feat: 添加对const关键字的支持，允许定义不可变变量并增强相关错误处理

src/parser/parser.rs

@@ -31,7 +31,9 @@ impl Parser {
 
     fn declaration(&mut self) -> Result<Stmt, RuntimeError> {
         if self.match_kind(&[TokenKind::Let]) {
-            self.let_declaration()
+            self.let_declaration(true)
+        } else if self.match_kind(&[TokenKind::Const]) {
+            self.let_declaration(false)
         } else if self.match_kind(&[TokenKind::Fn]) {
             self.fn_declaration()
         } else {
@@ -59,13 +61,13 @@ impl Parser {
         }
     }
 
-    fn let_declaration(&mut self) -> Result<Stmt, RuntimeError> {
+    fn let_declaration(&mut self, mutable: bool) -> Result<Stmt, RuntimeError> {
         let name = self.consume_ident("Expected variable name.")?;
         self.consume(TokenKind::Equal, "Expected '=' after variable name.")?;
         let initializer = self.expression()?;
         self.match_kind(&[TokenKind::Semicolon]); // 分号可选
 
-        Ok(Stmt::Let { name, initializer })
+        Ok(Stmt::Let { name, initializer, mutable })
     }
 
     fn fn_declaration(&mut self) -> Result<Stmt, RuntimeError> {
@@ -135,7 +137,7 @@ impl Parser {
         let initializer = if self.match_kind(&[TokenKind::Semicolon]) {
             None
         } else if self.match_kind(&[TokenKind::Let]) {
-            Some(self.let_declaration()?)
+            Some(self.let_declaration(true)?)
         } else {
             Some(self.expression_statement()?)
         };
@@ -565,6 +567,7 @@ impl Parser {
 
             match self.peek().kind {
                 TokenKind::Let
+                | TokenKind::Const
                 | TokenKind::Fn
                 | TokenKind::If
                 | TokenKind::While
@@ -1106,7 +1109,7 @@ mod tests {
         #[test]
         fn parse_let_statement() {
             match parse_stmt("let x = 5;") {
-                Stmt::Let { name, initializer } => {
+                Stmt::Let { name, initializer, .. } => {
                     assert_eq!(name, "x");
                     assert!(matches!(initializer, Expr::Literal(Literal::Number(5.0))));
                 }
@@ -1123,6 +1126,17 @@ mod tests {
             }
         }
 
+        #[test]
+        fn parse_const_statement() {
+            match parse_stmt("const x = 5;") {
+                Stmt::Let { name, mutable, .. } => {
+                    assert_eq!(name, "x");
+                    assert!(!mutable, "const should set mutable=false");
+                }
+                e => panic!("Expected Let (const), got {:?}", e),
+            }
+        }
+
         #[test]
         fn parse_expression_statement() {
             match parse_stmt("x;") {