refactor(compiler): Remove NullAstVisitor and visitAstChildren (#35619)

This commit removes the `NullAstVisitor` and `visitAstChildren` exported
from `packages/compiler/src/expression_parser/ast.ts` because they
contain duplicate and buggy implementation, and their use cases could be
sufficiently covered by `RecursiveAstVisitor` if the latter implements the
`visit` method. This use case is only needed in the language service.

With this change, any visitor that extends `RecursiveAstVisitor` could
just define their own `visit` function and the parent class will behave
correctly.

A bit of historical context:
In language service, we need a way to tranverse the expression AST in a
selective manner based on where the user's cursor is. This means we need a
"filtering" function to decide which node to visit and which node to not
visit. Instead of refactoring `RecursiveAstVisitor` to support this,
`visitAstChildren` was created. `visitAstChildren` duplicates the
implementation of `RecursiveAstVisitor`, but introduced some bugs along
the way. For example, in `visitKeyedWrite`, it visits
```
obj -> key -> obj
```
instead of
```
obj -> key -> value
```

Moreover, because of the following line
```
visitor.visit && visitor.visit(ast, context) || ast.visit(visitor, context);
```
`visitAstChildren` visits every node *twice*.

PR Close #35619

packages/language-service/src/expression_type.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {AST, AstVisitor, Binary, BindingPipe, Chain, Conditional, FunctionCall, ImplicitReceiver, Interpolation, KeyedRead, KeyedWrite, LiteralArray, LiteralMap, LiteralPrimitive, MethodCall, NonNullAssert, PrefixNot, PropertyRead, PropertyWrite, Quote, SafeMethodCall, SafePropertyRead, visitAstChildren} from '@angular/compiler';
+import {AST, AstVisitor, Binary, BindingPipe, Chain, Conditional, FunctionCall, ImplicitReceiver, Interpolation, KeyedRead, KeyedWrite, LiteralArray, LiteralMap, LiteralPrimitive, MethodCall, NonNullAssert, PrefixNot, PropertyRead, PropertyWrite, Quote, SafeMethodCall, SafePropertyRead} from '@angular/compiler';
 import * as ts from 'typescript';
 
 import {BuiltinType, Signature, Symbol, SymbolQuery, SymbolTable} from './symbols';
@@ -178,14 +178,18 @@ export class AstType implements AstVisitor {
 
   visitChain(ast: Chain) {
     // If we are producing diagnostics, visit the children
-    visitAstChildren(ast, this);
+    for (const expr of ast.expressions) {
+      expr.visit(this);
+    }
     // The type of a chain is always undefined.
     return this.query.getBuiltinType(BuiltinType.Undefined);
   }
 
   visitConditional(ast: Conditional) {
     // The type of a conditional is the union of the true and false conditions.
-    visitAstChildren(ast, this);
+    ast.condition.visit(this);
+    ast.trueExp.visit(this);
+    ast.falseExp.visit(this);
     return this.query.getTypeUnion(this.getType(ast.trueExp), this.getType(ast.falseExp));
   }
 
@@ -235,7 +239,9 @@ export class AstType implements AstVisitor {
 
   visitInterpolation(ast: Interpolation): Symbol {
     // If we are producing diagnostics, visit the children.
-    visitAstChildren(ast, this);
+    for (const expr of ast.expressions) {
+      expr.visit(this);
+    }
     return this.undefinedType;
   }
 
@@ -260,7 +266,9 @@ export class AstType implements AstVisitor {
 
   visitLiteralMap(ast: LiteralMap): Symbol {
     // If we are producing diagnostics, visit the children
-    visitAstChildren(ast, this);
+    for (const value of ast.values) {
+      value.visit(this);
+    }
     // TODO: Return a composite type.
     return this.anyType;
   }
@@ -312,7 +320,7 @@ export class AstType implements AstVisitor {
 
   visitPrefixNot(ast: PrefixNot) {
     // If we are producing diagnostics, visit the children
-    visitAstChildren(ast, this);
+    ast.expression.visit(this);
     // The type of a prefix ! is always boolean.
     return this.query.getBuiltinType(BuiltinType.Boolean);
   }

packages/language-service/src/expressions.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {AST, ASTWithSource, AstPath as AstPathBase, NullAstVisitor, visitAstChildren} from '@angular/compiler';
+import {AST, ASTWithSource, AstPath as AstPathBase, RecursiveAstVisitor} from '@angular/compiler';
 import {AstType} from './expression_type';
 
 import {BuiltinType, Span, Symbol, SymbolQuery, SymbolTable} from './types';
@@ -16,12 +16,12 @@ type AstPath = AstPathBase<AST>;
 
 function findAstAt(ast: AST, position: number, excludeEmpty: boolean = false): AstPath {
   const path: AST[] = [];
-  const visitor = new class extends NullAstVisitor {
+  const visitor = new class extends RecursiveAstVisitor {
     visit(ast: AST) {
       if ((!excludeEmpty || ast.sourceSpan.start < ast.sourceSpan.end) &&
           inSpan(position, ast.sourceSpan)) {
         path.push(ast);
-        visitAstChildren(ast, this);
+        ast.visit(this);
       }
     }
   };