Add early errors for `LexicalDeclaration` (#3207)

boa_parser/src/parser/statement/declaration/lexical.rs

@@ -21,6 +21,7 @@ use ast::operations::bound_names;
 use boa_ast::{self as ast, declaration::Variable, pattern::Pattern, Keyword, Punctuator};
 use boa_interner::{Interner, Sym};
 use boa_profiler::Profiler;
+use rustc_hash::FxHashSet;
 use std::io::Read;
 
 /// Parses a lexical declaration.
@@ -99,6 +100,25 @@ where
             cursor.expect_semicolon("lexical declaration", interner)?;
         }
 
+        // It is a Syntax Error if the BoundNames of BindingList contains "let".
+        // It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.
+        let bound_names = bound_names(&lexical_declaration);
+        let mut names = FxHashSet::default();
+        for name in bound_names {
+            if name.sym() == Sym::LET {
+                return Err(Error::general(
+                    "'let' is disallowed as a lexically bound name",
+                    tok.span().start(),
+                ));
+            }
+            if !names.insert(name) {
+                return Err(Error::general(
+                    "lexical name declared multiple times",
+                    tok.span().start(),
+                ));
+            }
+        }
+
         Ok(lexical_declaration)
     }
 }

boa_parser/src/parser/statement/declaration/tests.rs

@@ -349,3 +349,14 @@ fn multiple_const_declaration() {
         interner,
     );
 }
+
+/// Checks `LexicalDeclaration` early errors.
+#[test]
+fn lexical_declaration_early_errors() {
+    check_invalid_script("let let = 0");
+    check_invalid_script("let a = 0, a = 0");
+    check_invalid_script("const a = 0, a = 0");
+    check_invalid_script("for (let let = 0; ; ) {}");
+    check_invalid_script("for (let a = 0, a = 0; ; ) {}");
+    check_invalid_script("for (const a = 0, a = 0; ; ) {}");
+}