perf(@angular/build): decouple diagnostic type checking from build start

Previously, the compiler plugin sequentially awaited compilation.diagnoseFiles() in build.onStart before returning to esbuild.

Because build.onStart blocked until TypeScript diagnostic checks finished, esbuild's Go bundler sat idle while single-threaded type checking occurred, preventing module resolution and bundling from running concurrently.

To overlap bundling with type checking:
- Initiate compilation.diagnoseFiles() asynchronously during build.onStart without awaiting its completion.
- Allow build.onStart to return immediately once compilation emit is complete, enabling esbuild to begin bundling and file resolution concurrently.
- Await the diagnostics promise in build.onEnd, merging any diagnostic errors or warnings into the final build result.

In benchmarks on clean builds, this overlaps diagnostic checks with bundling, saving ~150 ms in small projects and 500 ms to 2,000+ ms in large enterprise codebases.

Author: alan-agius4

packages/angular/build/src/tools/esbuild/angular/compiler-plugin.ts

@@ -75,6 +75,7 @@ export function createCompilerPlugin(
     // eslint-disable-next-line max-lines-per-function
     async setup(build: PluginBuild): Promise<void> {
       let setupWarnings: PartialMessage[] | undefined = [];
+      let diagnosticsPromise: ReturnType<AngularCompilation['diagnoseFiles']> | undefined;
       const preserveSymlinks = build.initialOptions.preserveSymlinks;
 
       // Initialize a worker pool for JavaScript transformations.
@@ -402,15 +403,10 @@ export function createCompilerPlugin(
           }
         }
 
-        const diagnostics = await compilation.diagnoseFiles(
-          useTypeChecking ? DiagnosticModes.All : DiagnosticModes.All & ~DiagnosticModes.Semantic,
-        );
-        if (diagnostics.errors?.length) {
-          (result.errors ??= []).push(...diagnostics.errors);
-        }
-        if (diagnostics.warnings?.length) {
-          (result.warnings ??= []).push(...diagnostics.warnings);
-        }
+        const diagnosticModes = useTypeChecking
+          ? DiagnosticModes.All
+          : DiagnosticModes.All & ~DiagnosticModes.Semantic;
+        diagnosticsPromise = compilation.diagnoseFiles(diagnosticModes);
 
         // Add errors from failed additional results.
         // This must be done after emit to capture latest web worker results.
@@ -617,7 +613,7 @@ export function createCompilerPlugin(
         );
       }
 
-      build.onEnd((result) => {
+      build.onEnd(async (result) => {
         // Ensure other compilations are unblocked if the main compilation throws during start
         if (angularCompilationContext.isPrimary()) {
           angularCompilationContext.markAsReady(hasCompilationErrors);
@@ -640,6 +636,20 @@ export function createCompilerPlugin(
         }
 
         logCumulativeDurations();
+
+        if (diagnosticsPromise) {
+          try {
+            const diagnostics = await diagnosticsPromise;
+            const errors = diagnostics.errors?.length ? diagnostics.errors : undefined;
+            const warnings = diagnostics.warnings?.length ? diagnostics.warnings : undefined;
+
+            if (errors || warnings) {
+              return { errors, warnings };
+            }
+          } finally {
+            diagnosticsPromise = undefined;
+          }
+        }
       });
 
       build.onDispose(() => {