Don't add excess properties to type nodes in typeToTypeNode

src/compiler/binder.ts

@@ -2348,8 +2348,8 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
         const saveCurrentFlow = currentFlow;
         for (const typeAlias of delayedTypeAliases) {
             const host = typeAlias.parent.parent;
-            container = findAncestor(host.parent, n => !!(getContainerFlags(n) & ContainerFlags.IsContainer)) as IsContainer | undefined || file;
-            blockScopeContainer = getEnclosingBlockScopeContainer(host) as IsBlockScopedContainer | undefined || file;
+            container = (findAncestor(host.parent, n => !!(getContainerFlags(n) & ContainerFlags.IsContainer)) as IsContainer | undefined) || file;
+            blockScopeContainer = (getEnclosingBlockScopeContainer(host) as IsBlockScopedContainer | undefined) || file;
             currentFlow = initFlowNode({ flags: FlowFlags.Start });
             parent = typeAlias;
             bind(typeAlias.typeExpression);

src/compiler/checker.ts

@@ -57,8 +57,8 @@ import {
     canHaveJSDoc,
     canHaveLocals,
     canHaveModifiers,
-    canUsePropertyAccess,
     canHaveSymbol,
+    canUsePropertyAccess,
     cartesianProduct,
     CaseBlock,
     CaseClause,
@@ -6597,7 +6597,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         context.truncating = true;
                     }
                     context.approximateLength += cachedResult.addedLength;
-                    return deepCloneOrReuseNode(cachedResult) as TypeNode as T;
+                    return deepCloneOrReuseNode(cachedResult.node) as T;
                 }
 
                 let depth: number | undefined;
@@ -6613,11 +6613,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 const result = transform(type);
                 const addedLength = context.approximateLength - startLength;
                 if (!context.reportedDiagnostic && !context.encounteredError) {
-                    if (context.truncating) {
-                        (result as any).truncating = true;
-                    }
-                    (result as any).addedLength = addedLength;
-                    links?.serializedTypes?.set(key, result as TypeNode as TypeNode & {truncating?: boolean, addedLength: number});
+                    links?.serializedTypes?.set(key, { node: result, truncating: context.truncating, addedLength });
                 }
                 context.visitedTypes.delete(typeId);
                 if (id) {

src/compiler/factory/nodeFactory.ts

@@ -2319,9 +2319,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
         const node = createBaseDeclaration<TypeLiteralNode>(SyntaxKind.TypeLiteral);
         node.members = createNodeArray(members);
         node.transformFlags = TransformFlags.ContainsTypeScript;
-
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -2749,8 +2746,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
 
         node.jsDoc = undefined; // initialized by parser (JsDocContainer)
         node.jsDocCache = undefined; // initialized by parser (JsDocContainer)
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -3585,8 +3580,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
 
         node.jsDoc = undefined; // initialized by parser (JsDocContainer)
         node.jsDocCache = undefined; // initialized by parser (JsDocContainer)
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -4373,8 +4366,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
 
         node.jsDoc = undefined; // initialized by parser (JsDocContainer)
         node.jsDocCache = undefined; // initialized by parser (JsDocContainer)
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -4415,8 +4406,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
         node.illegalDecorators = undefined; // initialized by parser for grammar errors
         node.jsDoc = undefined; // initialized by parser (JsDocContainer)
         node.jsDocCache = undefined; // initialized by parser (JsDocContainer)
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -4510,8 +4499,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
         node.illegalDecorators = undefined; // initialized by parser for grammar errors
         node.jsDoc = undefined; // initialized by parser (JsDocContainer)
         node.jsDocCache = undefined; // initialized by parser (JsDocContainer)
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -5142,8 +5129,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
         const node = createBaseDeclaration<JSDocTypeLiteral>(SyntaxKind.JSDocTypeLiteral);
         node.jsDocPropertyTags = asNodeArray(propertyTags);
         node.isArrayType = isArrayType;
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -5738,8 +5723,6 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
         node.transformFlags |=
             propagateChildrenFlags(node.properties) |
             TransformFlags.ContainsJsx;
-        // node.locals = undefined; // initialized by binder (LocalsContainer)
-        // node.nextContainer = undefined; // initialized by binder (LocalsContainer)
         return node;
     }
 
@@ -6097,7 +6080,8 @@ export function createNodeFactory(flags: NodeFactoryFlags, baseFactory: BaseNode
     }
 
     function cloneSourceFileWorker(source: SourceFile) {
-        // TODO: explicit property assignments instead of for..in
+        // TODO: This mechanism for cloning results in megamorphic property reads and writes. In future perf-related
+        //       work, we should consider switching explicit property assignments instead of using `for..in`.
         const node = baseFactory.createBaseSourceFileNode(SyntaxKind.SourceFile) as Mutable<SourceFile>;
         node.flags |= source.flags & ~NodeFlags.Synthesized;
         for (const p in source) {

src/compiler/parser.ts

@@ -1580,7 +1580,7 @@ namespace Parser {
         // Prime the scanner.
         nextToken();
         const pos = getNodePos();
-        let statements, endOfFileToken: EndOfFileToken;
+        let statements, endOfFileToken;
         if (token() === SyntaxKind.EndOfFileToken) {
             statements = createNodeArray([], pos, pos);
             endOfFileToken = parseTokenNode<EndOfFileToken>();

src/compiler/types.ts

@@ -5953,12 +5953,19 @@ export interface NodeLinks {
     isExhaustive?: boolean | 0;         // Is node an exhaustive switch statement (0 indicates in-process resolution)
     skipDirectInference?: true;         // Flag set by the API `getContextualType` call on a node when `Completions` is passed to force the checker to skip making inferences to a node's type
     declarationRequiresScopeChange?: boolean; // Set by `useOuterVariableScopeInParameter` in checker when downlevel emit would change the name resolution scope inside of a parameter.
-    serializedTypes?: Map<string, TypeNode & {truncating?: boolean, addedLength: number}>; // Collection of types serialized at this location
+    serializedTypes?: Map<string, SerializedTypeEntry>; // Collection of types serialized at this location
 
     contextualType?: Type;              // Used to temporarily assign a contextual type during overload resolution
     inferenceContext?: InferenceContext; // Inference context for contextual type
 }
 
+/** @internal */
+export interface SerializedTypeEntry {
+    node: TypeNode;
+    truncating?: boolean;
+    addedLength: number;
+}
+
 export const enum TypeFlags {
     Any             = 1 << 0,
     Unknown         = 1 << 1,