feat: guard 语句 guard x >= 0 else { return -1; }

src/lexer/lexer.c

@@ -56,7 +56,7 @@ static TokenKind check_keyword(const Token* tok) {
     #define KW(s, k) if (tok->length == sizeof(s)-1 && memcmp(tok->start, s, sizeof(s)-1) == 0) return k
     KW("fn",     TOK_FN);     KW("let",    TOK_LET);
     KW("var",    TOK_VAR);
-    KW("if",     TOK_IF);     KW("else",   TOK_ELSE);
+    KW("if",     TOK_IF);     KW("else",   TOK_ELSE);  KW("guard",  TOK_GUARD);
     KW("while",  TOK_WHILE);  KW("for",    TOK_FOR);  KW("in",     TOK_IN);
     KW("to",     TOK_TO);
 	    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_FN] = "fn", [TOK_LET] = "let", [TOK_VAR] = "var", [TOK_IF] = "if", [TOK_GUARD] = "guard",
     [TOK_ELSE] = "else", [TOK_WHILE] = "while", [TOK_FOR] = "for", [TOK_IN] = "in", [TOK_RETURN] = "return",
     [TOK_STRUCT] = "struct", [TOK_TYPE] = "type", [TOK_ENUM] = "enum", [TOK_EXTEND] = "extend",
     [TOK_MATCH] = "match",

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_FN, TOK_LET, TOK_VAR, TOK_IF, TOK_ELSE, TOK_WHILE, TOK_FOR, TOK_IN, TOK_RETURN, TOK_GUARD,
     TOK_STRUCT, TOK_TYPE, TOK_ENUM, TOK_EXTEND, TOK_MATCH,
     // 类型关键字
     TOK_I32, TOK_I64, TOK_U64, TOK_F64, TOK_BOOL, TOK_CHAR, TOK_STR, TOK_VOID,

src/parser/parser.c

@@ -621,6 +621,19 @@ static AstNode* parse_statement(Parser* p, ErrorInfo* error) {
         return parse_match_stmt(p, error);
     }
 
+    if (t->kind == TOK_GUARD) {
+        // guard expr else { ... }  →  if !(expr) { ... }
+        const Token* guard_tok = advance(p);
+        AstNode* cond = parse_expr(p, error);
+        if (!cond) return NULL;
+        if (!expect(p, TOK_ELSE, error, "guard 缺少 'else'")) return NULL;
+        AstNode* body = parse_block(p, error);
+        if (!body) return NULL;
+        // 去糖: if !cond { body }
+        AstNode* not_cond = ast_make_unary(p->arena, OP_NOT, cond, tok_loc(guard_tok));
+        return ast_make_if(p->arena, not_cond, body, NULL, tok_loc(guard_tok));
+    }
+
     if (t->kind == TOK_RETURN) {
         advance(p);
         if (match(p, TOK_SEMICOLON)) {