fixup! perf(@angular/build): cache transformed configuration in TypeScript compilation

Author: alan-agius4

packages/angular/build/src/tools/angular/compilation/typescript-compilation.ts

@@ -25,6 +25,7 @@ export interface TransformedConfiguration {
 export abstract class TypeScriptCompilation extends AngularCompilation {
   static #angularCompilerCliModule?: typeof ng;
   #cachedConfiguration?: TransformedConfiguration;
+  #cachedRootFiles?: readonly string[];
 
   static async loadCompilerCli(): Promise<typeof ng> {
     TypeScriptCompilation.#angularCompilerCliModule ??= await import('@angular/compiler-cli');
@@ -39,7 +40,14 @@ export abstract class TypeScriptCompilation extends AngularCompilation {
   ): Promise<TransformedConfiguration> {
     // When `rootFiles` are explicitly provided (e.g., library builder), avoid re-parsing `tsconfig.json`
     // and walking the project directory tree via `readConfiguration` on every watch rebuild (~200-350ms on large libraries).
-    if (compilerOptionOverrides?.rootFiles?.length && this.#cachedConfiguration) {
+    const currentRootFiles = compilerOptionOverrides?.rootFiles;
+    if (
+      currentRootFiles &&
+      this.#cachedConfiguration &&
+      this.#cachedRootFiles &&
+      this.#cachedRootFiles.length === currentRootFiles.length &&
+      this.#cachedRootFiles.every((file, index) => file === currentRootFiles[index])
+    ) {
       return this.#cachedConfiguration;
     }
 
@@ -96,8 +104,9 @@ export abstract class TypeScriptCompilation extends AngularCompilation {
       warnings,
     };
 
-    if (compilerOptionOverrides?.rootFiles?.length) {
+    if (currentRootFiles?.length) {
       this.#cachedConfiguration = config;
+      this.#cachedRootFiles = currentRootFiles;
     }
 
     return config;
@@ -112,6 +121,7 @@ export abstract class TypeScriptCompilation extends AngularCompilation {
       if (/\.json$/i.test(posixFile)) {
         // If a tsconfig changes, we need to re-read the configuration.
         this.#cachedConfiguration = undefined;
+        this.#cachedRootFiles = undefined;
       }
     }
   }