-' + shared3();
+          }
+        `,
+        'src/shared-3.ts': `
+          import { shared4 } from './shared-4';
+          export function shared3(): string {
+            return 's3-' + shared4();
+          }
+        `,
+        'src/shared-4.ts': `
+          export function shared4(): string {
+            return 's4-' + Date.now().toString(36);
+          }
+        `,
+        'src/lazy-shared.ts': `
+          export function lazyOnlyShared(): string {
+            return 'lazy-only-' + Date.now().toString(36);
+          }
+        `,
+        'src/app/app.component.ts': `
+          import { Component } from '@angular/core';
+          import { RouterOutlet } from '@angular/router';
+          import { shared1 } from '../shared-1';
+
+          @Component({
+            selector: 'app-root',
+            imports: [RouterOutlet],
+            template: '<p>{{ val }}</p><router-outlet></router-outlet>',
+          })
+          export class AppComponent {
+            val = shared1();
+          }
+        `,
+        'src/app/lazy-a.component.ts': `
+          import { Component } from '@angular/core';
+          import { shared1 } from '../shared-1';
+          import { lazyOnlyShared } from '../lazy-shared';
+
+          @Component({ selector: 'app-lazy-a', template: '{{ val }}' })
+          export default class LazyAComponent {
+            val = shared1() + lazyOnlyShared();
+          }
+        `,
+        'src/app/lazy-b.component.ts': `
+          import { Component } from '@angular/core';
+          import { shared2 } from '../shared-2';
+          import { lazyOnlyShared } from '../lazy-shared';
+
+          @Component({ selector: 'app-lazy-b', template: '{{ val }}' })
+          export default class LazyBComponent {
+            val = shared2() + lazyOnlyShared();
+          }
+        `,
+        'src/app/lazy-c.component.ts': `
+          import { Component } from '@angular/core';
+          import { shared3 } from '../shared-3';
+
+          @Component({ selector: 'app-lazy-c', template: '{{ val }}' })
+          export default class LazyCComponent {
+            val = shared3();
+          }
+        `,
+        'src/app/lazy-d.component.ts': `
+          import { Component } from '@angular/core';
+          import { shared4 } from '../shared-4';
+
+          @Component({ selector: 'app-lazy-d', template: '{{ val }}' })
+          export default class LazyDComponent {
+            val = shared4();
+          }
+        `,
+      });
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        polyfills: ['zone.js'],
+        optimization: true,
+        namedChunks: true,
+      });
+
+      const { result } = await harness.executeOnce();
+      expect(result?.success).toBeTrue();
+
+      const mainContent = harness.readFile('dist/browser/main.js');
+      expect(mainContent).toContain('s1-');
+      expect(mainContent).toContain('s2-');
+      expect(mainContent).toContain('s3-');
+      expect(mainContent).toContain('s4-');
+      expect(mainContent).not.toContain('lazy-only-');
+
+      // No modulepreload hints should be needed in index.html because all initial shared
+      // modules (shared-1 through shared-4) were consolidated into main.js.
+      const indexContent = harness.readFile('dist/browser/index.html');
+      expect(indexContent).not.toContain('rel="modulepreload"');
+
+      // The lazy-only shared module should still be emitted as a separate shared chunk.
+      expect(harness.hasFileMatch('dist/browser', /^chunk-/)).toBeTrue();
+    });
   });
 });