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Fix using transform variable shadowing in for-of loops #61934

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Fix using transform variable shadowing in for-of loops #61934

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34e17b3
Initial plan for issue
Copilot Jun 24, 2025
237579e
Implement variable renaming for using declaration shadowing in for-of…
Copilot Jun 24, 2025
6fead46
Add test baselines for using declaration shadowing fix
Copilot Jun 24, 2025
2738f69
Add importHelpers directive and fix line endings
Copilot Jun 24, 2025
8f3df02
Implement block scoping approach for using declaration shadowing in f…
Copilot Jun 25, 2025
0b3bf33
Fix empty block generation in using declaration for-of loops and remo…
Copilot Jun 25, 2025
957cf79
Fix empty statement handling in using declaration for-of loops
Copilot Jun 25, 2025
88c9bfa
Fix trailing whitespace in esnext.ts
Copilot Jun 25, 2025
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231 changes: 119 additions & 112 deletions src/compiler/transformers/esnext.ts
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Original file line number Diff line number Diff line change
@@ -1,67 +1,67 @@
import {
addEmitHelpers,
addRange,
append,
arrayFrom,
BindingElement,
Block,
Bundle,
CaseOrDefaultClause,
chainBundle,
ClassDeclaration,
Debug,
EmitFlags,
ExportAssignment,
ExportSpecifier,
Expression,
firstOrUndefined,
ForOfStatement,
ForStatement,
GeneratedIdentifierFlags,
getEmitFlags,
hasSyntacticModifier,
Identifier,
IdentifierNameMap,
isArray,
isBindingPattern,
isBlock,
isCaseClause,
isCustomPrologue,
isExpression,
isGeneratedIdentifier,
isIdentifier,
isLocalName,
isNamedEvaluation,
isOmittedExpression,
isPrologueDirective,
isSourceFile,
isStatement,
isVariableDeclarationList,
isVariableStatement,
ModifierFlags,
Node,
NodeFlags,
setCommentRange,
setEmitFlags,
setOriginalNode,
setSourceMapRange,
setTextRange,
skipOuterExpressions,
SourceFile,
Statement,
SwitchStatement,
SyntaxKind,
TransformationContext,
TransformFlags,
transformNamedEvaluation,
VariableDeclaration,
VariableDeclarationList,
VariableStatement,
visitArray,
visitEachChild,
visitNode,
visitNodes,
VisitResult,
import {
addEmitHelpers,
addRange,
append,
arrayFrom,
BindingElement,
Block,
Bundle,
CaseOrDefaultClause,
chainBundle,
ClassDeclaration,
Debug,
EmitFlags,
ExportAssignment,
ExportSpecifier,
Expression,
firstOrUndefined,
ForOfStatement,
ForStatement,
GeneratedIdentifierFlags,
getEmitFlags,
hasSyntacticModifier,
Identifier,
IdentifierNameMap,
isArray,
isBindingPattern,
isBlock,
isCaseClause,
isCustomPrologue,
isExpression,
isGeneratedIdentifier,
isIdentifier,
isLocalName,
isNamedEvaluation,
isOmittedExpression,
isPrologueDirective,
isSourceFile,
isStatement,
isVariableDeclarationList,
isVariableStatement,
ModifierFlags,
Node,
NodeFlags,
setCommentRange,
setEmitFlags,
setOriginalNode,
setSourceMapRange,
setTextRange,
skipOuterExpressions,
SourceFile,
Statement,
SwitchStatement,
SyntaxKind,
TransformationContext,
TransformFlags,
transformNamedEvaluation,
VariableDeclaration,
VariableDeclarationList,
VariableStatement,
visitArray,
visitEachChild,
visitNode,
visitNodes,
VisitResult,
} from "../_namespaces/ts.js";

const enum UsingKind {
Expand Down Expand Up @@ -289,54 +289,61 @@ export function transformESNext(context: TransformationContext): (x: SourceFile
);
}

return visitEachChild(node, visitor, context);
}

function visitForOfStatement(node: ForOfStatement) {
if (isUsingVariableDeclarationList(node.initializer)) {
// given:
//
// for (using x of y) { ... }
//
// produces a shallow transformation to:
//
// for (const x_1 of y) {
// using x = x;
// ...
// }
//
// before handing the shallow transformation back to the visitor for an in-depth transformation.
const forInitializer = node.initializer;
const forDecl = firstOrUndefined(forInitializer.declarations) || factory.createVariableDeclaration(factory.createTempVariable(/*recordTempVariable*/ undefined));

const isAwaitUsing = getUsingKindOfVariableDeclarationList(forInitializer) === UsingKind.Async;
const temp = factory.getGeneratedNameForNode(forDecl.name);
const usingVar = factory.updateVariableDeclaration(forDecl, forDecl.name, /*exclamationToken*/ undefined, /*type*/ undefined, temp);
const usingVarList = factory.createVariableDeclarationList([usingVar], isAwaitUsing ? NodeFlags.AwaitUsing : NodeFlags.Using);
const usingVarStatement = factory.createVariableStatement(/*modifiers*/ undefined, usingVarList);
return visitNode(
factory.updateForOfStatement(
node,
node.awaitModifier,
factory.createVariableDeclarationList([
factory.createVariableDeclaration(temp),
], NodeFlags.Const),
node.expression,
isBlock(node.statement) ?
factory.updateBlock(node.statement, [
usingVarStatement,
...node.statement.statements,
]) :
factory.createBlock([
usingVarStatement,
node.statement,
], /*multiLine*/ true),
),
visitor,
isStatement,
);
}
return visitEachChild(node, visitor, context);
return visitEachChild(node, visitor, context);
}

/**
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Deleted the comment in commit 0b3bf33. The obsolete comment about collecting variable declarations has been removed from esnext.ts.

* Collects all variable declarations that shadow a given identifier name in a statement.
*/


function visitForOfStatement(node: ForOfStatement) {
if (isUsingVariableDeclarationList(node.initializer)) {
// given:
//
// for (using x of y) { ... }
//
// produces a shallow transformation to:
//
// for (const x_1 of y) {
// using x = x_1;
// { ... }
// }
//
// where the original loop body is wrapped in an additional block scope
// to handle shadowing variables naturally through block scoping.
const forInitializer = node.initializer;
const forDecl = firstOrUndefined(forInitializer.declarations) || factory.createVariableDeclaration(factory.createTempVariable(/*recordTempVariable*/ undefined));

const isAwaitUsing = getUsingKindOfVariableDeclarationList(forInitializer) === UsingKind.Async;
const temp = factory.getGeneratedNameForNode(forDecl.name);
const usingVar = factory.updateVariableDeclaration(forDecl, forDecl.name, /*exclamationToken*/ undefined, /*type*/ undefined, temp);
const usingVarList = factory.createVariableDeclarationList([usingVar], isAwaitUsing ? NodeFlags.AwaitUsing : NodeFlags.Using);
const usingVarStatement = factory.createVariableStatement(/*modifiers*/ undefined, usingVarList);

// Wrap the original loop body in an additional block scope to handle shadowing
const wrappedStatement = isBlock(node.statement) ?
node.statement :
factory.createBlock([node.statement], /*multiLine*/ true);

return visitNode(
factory.updateForOfStatement(
node,
node.awaitModifier,
factory.createVariableDeclarationList([
factory.createVariableDeclaration(temp),
], NodeFlags.Const),
node.expression,
factory.createBlock([
usingVarStatement,
wrappedStatement,
], /*multiLine*/ true),
),
visitor,
isStatement,
);
}
return visitEachChild(node, visitor, context);
}

function visitCaseOrDefaultClause(node: CaseOrDefaultClause, envBinding: Identifier) {
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForAwaitOf(target=es2015).js
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Original file line number Diff line number Diff line change
Expand Up @@ -88,6 +88,8 @@ function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, true);
{
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If the existing block is empty, we shouldn't bother adding a new, empty block.

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Fixed in commit 0b3bf33. The implementation now checks if the loop body is empty and skips adding the extra wrapping block in those cases. Empty using for-of loops no longer generate redundant empty blocks.

}
}
catch (e_2) {
env_1.error = e_2;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForAwaitOf(target=es2017).js
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Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,8 @@ async function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, true);
{
}
}
catch (e_2) {
env_1.error = e_2;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForAwaitOf(target=es2022).js
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,8 @@ async function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, true);
{
}
}
catch (e_1) {
env_1.error = e_1;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForAwaitOf(target=es5).js
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Original file line number Diff line number Diff line change
Expand Up @@ -128,6 +128,8 @@ function main() {
case 3:
_j.trys.push([3, 4, 5, 8]);
d1 = __addDisposableResource(env_1, d1_1, true);
{
}
return [3 /*break*/, 8];
case 4:
e_1 = _j.sent();
Expand Down
8 changes: 6 additions & 2 deletions tests/baselines/reference/awaitUsingDeclarationsInForAwaitOf.3(target=es5).js
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Original file line number Diff line number Diff line change
Expand Up @@ -118,7 +118,9 @@ try {
var env_1 = { stack: [], error: void 0, hasError: false };
try {
var _e = __addDisposableResource(env_1, _e_1, true);
;
{
;
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We should consider a block with just an empty statement the same as an empty block and avoid emitting the new block.

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Fixed in commit 957cf79. The implementation now considers blocks containing only empty statements as effectively empty and avoids generating the extra wrapping block in those cases.

}
}
catch (e_2) {
env_1.error = e_2;
Expand Down Expand Up @@ -159,7 +161,9 @@ export function test() {
case 3:
_f.trys.push([3, 4, 5, 8]);
_b = __addDisposableResource(env_2, _b_1, true);
;
{
;
}
return [3 /*break*/, 8];
case 4:
e_3 = _f.sent();
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForOf.1(target=es2015).js
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Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,8 @@ function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, true);
{
}
}
catch (e_1) {
env_1.error = e_1;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForOf.1(target=es2017).js
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,8 @@ async function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, true);
{
}
}
catch (e_1) {
env_1.error = e_1;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForOf.1(target=es2022).js
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,8 @@ async function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, true);
{
}
}
catch (e_1) {
env_1.error = e_1;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/awaitUsingDeclarationsInForOf.1(target=es5).js
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Original file line number Diff line number Diff line change
Expand Up @@ -117,6 +117,8 @@ function main() {
case 2:
_d.trys.push([2, 3, 4, 7]);
d1 = __addDisposableResource(env_1, d1_1, true);
{
}
return [3 /*break*/, 7];
case 3:
e_1 = _d.sent();
Expand Down
8 changes: 6 additions & 2 deletions tests/baselines/reference/awaitUsingDeclarationsInForOf.5(target=es5).js
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Original file line number Diff line number Diff line change
Expand Up @@ -107,7 +107,9 @@ for (var _i = 0, x_1 = x; _i < x_1.length; _i++) {
var env_1 = { stack: [], error: void 0, hasError: false };
try {
var _a = __addDisposableResource(env_1, _a_1, true);
;
{
;
}
}
catch (e_1) {
env_1.error = e_1;
Expand Down Expand Up @@ -135,7 +137,9 @@ export function test() {
case 2:
_b.trys.push([2, 3, 4, 7]);
_a = __addDisposableResource(env_2, _a_2, true);
;
{
;
}
return [3 /*break*/, 7];
case 3:
e_2 = _b.sent();
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/usingDeclarationsInForAwaitOf(target=es2015).js
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Original file line number Diff line number Diff line change
Expand Up @@ -87,6 +87,8 @@ function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, false);
{
}
}
catch (e_2) {
env_1.error = e_2;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/usingDeclarationsInForAwaitOf(target=es2017).js
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Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,8 @@ async function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, false);
{
}
}
catch (e_2) {
env_1.error = e_2;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/usingDeclarationsInForAwaitOf(target=es2022).js
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,8 @@ async function main() {
const env_1 = { stack: [], error: void 0, hasError: false };
try {
const d1 = __addDisposableResource(env_1, d1_1, false);
{
}
}
catch (e_1) {
env_1.error = e_1;
Expand Down
2 changes: 2 additions & 0 deletions tests/baselines/reference/usingDeclarationsInForAwaitOf(target=es5).js
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Original file line number Diff line number Diff line change
Expand Up @@ -123,6 +123,8 @@ function main() {
env_1 = { stack: [], error: void 0, hasError: false };
try {
d1 = __addDisposableResource(env_1, d1_1, false);
{
}
}
catch (e_2) {
env_1.error = e_2;
Expand Down
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