feat: const 编译期常量 — const N = 42;

Token(77): +TOK_CONST, AST_LET_STMT 新增 is_const, Symbol 新增 const_value
sema: 字面量初始化自动折叠为编译期常量值

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ast/ast.c

@@ -67,6 +67,7 @@ AstNode* ast_make_let(void* alloc, const char* name, TypeKind annot_type, bool h
     NEW(alloc, AST_LET_STMT);
     n->as.let_stmt.name = name; n->as.let_stmt.annot_type = annot_type;
     n->as.let_stmt.has_type_annot = has_type_annot; n->as.let_stmt.is_mut = is_mut;
+    n->as.let_stmt.is_const = false;
     n->as.let_stmt.init = init;
     n->as.let_stmt.struct_type_name = struct_type_name;
     n->as.let_stmt.annot_element_type = annot_elem_type;

src/ast/ast.h

@@ -78,7 +78,7 @@ struct AstNode {
         // AST_BLOCK
         struct { struct AstNode** stmts; size_t stmt_count; } block;
         // AST_LET_STMT
-        struct { const char* name; TypeKind annot_type; bool has_type_annot; bool is_mut; struct AstNode* init;
+        struct { const char* name; TypeKind annot_type; bool has_type_annot; bool is_mut; bool is_const; struct AstNode* init;
                  const char* struct_type_name;
                  TypeKind annot_element_type; const char* annot_element_struct_name; int64_t annot_array_size; } let_stmt;
         // AST_ASSIGN_STMT

src/lexer/lexer.c

@@ -57,6 +57,7 @@ static TokenKind check_keyword(const Token* tok) {
     KW("fn",     TOK_FN);     KW("let",    TOK_LET);
     KW("var",    TOK_VAR);
     KW("if",     TOK_IF);     KW("else",   TOK_ELSE);  KW("guard",  TOK_GUARD);
+    KW("const",  TOK_CONST);
     KW("while",  TOK_WHILE);  KW("for",    TOK_FOR);  KW("in",     TOK_IN);
     KW("to",     TOK_TO);    KW("step", TOK_STEP);
 	    KW("return", TOK_RETURN);

src/lexer/token.c

@@ -5,7 +5,7 @@
 #include <inttypes.h>
 
 static const char* NAMES[] = {
-    [TOK_FN] = "fn", [TOK_LET] = "let", [TOK_VAR] = "var", [TOK_IF] = "if", [TOK_GUARD] = "guard",
+    [TOK_FN] = "fn", [TOK_LET] = "let", [TOK_VAR] = "var", [TOK_CONST] = "const", [TOK_IF] = "if", [TOK_GUARD] = "guard",
     [TOK_PUB] = "pub", [TOK_MOD] = "mod", [TOK_USE] = "use",
     [TOK_TRAIT] = "trait", [TOK_SELF] = "Self",
     [TOK_ELSE] = "else", [TOK_WHILE] = "while", [TOK_FOR] = "for", [TOK_IN] = "in", [TOK_RETURN] = "return",

src/lexer/token.h

@@ -6,7 +6,7 @@
 // === Token 类型枚举 ===
 typedef enum {
     // 关键字
-    TOK_FN, TOK_LET, TOK_VAR, TOK_IF, TOK_ELSE, TOK_WHILE, TOK_FOR, TOK_IN, TOK_RETURN, TOK_GUARD,
+    TOK_FN, TOK_LET, TOK_VAR, TOK_CONST, TOK_IF, TOK_ELSE, TOK_WHILE, TOK_FOR, TOK_IN, TOK_RETURN, TOK_GUARD,
     TOK_STRUCT, TOK_TYPE, TOK_ENUM, TOK_EXTEND, TOK_DEFER, TOK_MATCH, TOK_PUB, TOK_MOD, TOK_USE,
     TOK_TRAIT, TOK_SELF,
     // 类型关键字

src/parser/parser.c

@@ -118,10 +118,11 @@ AstNode* parse_block(Parser* p, ErrorInfo* error) {
 AstNode* parse_statement(Parser* p, ErrorInfo* error) {
     const Token* t = peek(p);
 
-    if (t->kind == TOK_LET || t->kind == TOK_VAR) {
+    if (t->kind == TOK_LET || t->kind == TOK_VAR || t->kind == TOK_CONST) {
-        bool is_mut = (advance(p)->kind == TOK_VAR);
+        TokenKind decl_kind = advance(p)->kind;
-        const Token* name = expect(p, TOK_IDENT, error,
+        bool is_mut = (decl_kind == TOK_VAR);
-            is_mut ? "var 后应为变量名" : "let 后应为变量名");
+        bool is_const = (decl_kind == TOK_CONST);
+        const Token* name = expect(p, TOK_IDENT, error, "变量名");
         if (!name) return NULL;
         // 可选的类型标注
         TypeKind annot_type = TYPE_UNKNOWN;
@@ -144,10 +145,12 @@ AstNode* parse_statement(Parser* p, ErrorInfo* error) {
         AstNode* init = parse_expr(p, error);
         if (!init) return NULL;
         if (!expect(p, TOK_SEMICOLON, error, "缺少 ';'")) return NULL;
-        return ast_make_let(p->arena,
+        AstNode* let_node = ast_make_let(p->arena,
             arena_strdup_impl(p->arena, name->start, name->length),
             annot_type, has_type_annot, is_mut, init, struct_type_name,
             annot_elem_type, annot_elem_struct, annot_arr_size, tok_loc(t));
+        let_node->as.let_stmt.is_const = is_const;
+        return let_node;
     }
 
     if (t->kind == TOK_IF) {

src/sema/sema.c

@@ -319,6 +319,10 @@ void analyze_node(AstNode* node, Scope* scope, ErrorList* errors, Arena* a) {
                       "变量 '%s' 重复定义", node->as.let_stmt.name);
         } else {
             sym->is_mut = node->as.let_stmt.is_mut;
+            sym->is_const = node->as.let_stmt.is_const;
+            if (node->as.let_stmt.is_const && node->as.let_stmt.init->kind == AST_LITERAL_EXPR) {
+                sym->const_value = node->as.let_stmt.init->as.literal.i64_val;
+            }
             if (var_struct_name) {
                 sym->type = TYPE_STRUCT;
                 sym->struct_type_name = var_struct_name;

src/sema/symbol.h

@@ -11,6 +11,8 @@ typedef struct Symbol {
     SymbolKind  kind;
     TypeKind    type;          // 变量/参数的类型
     bool        is_mut;        // 变量是否可变（可被赋值）
+    bool        is_const;      // 编译期常量 (const 声明)
+    int64_t     const_value;   // 编译期常量值
     // 函数特有
     TypeKind    return_type;
     const char* return_struct_type_name;  // 返回类型为 struct 时的类型名