✨ add codegen for `??` operator

src/nasal_codegen.cpp

@@ -1059,6 +1059,9 @@ void codegen::binary_gen(binary_operator* node) {
             calc_gen(node->get_right());
             emit(op_btand, 0, node->get_location());
             return;
+        case binary_operator::binary_type::nullchain:
+            null_chain_gen(node);
+            return;
         default: break;
     }
     switch(node->get_operator_type()) {
@@ -1174,6 +1177,23 @@ void codegen::binary_gen(binary_operator* node) {
     }
 }
 
+void codegen::null_chain_gen(binary_operator* node) {
+    calc_gen(node->get_left());
+    emit(op_pnil, 0, node->get_location());
+    emit(op_eq, 0, node->get_location());
+
+    const auto jmp_false_point = code.size();
+    emit(op_jf, 0, node->get_location());
+
+    calc_gen(node->get_right());
+    const auto jmp_direct_point = code.size();
+    emit(op_jmp, 0, node->get_location());
+
+    code[jmp_false_point].num = code.size();
+    emit(op_pop, 0, node->get_location());
+    code[jmp_direct_point].num = code.size();
+}
+
 void codegen::trino_gen(ternary_operator* node) {
     calc_gen(node->get_condition());
     usize label_jump_false = code.size();

src/nasal_codegen.h

@@ -148,6 +148,7 @@ private:
     void and_gen(binary_operator*);
     void unary_gen(unary_operator*);
     void binary_gen(binary_operator*);
+    void null_chain_gen(binary_operator*);
     void trino_gen(ternary_operator*);
     void calc_gen(expr*);
     void repl_mode_info_output_gen(expr*);

test/scalar.nas

@@ -267,3 +267,10 @@ for(var i = 1; i<=10; i += 1) {
         die("test failed: expect " ~ i ~ ", but get " ~ closure_tester[1]());
     }
 }
+
+func() {
+    var a = nil;
+    var b = nil;
+    var c = nil;
+    println(a??b??c??"a??b??c?? -> should print this text");
+}();