From 95c28b8ba82bc1cc1aa011d2d4ad06edf440b5c9 Mon Sep 17 00:00:00 2001
From: Alan Agius <17563226+alan-agius4@users.noreply.github.com>
Date: Tue, 20 Oct 2026 10:21:07 +0000
Subject: [PATCH] perf(@angular/build): avoid TS AST eviction and d.ts re-emit
 on external resource edits

Register external stylesheet dependencies on `stylesheetFile ?? containingFile` in the library builder so modifying an external stylesheet or imported SCSS partial does not add the component `.ts` file to `effectiveModifiedFiles` and evict it from `TypeScriptCompilation`'s AST cache. Instead, `getResourceDependencies` maps the modified stylesheet back to the component `.ts` file via `resourceModifiedFiles` while reusing the cached `ts.SourceFile` AST.

Additionally, persist a `WeakSet<ts.SourceFile>` of emitted declaration files on `AngularCompilationState` across incremental watch builds so that unchanged `ts.SourceFile`s re-emitted solely due to external resource edits (`templateUrl` / `styleUrl`) pass `EmitOnly.Js` to `program.emit`, skipping redundant `.d.ts` generation and downstream declaration processing.
---
 .../builders/library/pipeline/compilation.ts  |  5 +-
 .../library/tests/behavior/watch_spec.ts      | 74 +++++++++++++++++++
 .../angular/compilation/aot-compilation.ts    |  5 +-
 3 files changed, 78 insertions(+), 6 deletions(-)

diff --git a/packages/angular/build/src/builders/library/pipeline/compilation.ts b/packages/angular/build/src/builders/library/pipeline/compilation.ts
index 1c026c5e9fec..163c6194d6ad 100644
--- a/packages/angular/build/src/builders/library/pipeline/compilation.ts
+++ b/packages/angular/build/src/builders/library/pipeline/compilation.ts
@@ -89,10 +89,7 @@ export async function compileLibrary(
           : await stylesheetBundler.bundleInline(data, containingFile);
 
         if (result.referencedFiles) {
-          buildState.addFileReferences(containingFile, result.referencedFiles);
-          if (stylesheetFile) {
-            buildState.addFileReferences(stylesheetFile, result.referencedFiles);
-          }
+          buildState.addFileReferences(stylesheetFile ?? containingFile, result.referencedFiles);
         }
 
         if (result.warnings.length > 0) {
diff --git a/packages/angular/build/src/builders/library/tests/behavior/watch_spec.ts b/packages/angular/build/src/builders/library/tests/behavior/watch_spec.ts
index 9f85102fe20c..b648c690951a 100644
--- a/packages/angular/build/src/builders/library/tests/behavior/watch_spec.ts
+++ b/packages/angular/build/src/builders/library/tests/behavior/watch_spec.ts
@@ -984,5 +984,79 @@ describeLibraryBuilder(executeLibraryBuilder, LIBRARY_BUILDER_INFO, (harness) =>
         { outputLogsOnFailure: false },
       );
     });
+
+    it('should rebuild when an external Sass partial or template is modified and recover from template errors', async () => {
+      await harness.writeFiles({
+        'projects/lib/src/lib/_vars.scss': '$primary-color: rgb(10, 20, 30);',
+        'projects/lib/src/lib/lib.component.scss':
+          '@use "./vars"; :host { color: vars.$primary-color; }',
+        'projects/lib/src/lib/lib.component.html': '<h1>{{ title }}</h1>',
+        'projects/lib/src/lib/lib.component.ts': `
+          import { Component } from '@angular/core';
+
+          @Component({
+            selector: 'lib-external-resources',
+            templateUrl: './lib.component.html',
+            styleUrl: './lib.component.scss',
+          })
+          export class LibComponent {
+            title = 'Hello';
+          }
+        `,
+      });
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        watch: true,
+      });
+
+      await harness.executeWithCases([
+        async ({ result }) => {
+          expect(result?.success).toBeTrue();
+          const content = harness.readFile('dist/lib/fesm2022/lib.mjs');
+          expect(content).toContain('h1');
+          expect(content).toMatch(/color:\s*(?:#0a141e|rgb\(10,\s*20,\s*30\))/);
+          expect(harness.readFile('dist/lib/types/lib.d.ts')).toContain('LibComponent');
+
+          // Modify the imported Sass partial
+          await harness.writeFile(
+            'projects/lib/src/lib/_vars.scss',
+            '$primary-color: rgb(200, 100, 50);',
+          );
+        },
+        async ({ result }) => {
+          expect(result?.success).toBeTrue();
+          expect(harness.readFile('dist/lib/fesm2022/lib.mjs')).toMatch(
+            /color:\s*(?:#c86432|rgb\(200,\s*100,\s*50\))/,
+          );
+          expect(harness.readFile('dist/lib/types/lib.d.ts')).toContain('LibComponent');
+
+          // Introduce a template type-checking error in the external HTML template
+          await harness.writeFile(
+            'projects/lib/src/lib/lib.component.html',
+            '<section>{{ nonExistentProp }}</section>',
+          );
+        },
+        async ({ result, logs }) => {
+          expect(result?.success).toBeFalse();
+          expect(logs).toContain(
+            jasmine.objectContaining({
+              message: jasmine.stringContaining('nonExistentProp'),
+            }),
+          );
+
+          // Fix the external HTML template error
+          await harness.writeFile(
+            'projects/lib/src/lib/lib.component.html',
+            '<footer>{{ title }}</footer>',
+          );
+        },
+        ({ result }) => {
+          expect(result?.success).toBeTrue();
+          expect(harness.readFile('dist/lib/fesm2022/lib.mjs')).toContain('footer');
+          expect(harness.readFile('dist/lib/types/lib.d.ts')).toContain('LibComponent');
+        },
+      ]);
+    });
   });
 });
diff --git a/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts b/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts
index 92684539c2ca..8a3e559c9c69 100644
--- a/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts
+++ b/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts
@@ -83,6 +83,7 @@ class AngularCompilationState {
     public readonly declarationDiagnostics = new WeakMap<ts.SourceFile, readonly ts.Diagnostic[]>(),
     public readonly filesWithoutDtsSignature = new Set<string>(),
     public readonly useFallbackRootNames = false,
+    public readonly emittedDtsSourceFiles = new WeakSet<ts.SourceFile>(),
   ) {}
 
   get angularCompiler() {
@@ -362,6 +363,7 @@ export class AotCompilation extends TypeScriptCompilation {
       this.#state?.declarationDiagnostics,
       filesWithoutDtsSignature,
       useFallbackRootNames,
+      this.#state?.emittedDtsSourceFiles,
     );
 
     return {
@@ -498,10 +500,10 @@ export class AotCompilation extends TypeScriptCompilation {
   override emitAffectedFiles(): Iterable<EmitFileResult> {
     assert(this.#state, 'Angular compilation must be initialized prior to emitting files.');
     const {
-      affectedFiles,
       angularCompiler,
       compilerHost,
       declarationDiagnostics,
+      emittedDtsSourceFiles,
       typeScriptProgram,
       webWorkerTransform,
       useTypeScriptTranspilation,
@@ -512,7 +514,6 @@ export class AotCompilation extends TypeScriptCompilation {
 
     const emittedFiles = new Map<string, EmitFileResult>();
     const emittedSourceFiles = new Set<ts.SourceFile>();
-    const emittedDtsSourceFiles = new Set<ts.SourceFile>();
     const writeFileCallback: ts.WriteFileCallback = (filename, contents, _a, _b, sourceFiles) => {
       if (!sourceFiles?.length && filename.endsWith(buildInfoFilename)) {
         // Save builder info contents to specified location