refactor(ivy): extract import rewriting into a separate interface (#27998)

Currently the ImportManager class handles various rewriting actions of
imports when compiling @angular/core. This is required as code compiled
within @angular/core cannot import from '@angular/core'. To work around
this, imports are rewritten to get core symbols from a particular file,
r3_symbols.ts.

In this refactoring, this rewriting logic is moved out of the ImportManager
and put behind an interface, ImportRewriter. There are three implementers
of the interface:

* NoopImportRewriter, used for compiling all non-core packages.
* R3SymbolsImportRewriter, used when ngtsc compiles @angular/core.
* NgccFlatImportRewriter, used when ngcc compiles @angular/core (special
  logic is needed because ngcc has to rewrite imports in flat bundles
  differently than in non-flat bundles).

This is a precursor to using this rewriting logic in other contexts besides
the ImportManager.

PR Close #27998

packages/compiler-cli/src/ngcc/src/rendering/ngcc_import_manager.ts

@@ -1,22 +0,0 @@
-
-/**
- * @license
- * Copyright Google Inc. All Rights Reserved.
- *
- * Use of this source code is governed by an MIT-style license that can be
- * found in the LICENSE file at https://angular.io/license
- */
-
-import {ImportManager} from '../../../ngtsc/translator';
-
-export class NgccImportManager extends ImportManager {
-  constructor(private isFlat: boolean, isCore: boolean, prefix?: string) { super(isCore, prefix); }
-
-  generateNamedImport(moduleName: string, symbol: string):
-      {moduleImport: string | null, symbol: string} {
-    if (this.isFlat && this.isCore && moduleName === '@angular/core') {
-      return {moduleImport: null, symbol: this.rewriteSymbol(moduleName, symbol)};
-    }
-    return super.generateNamedImport(moduleName, symbol);
-  }
-}

packages/compiler-cli/src/ngcc/src/rendering/ngcc_import_rewriter.ts

@@ -0,0 +1,34 @@
+
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {ImportRewriter, validateAndRewriteCoreSymbol} from '../../../ngtsc/imports';
+
+export class NgccFlatImportRewriter implements ImportRewriter {
+  shouldImportSymbol(symbol: string, specifier: string): boolean {
+    if (specifier === '@angular/core') {
+      // Don't use imports for @angular/core symbols in a flat bundle, as they'll be visible
+      // directly.
+      return false;
+    } else {
+      return true;
+    }
+  }
+
+  rewriteSymbol(symbol: string, specifier: string): string {
+    if (specifier === '@angular/core') {
+      return validateAndRewriteCoreSymbol(symbol);
+    } else {
+      return symbol;
+    }
+  }
+
+  rewriteSpecifier(originalModulePath: string, inContextOfFile: string): string {
+    return originalModulePath;
+  }
+}

packages/compiler-cli/src/ngcc/src/rendering/renderer.ts

@@ -13,9 +13,10 @@ import {basename, dirname, relative, resolve} from 'canonical-path';
 import {SourceMapConsumer, SourceMapGenerator, RawSourceMap} from 'source-map';
 import * as ts from 'typescript';
 
+import {NoopImportRewriter, ImportRewriter, R3SymbolsImportRewriter} from '@angular/compiler-cli/src/ngtsc/imports';
 import {CompileResult} from '@angular/compiler-cli/src/ngtsc/transform';
 import {translateStatement, translateType, ImportManager} from '../../../ngtsc/translator';
-import {NgccImportManager} from './ngcc_import_manager';
+import {NgccFlatImportRewriter} from './ngcc_import_rewriter';
 import {CompiledClass, CompiledFile, DecorationAnalyses} from '../analysis/decoration_analyzer';
 import {ModuleWithProvidersInfo, ModuleWithProvidersAnalyses} from '../analysis/module_with_providers_analyzer';
 import {PrivateDeclarationsAnalyses, ExportInfo} from '../analysis/private_declarations_analyzer';
@@ -135,7 +136,8 @@ export abstract class Renderer {
     }
 
     if (compiledFile) {
-      const importManager = new NgccImportManager(this.bundle.isFlat, this.isCore, IMPORT_PREFIX);
+      const importManager = new ImportManager(
+          this.getImportRewriter(this.bundle.src.r3SymbolsFile, this.bundle.isFlat), IMPORT_PREFIX);
 
       // TODO: remove constructor param metadata and property decorators (we need info from the
       // handlers to do this)
@@ -152,9 +154,7 @@ export abstract class Renderer {
           renderConstantPool(compiledFile.sourceFile, compiledFile.constantPool, importManager),
           compiledFile.sourceFile);
 
-      this.addImports(
-          outputText, importManager.getAllImports(
-                          compiledFile.sourceFile.fileName, this.bundle.src.r3SymbolsFile));
+      this.addImports(outputText, importManager.getAllImports(compiledFile.sourceFile.fileName));
     }
 
     // Add exports to the entry-point file
@@ -169,7 +169,8 @@ export abstract class Renderer {
   renderDtsFile(dtsFile: ts.SourceFile, renderInfo: DtsRenderInfo): FileInfo[] {
     const input = this.extractSourceMap(dtsFile);
     const outputText = new MagicString(input.source);
-    const importManager = new NgccImportManager(false, this.isCore, IMPORT_PREFIX);
+    const importManager = new ImportManager(
+        this.getImportRewriter(this.bundle.dts !.r3SymbolsFile, false), IMPORT_PREFIX);
 
     renderInfo.classInfo.forEach(dtsClass => {
       const endOfClass = dtsClass.dtsDeclaration.getEnd();
@@ -181,8 +182,7 @@ export abstract class Renderer {
     });
 
     this.addModuleWithProvidersParams(outputText, renderInfo.moduleWithProviders, importManager);
-    this.addImports(
-        outputText, importManager.getAllImports(dtsFile.fileName, this.bundle.dts !.r3SymbolsFile));
+    this.addImports(outputText, importManager.getAllImports(dtsFile.fileName));
 
     this.addExports(outputText, dtsFile.fileName, renderInfo.privateExports);
 
@@ -199,7 +199,7 @@ export abstract class Renderer {
    */
   protected addModuleWithProvidersParams(
       outputText: MagicString, moduleWithProviders: ModuleWithProvidersInfo[],
-      importManager: NgccImportManager): void {
+      importManager: ImportManager): void {
     moduleWithProviders.forEach(info => {
       const ngModuleName = (info.ngModule.node as ts.ClassDeclaration).name !.text;
       const declarationFile = info.declaration.getSourceFile().fileName;
@@ -417,6 +417,16 @@ export abstract class Renderer {
     return (
         id && id.name === 'ModuleWithProviders' && (this.isCore || id.from === '@angular/core'));
   }
+
+  private getImportRewriter(r3SymbolsFile: ts.SourceFile|null, isFlat: boolean): ImportRewriter {
+    if (this.isCore && isFlat) {
+      return new NgccFlatImportRewriter();
+    } else if (this.isCore) {
+      return new R3SymbolsImportRewriter(r3SymbolsFile !.fileName);
+    } else {
+      return new NoopImportRewriter();
+    }
+  }
 }
 
 /**
@@ -451,7 +461,7 @@ export function mergeSourceMaps(
  * Render the constant pool as source code for the given class.
  */
 export function renderConstantPool(
-    sourceFile: ts.SourceFile, constantPool: ConstantPool, imports: NgccImportManager): string {
+    sourceFile: ts.SourceFile, constantPool: ConstantPool, imports: ImportManager): string {
   const printer = ts.createPrinter();
   return constantPool.statements.map(stmt => translateStatement(stmt, imports))
       .map(stmt => printer.printNode(ts.EmitHint.Unspecified, stmt, sourceFile))
@@ -467,7 +477,7 @@ export function renderConstantPool(
  * @param imports An object that tracks the imports that are needed by the rendered definitions.
  */
 export function renderDefinitions(
-    sourceFile: ts.SourceFile, compiledClass: CompiledClass, imports: NgccImportManager): string {
+    sourceFile: ts.SourceFile, compiledClass: CompiledClass, imports: ImportManager): string {
   const printer = ts.createPrinter();
   const name = (compiledClass.declaration as ts.NamedDeclaration).name !;
   const definitions =

packages/compiler-cli/src/ngtsc/annotations/test/metadata_spec.ts

@@ -8,6 +8,7 @@
 
 import * as ts from 'typescript';
 
+import {NoopImportRewriter} from '../../imports';
 import {TypeScriptReflectionHost} from '../../reflection';
 import {getDeclaration, makeProgram} from '../../testing/in_memory_typescript';
 import {ImportManager, translateStatement} from '../../translator';
@@ -87,7 +88,7 @@ function compileAndPrint(contents: string): string {
     return '';
   }
   const sf = program.getSourceFile('index.ts') !;
-  const im = new ImportManager(false, 'i');
+  const im = new ImportManager(new NoopImportRewriter(), 'i');
   const tsStatement = translateStatement(call, im);
   const res = ts.createPrinter().printNode(ts.EmitHint.Unspecified, tsStatement, sf);
   return res.replace(/\s+/g, ' ');

packages/compiler-cli/src/ngtsc/imports/index.ts

@@ -6,5 +6,6 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+export {ImportRewriter, NoopImportRewriter, R3SymbolsImportRewriter, validateAndRewriteCoreSymbol} from './src/core';
 export {AbsoluteReference, ImportMode, NodeReference, Reference, ResolvedReference} from './src/references';
 export {ReferenceResolver, TsReferenceResolver} from './src/resolver';

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

@@ -0,0 +1,103 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {relativePathBetween} from '../../util/src/path';
+
+/**
+ * Rewrites imports of symbols being written into generated code.
+ */
+export interface ImportRewriter {
+  /**
+   * Should the given symbol be imported at all?
+   *
+   * If `true`, the symbol should be imported from the given specifier. If `false`, the symbol
+   * should be referenced directly, without an import.
+   */
+  shouldImportSymbol(symbol: string, specifier: string): boolean;
+
+  /**
+   * Optionally rewrite a reference to an imported symbol, changing either the binding prefix or the
+   * symbol name itself.
+   */
+  rewriteSymbol(symbol: string, specifier: string): string;
+
+  /**
+   * Optionally rewrite the given module specifier in the context of a given file.
+   */
+  rewriteSpecifier(specifier: string, inContextOfFile: string): string;
+}
+
+/**
+ * `ImportRewriter` that does no rewriting.
+ */
+export class NoopImportRewriter implements ImportRewriter {
+  shouldImportSymbol(symbol: string, specifier: string): boolean { return true; }
+
+  rewriteSymbol(symbol: string, specifier: string): string { return symbol; }
+
+  rewriteSpecifier(specifier: string, inContextOfFile: string): string { return specifier; }
+}
+
+/**
+ * A mapping of supported symbols that can be imported from within @angular/core, and the names by
+ * which they're exported from r3_symbols.
+ */
+const CORE_SUPPORTED_SYMBOLS = new Map<string, string>([
+  ['defineInjectable', 'defineInjectable'],
+  ['defineInjector', 'defineInjector'],
+  ['ɵdefineNgModule', 'defineNgModule'],
+  ['inject', 'inject'],
+  ['ɵsetClassMetadata', 'setClassMetadata'],
+  ['ɵInjectableDef', 'InjectableDef'],
+  ['ɵInjectorDef', 'InjectorDef'],
+  ['ɵNgModuleDefWithMeta', 'NgModuleDefWithMeta'],
+  ['ɵNgModuleFactory', 'NgModuleFactory'],
+]);
+
+const CORE_MODULE = '@angular/core';
+
+/**
+ * `ImportRewriter` that rewrites imports from '@angular/core' to be imported from the r3_symbols.ts
+ * file instead.
+ */
+export class R3SymbolsImportRewriter implements ImportRewriter {
+  constructor(private r3SymbolsPath: string) {}
+
+  shouldImportSymbol(symbol: string, specifier: string): boolean { return true; }
+
+  rewriteSymbol(symbol: string, specifier: string): string {
+    if (specifier !== CORE_MODULE) {
+      // This import isn't from core, so ignore it.
+      return symbol;
+    }
+
+    return validateAndRewriteCoreSymbol(symbol);
+  }
+
+  rewriteSpecifier(specifier: string, inContextOfFile: string): string {
+    if (specifier !== CORE_MODULE) {
+      // This module isn't core, so ignore it.
+      return specifier;
+    }
+
+    const relativePathToR3Symbols = relativePathBetween(inContextOfFile, this.r3SymbolsPath);
+    if (relativePathToR3Symbols === null) {
+      throw new Error(
+          `Failed to rewrite import inside ${CORE_MODULE}: ${inContextOfFile} -> ${this.r3SymbolsPath}`);
+    }
+
+    return relativePathToR3Symbols;
+  }
+}
+
+export function validateAndRewriteCoreSymbol(name: string): string {
+  if (!CORE_SUPPORTED_SYMBOLS.has(name)) {
+    throw new Error(`Importing unexpected symbol ${name} while compiling ${CORE_MODULE}`);
+  }
+  return CORE_SUPPORTED_SYMBOLS.get(name) !;
+}

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

@@ -16,7 +16,7 @@ import {ComponentDecoratorHandler, DirectiveDecoratorHandler, InjectableDecorato
 import {BaseDefDecoratorHandler} from './annotations/src/base_def';
 import {ErrorCode, ngErrorCode} from './diagnostics';
 import {FlatIndexGenerator, ReferenceGraph, checkForPrivateExports, findFlatIndexEntryPoint} from './entry_point';
-import {Reference, TsReferenceResolver} from './imports';
+import {ImportRewriter, NoopImportRewriter, R3SymbolsImportRewriter, Reference, TsReferenceResolver} from './imports';
 import {PartialEvaluator} from './partial_evaluator';
 import {TypeScriptReflectionHost} from './reflection';
 import {FileResourceLoader, HostResourceLoader} from './resource_loader';
@@ -34,6 +34,7 @@ export class NgtscProgram implements api.Program {
   private sourceToFactorySymbols: Map<string, Set<string>>|null = null;
   private host: ts.CompilerHost;
   private _coreImportsFrom: ts.SourceFile|null|undefined = undefined;
+  private _importRewriter: ImportRewriter|undefined = undefined;
   private _reflector: TypeScriptReflectionHost|undefined = undefined;
   private _isCore: boolean|undefined = undefined;
   private rootDirs: string[];
@@ -234,7 +235,7 @@ export class NgtscProgram implements api.Program {
 
     const customTransforms = opts && opts.customTransformers;
     const beforeTransforms =
-        [ivyTransformFactory(this.compilation !, this.reflector, this.coreImportsFrom)];
+        [ivyTransformFactory(this.compilation !, this.reflector, this.importRewriter, this.isCore)];
 
     if (this.factoryToSourceInfo !== null) {
       beforeTransforms.push(
@@ -303,7 +304,7 @@ export class NgtscProgram implements api.Program {
     ];
 
     return new IvyCompilation(
-        handlers, checker, this.reflector, this.coreImportsFrom, this.sourceToFactorySymbols);
+        handlers, checker, this.reflector, this.importRewriter, this.sourceToFactorySymbols);
   }
 
   private get reflector(): TypeScriptReflectionHost {
@@ -326,6 +327,16 @@ export class NgtscProgram implements api.Program {
     }
     return this._isCore;
   }
+
+  private get importRewriter(): ImportRewriter {
+    if (this._importRewriter === undefined) {
+      const coreImportsFrom = this.coreImportsFrom;
+      this._importRewriter = coreImportsFrom !== null ?
+          new R3SymbolsImportRewriter(coreImportsFrom.fileName) :
+          new NoopImportRewriter();
+    }
+    return this._importRewriter;
+  }
 }
 
 const defaultEmitCallback: api.TsEmitCallback =

packages/compiler-cli/src/ngtsc/transform/BUILD.bazel

@@ -12,6 +12,7 @@ ts_library(
     deps = [
         "//packages/compiler",
         "//packages/compiler-cli/src/ngtsc/diagnostics",
+        "//packages/compiler-cli/src/ngtsc/imports",
         "//packages/compiler-cli/src/ngtsc/reflection",
         "//packages/compiler-cli/src/ngtsc/translator",
         "//packages/compiler-cli/src/ngtsc/typecheck",

packages/compiler-cli/src/ngtsc/transform/src/compilation.ts

@@ -10,6 +10,7 @@ import {ConstantPool} from '@angular/compiler';
 import * as ts from 'typescript';
 
 import {FatalDiagnosticError} from '../../diagnostics';
+import {ImportRewriter} from '../../imports';
 import {Decorator, ReflectionHost, reflectNameOfDeclaration} from '../../reflection';
 import {TypeCheckContext} from '../../typecheck';
 
@@ -17,6 +18,7 @@ import {AnalysisOutput, CompileResult, DecoratorHandler} from './api';
 import {DtsFileTransformer} from './declaration';
 
 
+
 /**
  * Record of an adapter which decided to emit a static field, and the analysis it performed to
  * prepare for that operation.
@@ -63,7 +65,7 @@ export class IvyCompilation {
    */
   constructor(
       private handlers: DecoratorHandler<any, any>[], private checker: ts.TypeChecker,
-      private reflector: ReflectionHost, private coreImportsFrom: ts.SourceFile|null,
+      private reflector: ReflectionHost, private importRewriter: ImportRewriter,
       private sourceToFactorySymbols: Map<string, Set<string>>|null) {}
 
 
@@ -227,7 +229,7 @@ export class IvyCompilation {
 
   private getDtsTransformer(tsFileName: string): DtsFileTransformer {
     if (!this.dtsMap.has(tsFileName)) {
-      this.dtsMap.set(tsFileName, new DtsFileTransformer(this.coreImportsFrom));
+      this.dtsMap.set(tsFileName, new DtsFileTransformer(this.importRewriter));
     }
     return this.dtsMap.get(tsFileName) !;
   }

packages/compiler-cli/src/ngtsc/transform/src/declaration.ts

@@ -8,6 +8,7 @@
 
 import * as ts from 'typescript';
 
+import {ImportRewriter} from '../../imports';
 import {ImportManager, translateType} from '../../translator';
 
 import {CompileResult} from './api';
@@ -21,8 +22,8 @@ export class DtsFileTransformer {
   private ivyFields = new Map<string, CompileResult[]>();
   private imports: ImportManager;
 
-  constructor(private coreImportsFrom: ts.SourceFile|null, importPrefix?: string) {
-    this.imports = new ImportManager(coreImportsFrom !== null, importPrefix);
+  constructor(private importRewriter: ImportRewriter, importPrefix?: string) {
+    this.imports = new ImportManager(importRewriter, importPrefix);
   }
 
   /**
@@ -56,7 +57,7 @@ export class DtsFileTransformer {
       }
     }
 
-    const imports = this.imports.getAllImports(tsPath, this.coreImportsFrom);
+    const imports = this.imports.getAllImports(tsPath);
     if (imports.length !== 0) {
       dts = imports.map(i => `import * as ${i.as} from '${i.name}';\n`).join('') + dts;
     }

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

@@ -9,6 +9,7 @@
 import {ConstantPool} from '@angular/compiler';
 import * as ts from 'typescript';
 
+import {ImportRewriter} from '../../imports';
 import {Decorator, ReflectionHost} from '../../reflection';
 import {ImportManager, translateExpression, translateStatement} from '../../translator';
 import {VisitListEntryResult, Visitor, visit} from '../../util/src/visitor';
@@ -19,11 +20,11 @@ import {IvyCompilation} from './compilation';
 const NO_DECORATORS = new Set<ts.Decorator>();
 
 export function ivyTransformFactory(
-    compilation: IvyCompilation, reflector: ReflectionHost,
-    coreImportsFrom: ts.SourceFile | null): ts.TransformerFactory<ts.SourceFile> {
+    compilation: IvyCompilation, reflector: ReflectionHost, importRewriter: ImportRewriter,
+    isCore: boolean): ts.TransformerFactory<ts.SourceFile> {
   return (context: ts.TransformationContext): ts.Transformer<ts.SourceFile> => {
     return (file: ts.SourceFile): ts.SourceFile => {
-      return transformIvySourceFile(compilation, context, reflector, coreImportsFrom, file);
+      return transformIvySourceFile(compilation, context, reflector, importRewriter, isCore, file);
     };
   };
 }
@@ -188,13 +189,12 @@ class IvyVisitor extends Visitor {
  */
 function transformIvySourceFile(
     compilation: IvyCompilation, context: ts.TransformationContext, reflector: ReflectionHost,
-    coreImportsFrom: ts.SourceFile | null, file: ts.SourceFile): ts.SourceFile {
+    importRewriter: ImportRewriter, isCore: boolean, file: ts.SourceFile): ts.SourceFile {
   const constantPool = new ConstantPool();
-  const importManager = new ImportManager(coreImportsFrom !== null);
+  const importManager = new ImportManager(importRewriter);
 
   // Recursively scan through the AST and perform any updates requested by the IvyCompilation.
-  const visitor =
-      new IvyVisitor(compilation, reflector, importManager, coreImportsFrom !== null, constantPool);
+  const visitor = new IvyVisitor(compilation, reflector, importManager, isCore, constantPool);
   const sf = visit(file, visitor, context);
 
   // Generate the constant statements first, as they may involve adding additional imports
@@ -202,7 +202,7 @@ function transformIvySourceFile(
   const constants = constantPool.statements.map(stmt => translateStatement(stmt, importManager));
 
   // Generate the import statements to prepend.
-  const addedImports = importManager.getAllImports(file.fileName, coreImportsFrom).map(i => {
+  const addedImports = importManager.getAllImports(file.fileName).map(i => {
     return ts.createImportDeclaration(
         undefined, undefined,
         ts.createImportClause(undefined, ts.createNamespaceImport(ts.createIdentifier(i.as))),

packages/compiler-cli/src/ngtsc/translator/BUILD.bazel

@@ -9,6 +9,7 @@ ts_library(
     deps = [
         "//packages:types",
         "//packages/compiler",
+        "//packages/compiler-cli/src/ngtsc/imports",
         "//packages/compiler-cli/src/ngtsc/util",
         "@ngdeps//typescript",
     ],

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

@@ -9,7 +9,7 @@
 import {ArrayType, AssertNotNull, BinaryOperator, BinaryOperatorExpr, BuiltinType, BuiltinTypeName, CastExpr, ClassStmt, CommaExpr, CommentStmt, ConditionalExpr, DeclareFunctionStmt, DeclareVarStmt, Expression, ExpressionStatement, ExpressionType, ExpressionVisitor, ExternalExpr, ExternalReference, FunctionExpr, IfStmt, InstantiateExpr, InvokeFunctionExpr, InvokeMethodExpr, JSDocCommentStmt, LiteralArrayExpr, LiteralExpr, LiteralMapExpr, MapType, NotExpr, ReadKeyExpr, ReadPropExpr, ReadVarExpr, ReturnStatement, Statement, StatementVisitor, StmtModifier, ThrowStmt, TryCatchStmt, Type, TypeVisitor, TypeofExpr, WrappedNodeExpr, WriteKeyExpr, WritePropExpr, WriteVarExpr} from '@angular/compiler';
 import * as ts from 'typescript';
 
-import {relativePathBetween} from '../../util/src/path';
+import {ImportRewriter, NoopImportRewriter} from '../../imports';
 
 export class Context {
   constructor(readonly isStatement: boolean) {}
@@ -38,60 +38,41 @@ const BINARY_OPERATORS = new Map<BinaryOperator, ts.BinaryOperator>([
   [BinaryOperator.Plus, ts.SyntaxKind.PlusToken],
 ]);
 
-const CORE_SUPPORTED_SYMBOLS = new Map<string, string>([
-  ['defineInjectable', 'defineInjectable'],
-  ['defineInjector', 'defineInjector'],
-  ['ɵdefineNgModule', 'defineNgModule'],
-  ['inject', 'inject'],
-  ['ɵsetClassMetadata', 'setClassMetadata'],
-  ['ɵInjectableDef', 'InjectableDef'],
-  ['ɵInjectorDef', 'InjectorDef'],
-  ['ɵNgModuleDefWithMeta', 'NgModuleDefWithMeta'],
-  ['ɵNgModuleFactory', 'NgModuleFactory'],
-]);
+
 
 export class ImportManager {
   private moduleToIndex = new Map<string, string>();
+  private importedModules = new Set<string>();
   private nextIndex = 0;
 
-  constructor(protected isCore: boolean, private prefix = 'i') {}
+  constructor(protected rewriter: ImportRewriter = new NoopImportRewriter(), private prefix = 'i') {
+  }
 
-  generateNamedImport(moduleName: string, symbol: string):
+  generateNamedImport(moduleName: string, originalSymbol: string):
       {moduleImport: string | null, symbol: string} {
+    // First, rewrite the symbol name.
+    const symbol = this.rewriter.rewriteSymbol(originalSymbol, moduleName);
+
+    // Ask the rewriter if this symbol should be imported at all. If not, it can be referenced
+    // directly (moduleImport: null).
+    if (!this.rewriter.shouldImportSymbol(symbol, moduleName)) {
+      // The symbol should be referenced directly.
+      return {moduleImport: null, symbol};
+    }
+
+    // If not, this symbol will be imported. Allocate a prefix for the imported module if needed.
     if (!this.moduleToIndex.has(moduleName)) {
       this.moduleToIndex.set(moduleName, `${this.prefix}${this.nextIndex++}`);
     }
+    const moduleImport = this.moduleToIndex.get(moduleName) !;
 
-    return {
-      moduleImport: this.moduleToIndex.get(moduleName) !,
-      symbol: this.rewriteSymbol(moduleName, symbol)
-    };
+    return {moduleImport, symbol};
   }
 
-  protected rewriteSymbol(moduleName: string, symbol: string): string {
-    if (this.isCore && moduleName === '@angular/core') {
-      if (!CORE_SUPPORTED_SYMBOLS.has(symbol)) {
-        throw new Error(`Importing unexpected symbol ${symbol} while compiling core`);
-      }
-
-      symbol = CORE_SUPPORTED_SYMBOLS.get(symbol) !;
-    }
-
-    return symbol;
-  }
-
-  getAllImports(contextPath: string, rewriteCoreImportsTo: ts.SourceFile|null):
-      {name: string, as: string}[] {
+  getAllImports(contextPath: string): {name: string, as: string}[] {
     return Array.from(this.moduleToIndex.keys()).map(name => {
-      const as: string|null = this.moduleToIndex.get(name) !;
-      if (rewriteCoreImportsTo !== null && name === '@angular/core') {
-        const relative = relativePathBetween(contextPath, rewriteCoreImportsTo.fileName);
-        if (relative === null) {
-          throw new Error(
-              `Failed to rewrite import inside core: ${contextPath} -> ${rewriteCoreImportsTo.fileName}`);
-        }
-        name = relative;
-      }
+      const as = this.moduleToIndex.get(name) !;
+      name = this.rewriter.rewriteSpecifier(name, contextPath);
       return {name, as};
     });
   }

packages/compiler-cli/src/ngtsc/typecheck/src/context.ts

@@ -9,6 +9,7 @@
 import {R3TargetBinder, SelectorMatcher, TmplAstNode} from '@angular/compiler';
 import * as ts from 'typescript';
 
+import {NoopImportRewriter} from '../../imports';
 import {ImportManager} from '../../translator';
 
 import {TypeCheckBlockMetadata, TypeCheckableDirectiveMeta, TypeCtorMetadata} from './api';
@@ -16,6 +17,7 @@ import {generateTypeCheckBlock} from './type_check_block';
 import {generateTypeCtor} from './type_constructor';
 
 
+
 /**
  * A template type checking context for a program.
  *
@@ -117,7 +119,7 @@ export class TypeCheckContext {
 
     // Imports may need to be added to the file to support type-checking of directives used in the
     // template within it.
-    const importManager = new ImportManager(false, '_i');
+    const importManager = new ImportManager(new NoopImportRewriter(), '_i');
 
     // Each Op has a splitPoint index into the text where it needs to be inserted. Split the
     // original source text into chunks at these split points, where code will be inserted between
@@ -139,7 +141,7 @@ export class TypeCheckContext {
     });
 
     // Write out the imports that need to be added to the beginning of the file.
-    let imports = importManager.getAllImports(sf.fileName, null)
+    let imports = importManager.getAllImports(sf.fileName)
                       .map(i => `import * as ${i.as} from '${i.name}';`)
                       .join('\n');
     code = imports + '\n' + code;

packages/core/src/r3_symbols.ts

@@ -14,8 +14,9 @@
  * compiler writes imports to this file.
  *
  * Only a subset of such imports are supported - core is not allowed to declare components or pipes.
- * A check in ngtsc's translator.ts validates this condition. The translator is responsible for
- * translating an external name (prefixed with ɵ) to the internal symbol name as exported below.
+ * A check in ngtsc's `R3SymbolsImportRewriter` validates this condition. The rewriter is only used
+ * when compiling @angular/core and is responsible for translating an external name (prefixed with
+ * ɵ) to the internal symbol name as exported below.
  *
  * The below symbols are used for @Injectable and @NgModule compilation.
  */