fix(parser): handle escaped let in for loops (#26583)

Escaped `let` is an identifier in non-strict code, but `for` headers treated it as the declaration keyword or applied the restriction on expressions beginning with unescaped `let` in `for…of`. Check for escapes in both paths so valid loops such as `for (l\u0065t[0];;);` and `for (l\u0065t of []);` parse successfully.

Adds coverage for classic, `for…in`, `for…of`, and `for await…of` loops, while retaining rejection of escaped declaration keywords.

crates/oxc_parser/src/js/statement.rs

@@ -403,7 +403,7 @@ impl<'a, C: Config> ParserImpl<'a, C> {
                     r#await,
                 );
             }
-            Kind::Let => {
+            Kind::Let if !self.cur_token().escaped() => {
                 // `for (let`
                 let decl_start = self.cur_start();
                 // disallow `for (let in ...`
@@ -460,7 +460,7 @@ impl<'a, C: Config> ParserImpl<'a, C> {
             return self.parse_for_loop(for_start, parenthesis_opening_span, None, r#await);
         }
 
-        let is_let = self.at(Kind::Let);
+        let is_let = self.at(Kind::Let) && !self.cur_token().escaped();
         // `async` is allowed as `for (async of ...)` if `async` is escaped
         let is_async = self.at(Kind::Async) && !self.cur_token().escaped();
         let expr_start = self.cur_start();

tasks/coverage/misc/fail/for-escaped-let-declaration.cjs

@@ -0,0 +1 @@
+for (l\u0065t x = 0;;);

tasks/coverage/misc/fail/for-in-escaped-let-declaration.cjs

@@ -0,0 +1 @@
+for (l\u0065t x in {});

tasks/coverage/misc/fail/for-of-escaped-let-declaration.cjs

@@ -0,0 +1 @@
+for (l\u0065t x of []);

tasks/coverage/misc/pass/for-escaped-let.cjs

@@ -0,0 +1,9 @@
+for (l\u0065t[0];;);
+for (l\u0065t[0] in {});
+for (l\u0065t of []);
+for (l\u0065t[0] of []);
+for (l\u0065t.x of []);
+async function f() {
+  for await (l\u0065t of []);
+  for await (l\u0065t[0] of []);
+}

tasks/coverage/snapshots/codegen_misc.snap

@@ -1,3 +1,3 @@
 codegen_misc Summary:
-AST Parsed     : 107/107 (100.00%)
-Positive Passed: 107/107 (100.00%)
+AST Parsed     : 108/108 (100.00%)
+Positive Passed: 108/108 (100.00%)

tasks/coverage/snapshots/formatter_misc.snap

@@ -1,3 +1,3 @@
 formatter_misc Summary:
-AST Parsed     : 107/107 (100.00%)
-Positive Passed: 107/107 (100.00%)
+AST Parsed     : 108/108 (100.00%)
+Positive Passed: 108/108 (100.00%)

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 322/322 (100.00%)
-Positive Passed: 322/322 (100.00%)
+AST Parsed     : 326/326 (100.00%)
+Positive Passed: 326/326 (100.00%)

tasks/coverage/snapshots/parser_misc.snap

@@ -1,7 +1,7 @@
 parser_misc Summary:
-AST Parsed     : 107/107 (100.00%)
-Positive Passed: 107/107 (100.00%)
-Negative Passed: 215/215 (100.00%)
+AST Parsed     : 108/108 (100.00%)
+Positive Passed: 108/108 (100.00%)
+Negative Passed: 218/218 (100.00%)
 
   × Cannot assign to 'arguments' in strict mode
    ╭─[misc/fail/arguments-eval-ambient.ts:2:13]
@@ -705,6 +705,27 @@ Negative Passed: 215/215 (100.00%)
  3 │ }
    ╰────
 
+  × Expected `;` but found `Identifier`
+   ╭─[misc/fail/for-escaped-let-declaration.cjs:1:15]
+ 1 │ for (l\u0065t x = 0;;);
+   ·               ┬
+   ·               ╰── `;` expected
+   ╰────
+
+  × Expected `;` but found `Identifier`
+   ╭─[misc/fail/for-in-escaped-let-declaration.cjs:1:15]
+ 1 │ for (l\u0065t x in {});
+   ·               ┬
+   ·               ╰── `;` expected
+   ╰────
+
+  × Expected `;` but found `Identifier`
+   ╭─[misc/fail/for-of-escaped-let-declaration.cjs:1:15]
+ 1 │ for (l\u0065t x of []);
+   ·               ┬
+   ·               ╰── `;` expected
+   ╰────
+
   × The left-hand side of a for...in statement cannot be an using declaration.
    ╭─[misc/fail/for-using-in-initializer.js:1:6]
  1 │ for (using x = a in b;;);

tasks/coverage/snapshots/semantic_misc.snap

@@ -1,6 +1,6 @@
 semantic_misc Summary:
-AST Parsed     : 107/107 (100.00%)
-Positive Passed: 103/107 (96.26%)
+AST Parsed     : 108/108 (100.00%)
+Positive Passed: 104/108 (96.30%)
 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     : 107/107 (100.00%)
-Positive Passed: 107/107 (100.00%)
+AST Parsed     : 108/108 (100.00%)
+Positive Passed: 108/108 (100.00%)