fix(parser): disambiguate await using in for initializers (#26527)

Allow classic `for` initializers such as `for (await using;;)` when `using` is an ordinary identifier. Reuse the statement parser’s `await using` lookahead so resource declarations require a following binding identifier, preserving the existing line-break restrictions.

crates/oxc_parser/src/js/expression.rs

@@ -1664,7 +1664,7 @@ impl<'a, C: Config> ParserImpl<'a, C> {
     /// Returns `true` when await is definitely an await expression (not ambiguous).
     ///
     /// Unambiguous cases (returns `true`):
-    /// - Next token is identifier, keyword (except `of`), or literal on same line
+    /// - Next token is identifier, keyword, or literal on same line, except the cases below
     ///
     /// Ambiguous cases (returns `false`):
     /// - Line break after `await` (could be ASI)
@@ -1672,6 +1672,7 @@ impl<'a, C: Config> ParserImpl<'a, C> {
     /// - Next token is `(` `[` (could be call/member or grouping/array)
     /// - Next token is template literal
     /// - Next token is `of` (for-await-of ambiguity: `for (await of [])`)
+    /// - Next tokens form an `await using` declaration
     /// - Next token is `/` (division or regex literal)
     /// - Next token cannot start an expression (`)`, `}`, `;`, etc.)
     fn is_unambiguous_await(&mut self) -> bool {
@@ -1685,8 +1686,12 @@ impl<'a, C: Config> ParserImpl<'a, C> {
         let kind = token.kind();
 
         // Special case: `await of` is ambiguous (for-await-of loop)
-        // Special case: `await using` should be handled as a declaration, not `await (using)`
-        if matches!(kind, Kind::Of | Kind::Using) {
+        if kind == Kind::Of {
+            return false;
+        }
+
+        // Only reserve `await using` for a declaration when a binding identifier follows.
+        if kind == Kind::Using && self.is_using_statement() {
             return false;
         }
 

crates/oxc_parser/src/js/statement.rs

@@ -422,17 +422,8 @@ impl<'a, C: Config> ParserImpl<'a, C> {
             _ => {}
         }
 
-        // [+Using, +Await] await [no LineTerminator here] using [no LineTerminator here]
-        if self.at(Kind::Await)
-            && self.lookahead(|p| {
-                p.bump_any();
-                if !p.at(Kind::Using) || p.cur_token().is_on_new_line() {
-                    return false;
-                }
-                p.bump_any();
-                !p.cur_token().is_on_new_line()
-            })
-        {
+        // [+Using, +Await] await [no LineTerminator here] using [no LineTerminator here] ForBinding[?Yield, ?Await, ~Pattern]
+        if self.at(Kind::Await) && self.is_using_statement() {
             return self.parse_using_declaration_for_statement(
                 for_start,
                 parenthesis_opening_span,

tasks/coverage/misc/pass/for-await-using-expression-before-export.js

@@ -0,0 +1,2 @@
+for (await using;;);
+export {};

tasks/coverage/misc/pass/for-await-using-expression-top-level.js

@@ -0,0 +1 @@
+for (await using;;);

tasks/coverage/misc/pass/for-await-using-expression.js

@@ -0,0 +1,11 @@
+async function f(using) {
+  for (await using;;) { break; }
+  for (await using + 1;;) { break; }
+  for (await using.foo;;) { break; }
+  for (await using(1);;) { break; }
+  for (await (using);;) { break; }
+  for (await using
+    ;;) { break; }
+  for (await
+    using;;) { break; }
+}

tasks/coverage/snapshots/codegen_misc.snap

@@ -1,3 +1,3 @@
 codegen_misc Summary:
-AST Parsed     : 99/99 (100.00%)
-Positive Passed: 99/99 (100.00%)
+AST Parsed     : 103/103 (100.00%)
+Positive Passed: 103/103 (100.00%)

tasks/coverage/snapshots/formatter_misc.snap

@@ -1,3 +1,3 @@
 formatter_misc Summary:
-AST Parsed     : 99/99 (100.00%)
-Positive Passed: 99/99 (100.00%)
+AST Parsed     : 103/103 (100.00%)
+Positive Passed: 103/103 (100.00%)

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 302/302 (100.00%)
-Positive Passed: 302/302 (100.00%)
+AST Parsed     : 305/305 (100.00%)
+Positive Passed: 305/305 (100.00%)

tasks/coverage/snapshots/parser_misc.snap

@@ -1,7 +1,7 @@
 parser_misc Summary:
-AST Parsed     : 99/99 (100.00%)
-Positive Passed: 99/99 (100.00%)
-Negative Passed: 203/203 (100.00%)
+AST Parsed     : 103/103 (100.00%)
+Positive Passed: 103/103 (100.00%)
+Negative Passed: 202/202 (100.00%)
 
   × Cannot assign to 'arguments' in strict mode
    ╭─[misc/fail/arguments-eval-ambient.ts:2:13]
@@ -1071,15 +1071,6 @@ Negative Passed: 203/203 (100.00%)
    ·                      ╰── `b` has already been declared here
    ╰────
 
-  × Expected a semicolon or an implicit semicolon after a statement, but found none
-   ╭─[misc/fail/oxc-10503.ts:4:6]
- 3 │ 
- 4 │ await using
-   ·      ▲
- 5 │ foo = bar();
-   ╰────
-  help: Try inserting a semicolon here
-
   × Unexpected token
    ╭─[misc/fail/oxc-10638.js:2:1]
  1 │ for(

tasks/coverage/snapshots/parser_typescript.snap

@@ -28867,14 +28867,13 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
     ╰────
   help: Wrap this declaration in a block statement
 
-  × Expected a semicolon or an implicit semicolon after a statement, but found none
-   ╭─[typescript/tests/cases/conformance/statements/VariableStatements/usingDeclarations/awaitUsingDeclarations.4.ts:2:10]
+  × Cannot assign to this expression
+   ╭─[typescript/tests/cases/conformance/statements/VariableStatements/usingDeclarations/awaitUsingDeclarations.4.ts:2:5]
  1 │ {
  2 │     await using [a] = null;
-   ·          ▲
+   ·     ───────────────
  3 │ }
    ╰────
-  help: Try inserting a semicolon here
 
   × Using declarations may not have binding patterns.
    ╭─[typescript/tests/cases/conformance/statements/VariableStatements/usingDeclarations/awaitUsingDeclarations.5.ts:3:17]
@@ -28885,10 +28884,10 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
    ╰────
 
   × Expected a semicolon or an implicit semicolon after a statement, but found none
-   ╭─[typescript/tests/cases/conformance/statements/VariableStatements/usingDeclarations/awaitUsingDeclarations.6.ts:2:10]
+   ╭─[typescript/tests/cases/conformance/statements/VariableStatements/usingDeclarations/awaitUsingDeclarations.6.ts:2:16]
  1 │ {
  2 │     await using {a} = null;
-   ·          ▲
+   ·                ▲
  3 │ }
    ╰────
   help: Try inserting a semicolon here
@@ -28928,11 +28927,12 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
    ╰────
   help: Did you mean to use a for...of statement?
 
-  × Using declarations may not have binding patterns.
+  × Expected `;` but found `{`
    ╭─[typescript/tests/cases/conformance/statements/VariableStatements/usingDeclarations/awaitUsingDeclarationsInForOf.3.ts:2:22]
  1 │ async function main() {
  2 │     for (await using {} of []) {
-   ·                      ──
+   ·                      ┬
+   ·                      ╰── `;` expected
  3 │     }
    ╰────
 

tasks/coverage/snapshots/semantic_misc.snap

@@ -1,6 +1,6 @@
 semantic_misc Summary:
-AST Parsed     : 99/99 (100.00%)
-Positive Passed: 95/99 (95.96%)
+AST Parsed     : 103/103 (100.00%)
+Positive Passed: 99/103 (96.12%)
 semantic Error: tasks/coverage/misc/pass/declare-let-private.ts
 Bindings mismatch:
 after transform: ScopeId(0): ["private"]

tasks/coverage/snapshots/transformer_misc.snap

@@ -1,3 +1,3 @@
 transformer_misc Summary:
-AST Parsed     : 99/99 (100.00%)
-Positive Passed: 99/99 (100.00%)
+AST Parsed     : 103/103 (100.00%)
+Positive Passed: 103/103 (100.00%)