perf(@angular-devkit/build-angular): filter postcss usage based on content in esbuild builder

When using the esbuild-based browser application builder, stylesheets that do not need any
postcss processing will now skip the postcss step of the stylesheet build pipeline. Currently,
only Tailwind CSS leverages postcss and Tailwind CSS is an opt-in feature. As a result, postcss
will only be used if Tailwind CSS is configured for the project and a stylesheet contains one or
more of the directives/functions specific to Tailwind as provided by https://tailwindcss.com/docs/functions-and-directives.
This change should be most beneficial for component stylesheets which will rarely contain Tailwind
specific rules that need additional processing.

Author: clydin

Diff for: packages/angular_devkit/build_angular/src/tools/esbuild/stylesheets/stylesheet-plugin-factory.ts

@@ -29,6 +29,10 @@ export interface StylesheetPluginOptions {
    */
   sourcemap: boolean;
 
+  /**
+   * An optional array of paths that will be searched for stylesheets if the default
+   * resolution process for the stylesheet language does not succeed.
+   */
   includePaths?: string[];
 
   /**
@@ -37,9 +41,21 @@ export interface StylesheetPluginOptions {
    */
   inlineComponentData?: Record<string, string>;
 
+  /**
+   * Optional information used to load and configure Tailwind CSS. If present, the postcss
+   * will be added to the stylesheet processing with the Tailwind plugin setup as provided
+   * by the configuration file.
+   */
   tailwindConfiguration?: { file: string; package: string };
 }
 
+/**
+ * An array of keywords that indicate Tailwind CSS processing is required for a stylesheet.
+ *
+ * Based on https://tailwindcss.com/docs/functions-and-directives
+ */
+const TAILWIND_KEYWORDS = ['@tailwind', '@layer', '@apply', '@config', 'theme(', 'screen('];
+
 export interface StylesheetLanguage {
   name: string;
   componentFilter: RegExp;
@@ -54,6 +70,8 @@ export interface StylesheetLanguage {
 }
 
 export class StylesheetPluginFactory {
+  private postcssProcessor?: import('postcss').Processor;
+
   constructor(
     private readonly options: StylesheetPluginOptions,
     private readonly cache?: LoadResultCache,
@@ -69,21 +87,28 @@ export class StylesheetPluginFactory {
     }
 
     const { cache, options } = this;
+    const setupPostcss = async () => {
+      // Return already created processor if present
+      if (this.postcssProcessor) {
+        return this.postcssProcessor;
+      }
+
+      if (options.tailwindConfiguration) {
+        postcss ??= (await import('postcss')).default;
+        const tailwind = await import(options.tailwindConfiguration.package);
+        this.postcssProcessor = postcss().use(
+          tailwind.default({ config: options.tailwindConfiguration.file }),
+        );
+      }
+
+      return this.postcssProcessor;
+    };
 
     return {
       name: 'angular-' + language.name,
       async setup(build) {
-        // Setup postcss if needed by tailwind
-        // TODO: Move this into the plugin factory to avoid repeat setup per created plugin
-        let postcssProcessor: import('postcss').Processor | undefined;
-        if (options.tailwindConfiguration) {
-          postcss ??= (await import('postcss')).default;
-          postcssProcessor = postcss();
-          if (options.tailwindConfiguration) {
-            const tailwind = await import(options.tailwindConfiguration.package);
-            postcssProcessor.use(tailwind.default({ config: options.tailwindConfiguration.file }));
-          }
-        }
+        // Setup postcss if needed
+        const postcssProcessor = await setupPostcss();
 
         // Add a load callback to support inline Component styles
         build.onLoad(
@@ -96,6 +121,12 @@ export class StylesheetPluginFactory {
             );
 
             const [format, , filename] = args.path.split(';', 3);
+            // Only use postcss if Tailwind processing is required.
+            // NOTE: If postcss is used for more than just Tailwind in the future this check MUST
+            // be updated to account for the additional use.
+            // TODO: use better search algorithm for keywords
+            const needsPostcss =
+              !!postcssProcessor && TAILWIND_KEYWORDS.some((keyword) => data.includes(keyword));
 
             return processStylesheet(
               language,
@@ -104,7 +135,7 @@ export class StylesheetPluginFactory {
               format,
               options,
               build,
-              postcssProcessor,
+              needsPostcss ? postcssProcessor : undefined,
             );
           }),
         );
@@ -114,6 +145,8 @@ export class StylesheetPluginFactory {
           { filter: language.fileFilter },
           createCachedLoad(cache, async (args) => {
             const data = await readFile(args.path, 'utf-8');
+            const needsPostcss =
+              !!postcssProcessor && TAILWIND_KEYWORDS.some((keyword) => data.includes(keyword));
 
             return processStylesheet(
               language,
@@ -122,7 +155,7 @@ export class StylesheetPluginFactory {
               extname(args.path).toLowerCase().slice(1),
               options,
               build,
-              postcssProcessor,
+              needsPostcss ? postcssProcessor : undefined,
             );
           }),
         );