fix(parser): parse sloppy let expressions and labels (#28438)

Handle sloppy-mode `let` expressions, comma expressions, and labels
through the ordinary statement parser while preserving
lexical-declaration restrictions.

Validated with scoped Rust checks, full conformance, parser bindings,
allocation tracking.

AI-assisted (Codex).

crates/oxc_parser/src/js/declaration.rs

@@ -9,35 +9,22 @@ impl<'a, C: Config> ParserImpl<'a, C> {
     pub(crate) fn parse_let(&mut self, stmt_ctx: StatementContext) -> Statement<'a> {
         let start = self.cur_start();
 
-        let peeked = self.lexer.peek_token().kind();
+        let next = self.lexer.peek_token();
+        let peeked = next.kind();
 
-        // Fast path: avoid rewind.
-        if !stmt_ctx.is_single_statement() && peeked.is_after_let() {
+        // `let [` cannot start an expression statement, even in a single-statement
+        // position. Otherwise, a declaration needs a binding after `let`.
+        if peeked == Kind::LBrack
+            || (!stmt_ctx.is_single_statement()
+                && (peeked == Kind::LCurly
+                    || peeked.is_binding_identifier()
+                    || (!next.is_on_new_line() && peeked.is_after_let())))
+        {
             self.bump_any(); // bump `let`
             return self.parse_variable_statement(start, VariableDeclarationKind::Let, stmt_ctx);
         }
 
-        // let = foo, let instanceof x, let + 1
-        if peeked.is_assignment_operator() || peeked.is_binary_operator() {
-            let expr = self.parse_assignment_expression_or_higher();
-            self.parse_expression_statement(start, expr)
-        // let.a = 1, let?.a = 1, let()[a] = 1, let++, let--
-        } else if matches!(
-            peeked,
-            Kind::Dot | Kind::QuestionDot | Kind::LParen | Kind::Plus2 | Kind::Minus2
-        ) {
-            let expr = self.parse_expr();
-            self.parse_expression_statement(start, expr)
-        // single statement let declaration: while (0) let
-        } else if (stmt_ctx.is_single_statement() && peeked != Kind::LBrack)
-            || peeked == Kind::Semicolon
-        {
-            let expr = self.parse_identifier_expression();
-            self.parse_expression_statement(start, expr)
-        } else {
-            self.bump_any();
-            self.parse_variable_statement(start, VariableDeclarationKind::Let, stmt_ctx)
-        }
+        self.parse_expression_or_labeled_statement()
     }
 
     pub(crate) fn is_using_statement(&mut self) -> bool {

crates/oxc_parser/src/js/statement.rs

@@ -263,7 +263,7 @@ impl<'a, C: Config> ParserImpl<'a, C> {
         }
     }
 
-    fn parse_expression_or_labeled_statement(&mut self) -> Statement<'a> {
+    pub(super) fn parse_expression_or_labeled_statement(&mut self) -> Statement<'a> {
         let start = self.cur_start();
         let expr = self.parse_expr();
         if let Expression::Identifier(ident) = &expr {

tasks/coverage/misc/pass/let-expression-statements.cjs

@@ -0,0 +1,8 @@
+let, other;
+let ? yes : no;
+let: { break let; }
+let
+42;
+let
+const value = 1;
+let

tasks/coverage/snapshots/codegen_misc.snap

@@ -1,3 +1,3 @@
 codegen_misc Summary:
-AST Parsed     : 128/128 (100.00%)
-Positive Passed: 128/128 (100.00%)
+AST Parsed     : 129/129 (100.00%)
+Positive Passed: 129/129 (100.00%)

tasks/coverage/snapshots/formatter_misc.snap

@@ -1,3 +1,3 @@
 formatter_misc Summary:
-AST Parsed     : 128/128 (100.00%)
-Positive Passed: 128/128 (100.00%)
+AST Parsed     : 129/129 (100.00%)
+Positive Passed: 129/129 (100.00%)

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 391/391 (100.00%)
-Positive Passed: 391/391 (100.00%)
+AST Parsed     : 392/392 (100.00%)
+Positive Passed: 392/392 (100.00%)

tasks/coverage/snapshots/parser_babel.snap

@@ -1,8 +1,8 @@
 commit: b835f0c6
 
 parser_babel Summary:
-AST Parsed     : 2292/2304 (99.48%)
-Positive Passed: 2271/2304 (98.57%)
+AST Parsed     : 2304/2304 (100.00%)
+Positive Passed: 2283/2304 (99.09%)
 Negative Passed: 1828/1843 (99.19%)
 Expect Syntax Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/typescript/cast/unparenthesized-assert-and-assign/input.ts
 
@@ -34,15 +34,6 @@ Expect Syntax Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/ty
 
 Expect Syntax Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/typescript/module-namespace/module-identifier-invalid/input.ts
 
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-close-let-keyword/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-close-let-keyword/input.js:3:1]
- 2 │ */ --> c
- 3 │ if (x) {}
-   · ──
-   ╰────
-
 Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-open-export-default-from/input.js
 
   × Cannot use export statement outside a module
@@ -52,15 +43,6 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-
  2 │ "x";
    ╰────
 
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-open-let-keyword/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-open-let-keyword/input.js:2:1]
- 1 │ let <!-- c
- 2 │ if (x) {}
-   · ──
-   ╰────
-
 Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/3.3-function-in-if-body-lexical-redeclaration/input.js
 
   × Identifier `f` has already been declared
@@ -90,87 +72,6 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015
  2 │ if (x) {}
    ╰────
 
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword/input.js:2:1]
- 1 │ let
- 2 │ if (x) {}
-   · ──
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-cr/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-cr/input.js:2:1]
- 1 │ let
if (x) {}
-   ·     ──
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-declaration/input.js
-
-  × Identifier expected. 'function' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-declaration/input.js:2:1]
- 1 │ let
- 2 │ function f() {}
-   · ────────
- 3 │ let
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-expression/input.js
-
-  × Identifier expected. 'this' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-expression/input.js:2:1]
- 1 │ let // comment
- 2 │ this.x = 1;
-   · ────
- 3 │ let /*
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-line-separator/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-line-separator/input.js:1:7]
- 1 │ let
if (x) {}
-   ·     ──
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-nested/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-nested/input.js:3:3]
- 2 │   let
- 3 │   if (x) {}
-   ·   ──
- 4 │ }
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-paragraph-separator/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-paragraph-separator/input.js:1:7]
- 1 │ let
if (x) {}
-   ·     ──
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-void/input.js
-
-  × Identifier expected. 'void' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-void/input.js:2:1]
- 1 │ let
- 2 │ void 0;
-   · ────
-   ╰────
-
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-multiline-comment-and-keyword/input.js
-
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-multiline-comment-and-keyword/input.js:2:4]
- 1 │ let /*
- 2 │ */ if (x) {}
-   ·    ──
-   ╰────
-
 Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/estree/class-private-method/typescript-invalid-abstract/input.ts
 
   × TS(18019): 'abstract' modifier cannot be used with a private identifier.
@@ -424,15 +325,6 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
  6 │ }
    ╰────
 
-Fatal error: tasks/coverage/babel/packages/babel-parser/test/fixtures/typescript/enum/let-with-linebreak/input.js
-
-  × Identifier expected. 'enum' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/typescript/enum/let-with-linebreak/input.js:2:1]
- 1 │ let
- 2 │ enum E {}
-   · ────
-   ╰────
-
 Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typescript/function/declare-pattern-parameters/input.ts
 
   × TS(1016): A required parameter cannot follow an optional parameter.
@@ -3771,33 +3663,36 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
    ╰────
   help: Try inserting a semicolon here
 
-  × Identifier expected. 'enum' is a reserved word that cannot be used here.
+  × Unexpected token
    ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-enum/input.js:2:1]
  1 │ let
  2 │ enum;
    · ────
    ╰────
 
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
+  × Keywords cannot contain escape characters
    ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-escaped-keyword/input.js:2:1]
  1 │ let
  2 │ i\u0066 (x) {}
    · ───────
    ╰────
 
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-module/input.js:2:1]
+  × The keyword 'let' is reserved
+   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-module/input.js:1:1]
  1 │ let
+   · ───
  2 │ if (x) {}
-   · ──
    ╰────
+  note: Modules are always strict mode code
 
-  × Identifier expected. 'if' is a reserved word that cannot be used here.
-   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-strict/input.js:3:1]
+  × The keyword 'let' is reserved
+   ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-strict/input.js:2:1]
+ 1 │ "use strict";
  2 │ let
+   · ───
  3 │ if (x) {}
-   · ──
    ╰────
+  note: This identifier is reserved in strict mode code
 
   × Expected a semicolon or an implicit semicolon after a statement, but found none
    ╭─[babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-yield/input.js:3:8]

tasks/coverage/snapshots/parser_misc.snap

@@ -1,6 +1,6 @@
 parser_misc Summary:
-AST Parsed     : 128/128 (100.00%)
-Positive Passed: 128/128 (100.00%)
+AST Parsed     : 129/129 (100.00%)
+Positive Passed: 129/129 (100.00%)
 Negative Passed: 265/265 (100.00%)
 
   × TS(1275): 'accessor' modifier can only appear on a property declaration.

tasks/coverage/snapshots/parser_typescript.snap

@@ -6544,11 +6544,13 @@ Expect to Parse: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/
  1 │ const
    ╰────
 
-  × Unexpected token
-   ╭─[typescript/tsc/testdata/tests/cases/compiler/downlevelLetConst11.ts:2:4]
+  × The keyword 'let' is reserved
+   ╭─[typescript/tsc/testdata/tests/cases/compiler/downlevelLetConst11.ts:2:1]
  1 │ "use strict";
  2 │ let
+   · ───
    ╰────
+  note: This identifier is reserved in strict mode code
 
   × Missing initializer in const declaration
    ╭─[typescript/tsc/testdata/tests/cases/compiler/downlevelLetConst2.ts:1:7]
@@ -21757,11 +21759,13 @@ Expect to Parse: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/
    ·          ────
    ╰────
 
-  × Unexpected token
-   ╭─[typescript/tsc/testdata/tests/cases/conformance/es6/variableDeclarations/VariableDeclaration11_es6.ts:2:4]
+  × The keyword 'let' is reserved
+   ╭─[typescript/tsc/testdata/tests/cases/conformance/es6/variableDeclarations/VariableDeclaration11_es6.ts:2:1]
  1 │ "use strict";
  2 │ let
+   · ───
    ╰────
+  note: This identifier is reserved in strict mode code
 
   × Unexpected token
    ╭─[typescript/tsc/testdata/tests/cases/conformance/es6/variableDeclarations/VariableDeclaration13_es6.ts:4:5]

tasks/coverage/snapshots/semantic_babel.snap

@@ -2,16 +2,10 @@ commit: b835f0c6
 
 semantic_babel Summary:
 AST Parsed     : 2304/2304 (100.00%)
-Positive Passed: 2197/2304 (95.36%)
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-close-let-keyword/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
+Positive Passed: 2209/2304 (95.88%)
 semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-open-export-default-from/input.js
 Cannot use export statement outside a module
 
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/1.1-html-comments-open-let-keyword/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
 semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/annex-b/enabled/3.3-function-in-if-body-lexical-redeclaration/input.js
 Identifier `f` has already been declared
 Identifier `g` has already been declared
@@ -29,33 +23,6 @@ rebuilt        : SymbolId(3): SymbolFlags(BlockScopedVariable)
 semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/export-default-let-with-linebreak-and-keyword/input.js
 Cannot use export statement outside a module
 
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-cr/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-declaration/input.js
-Identifier expected. 'function' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-expression/input.js
-Identifier expected. 'this' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-line-separator/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-nested/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-keyword-paragraph-separator/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-linebreak-and-void/input.js
-Identifier expected. 'void' is a reserved word that cannot be used here.
-
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2015/let/let-with-multiline-comment-and-keyword/input.js
-Identifier expected. 'if' is a reserved word that cannot be used here.
-
 semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2022/class-private-properties/await-in-private-property-in-params-of-async-arrow/input.js
 Bindings mismatch:
 after transform: ScopeId(0): ["_asyncToGenerator", "_classPrivateFieldInitSpec"]
@@ -249,9 +216,6 @@ Bindings mismatch:
 after transform: ScopeId(0): ["E"]
 rebuilt        : ScopeId(0): []
 
-semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/typescript/enum/let-with-linebreak/input.js
-Identifier expected. 'enum' is a reserved word that cannot be used here.
-
 semantic Error: tasks/coverage/babel/packages/babel-parser/test/fixtures/typescript/export/declare/input.ts
 Bindings mismatch:
 after transform: ScopeId(0): ["C", "N", "_bar", "bar", "f", "foo", "x"]

tasks/coverage/snapshots/semantic_misc.snap

@@ -1,6 +1,6 @@
 semantic_misc Summary:
-AST Parsed     : 128/128 (100.00%)
-Positive Passed: 122/128 (95.31%)
+AST Parsed     : 129/129 (100.00%)
+Positive Passed: 123/129 (95.35%)
 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     : 128/128 (100.00%)
-Positive Passed: 128/128 (100.00%)
+AST Parsed     : 129/129 (100.00%)
+Positive Passed: 129/129 (100.00%)