refactor(compiler-cli): add compiler flag for testing let declarations (#56199)

Adds a private `_enableLetSyntax` flag that allows for let declarations to be enabled in tests.

PR Close #56199

packages/compiler-cli/src/ngtsc/annotations/component/src/handler.ts

@@ -247,6 +247,7 @@ export class ComponentDecoratorHandler
     private readonly deferredSymbolTracker: DeferredSymbolTracker,
     private readonly forbidOrphanRendering: boolean,
     private readonly enableBlockSyntax: boolean,
+    private readonly enableLetSyntax: boolean,
     private readonly localCompilationExtraImportsTracker: LocalCompilationExtraImportsTracker | null,
   ) {
     this.extractTemplateOptions = {
@@ -254,6 +255,7 @@ export class ComponentDecoratorHandler
       i18nNormalizeLineEndingsInICUs: this.i18nNormalizeLineEndingsInICUs,
       usePoisonedData: this.usePoisonedData,
       enableBlockSyntax: this.enableBlockSyntax,
+      enableLetSyntax: this.enableLetSyntax,
     };
   }
 
@@ -273,6 +275,7 @@ export class ComponentDecoratorHandler
     i18nNormalizeLineEndingsInICUs: boolean;
     usePoisonedData: boolean;
     enableBlockSyntax: boolean;
+    enableLetSyntax: boolean;
   };
 
   readonly precedence = HandlerPrecedence.PRIMARY;
@@ -622,6 +625,7 @@ export class ComponentDecoratorHandler
           i18nNormalizeLineEndingsInICUs: this.i18nNormalizeLineEndingsInICUs,
           usePoisonedData: this.usePoisonedData,
           enableBlockSyntax: this.enableBlockSyntax,
+          enableLetSyntax: this.enableLetSyntax,
         },
         this.compilationMode,
       );

packages/compiler-cli/src/ngtsc/annotations/component/src/resources.ts

@@ -13,6 +13,7 @@ import {
   ParsedTemplate,
   ParseSourceFile,
   parseTemplate,
+  ParseTemplateOptions,
   TmplAstNode,
 } from '@angular/compiler';
 import ts from 'typescript';
@@ -131,6 +132,7 @@ export interface ExtractTemplateOptions {
   enableI18nLegacyMessageIdFormat: boolean;
   i18nNormalizeLineEndingsInICUs: boolean;
   enableBlockSyntax: boolean;
+  enableLetSyntax: boolean;
 }
 
 export function extractTemplate(
@@ -259,16 +261,20 @@ function parseExtractedTemplate(
 ): ParsedComponentTemplate {
   // We always normalize line endings if the template has been escaped (i.e. is inline).
   const i18nNormalizeLineEndingsInICUs = escapedString || options.i18nNormalizeLineEndingsInICUs;
-
-  const parsedTemplate = parseTemplate(sourceStr, sourceMapUrl ?? '', {
-    preserveWhitespaces: template.preserveWhitespaces,
+  const commonParseOptions: ParseTemplateOptions = {
     interpolationConfig: template.interpolationConfig,
     range: sourceParseRange ?? undefined,
-    escapedString,
     enableI18nLegacyMessageIdFormat: options.enableI18nLegacyMessageIdFormat,
     i18nNormalizeLineEndingsInICUs,
     alwaysAttemptHtmlToR3AstConversion: options.usePoisonedData,
+    escapedString,
     enableBlockSyntax: options.enableBlockSyntax,
+    enableLetSyntax: options.enableLetSyntax,
+  };
+
+  const parsedTemplate = parseTemplate(sourceStr, sourceMapUrl ?? '', {
+    ...commonParseOptions,
+    preserveWhitespaces: template.preserveWhitespaces,
   });
 
   // Unfortunately, the primary parse of the template above may not contain accurate source map
@@ -287,16 +293,10 @@ function parseExtractedTemplate(
   // with the above options set to preserve source mappings.
 
   const {nodes: diagNodes} = parseTemplate(sourceStr, sourceMapUrl ?? '', {
+    ...commonParseOptions,
     preserveWhitespaces: true,
     preserveLineEndings: true,
-    interpolationConfig: template.interpolationConfig,
-    range: sourceParseRange ?? undefined,
-    escapedString,
-    enableI18nLegacyMessageIdFormat: options.enableI18nLegacyMessageIdFormat,
-    i18nNormalizeLineEndingsInICUs,
     leadingTriviaChars: [],
-    alwaysAttemptHtmlToR3AstConversion: options.usePoisonedData,
-    enableBlockSyntax: options.enableBlockSyntax,
   });
 
   return {

packages/compiler-cli/src/ngtsc/annotations/component/test/component_spec.ts

@@ -142,6 +142,7 @@ function setup(
     new DeferredSymbolTracker(checker, /* onlyExplicitDeferDependencyImports */ false),
     /* forbidOrphanRenderering */ false,
     /* enableBlockSyntax */ true,
+    /* enableLetSyntax */ false,
     /* localCompilationExtraImportsTracker */ null,
   );
   return {reflectionHost, handler, resourceLoader, metaRegistry};

packages/compiler-cli/src/ngtsc/core/api/src/options.ts

@@ -82,6 +82,14 @@ export interface InternalOptions {
    */
   _enableBlockSyntax?: boolean;
 
+  /**
+   * Whether `@let` syntax is enabled in the compiler.
+   * Defaults to false while the feature is being developed.
+   *
+   * @internal
+   */
+  _enableLetSyntax?: boolean;
+
   /**
    * Detected version of `@angular/core` in the workspace. Used by the
    * compiler to adjust the output depending on the available symbols.

packages/compiler-cli/src/ngtsc/core/src/compiler.ts

@@ -384,6 +384,7 @@ export class NgCompiler {
   readonly ignoreForEmit: Set<ts.SourceFile>;
   readonly enableTemplateTypeChecker: boolean;
   private readonly enableBlockSyntax: boolean;
+  private readonly enableLetSyntax: boolean;
   private readonly angularCoreVersion: string | null;
 
   /**
@@ -454,6 +455,7 @@ export class NgCompiler {
       enableTemplateTypeChecker || (options['_enableTemplateTypeChecker'] ?? false);
     // TODO(crisbeto): remove this flag and base `enableBlockSyntax` on the `angularCoreVersion`.
     this.enableBlockSyntax = options['_enableBlockSyntax'] ?? true;
+    this.enableLetSyntax = options['_enableLetSyntax'] ?? false;
     this.angularCoreVersion = options['_angularCoreVersion'] ?? null;
     this.constructionDiagnostics.push(
       ...this.adapter.constructionDiagnostics,
@@ -1397,6 +1399,7 @@ export class NgCompiler {
         deferredSymbolsTracker,
         !!this.options.forbidOrphanComponents,
         this.enableBlockSyntax,
+        this.enableLetSyntax,
         localCompilationExtraImportsTracker,
       ),