mruby-compiler: simplify NODE_KW_ARG to direct cons structure

Replace NODE_KW_ARG wrapper with direct (key . value) cons structure,
eliminating unnecessary memory overhead and access indirection.

Changes:
- Remove NODE_KW_ARG node type from enum
- Modify new_kw_arg() to return direct cons instead of wrapped structure
- Update codegen.c to use simplified access patterns (k->car->car, k->car->cdr)
- Fix new_args_tail() to handle simplified keyword argument structure
- Remove NODE_KW_ARG case from parser dump function

This reduces memory usage from 3 cons cells to 1 per keyword argument
while maintaining full functionality and following mruby's design priority
of memory efficiency over complexity.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-compiler/core/codegen.c

@@ -2410,10 +2410,9 @@ lambda_body(codegen_scope *s, node *locals, struct mrb_ast_args *args, node *bod
 
       while (kwds) {
         int jmpif_key_p, jmp_def_set = -1;
-        node *kwd = kwds->car, *def_arg = kwd->cdr->cdr->car;
-        mrb_sym kwd_sym = node_to_sym(kwd->cdr->car);
-
-        mrb_assert(node_to_int(kwd->car) == NODE_KW_ARG);
+        node *kwd = kwds->car;
+        mrb_sym kwd_sym = node_to_sym(kwd->car);   /* Direct access to key */
+        node *def_arg = kwd->cdr;                  /* Direct access to value */
 
         if (def_arg) {
           int idx;

mrbgems/mruby-compiler/core/node.h

@@ -61,7 +61,6 @@ enum node_type {
   NODE_REGX,
   NODE_DREGX,
   NODE_CALLARGS,
-  NODE_KW_ARG,
   NODE_KW_REST_ARGS,
   NODE_SPLAT,
   NODE_TO_ARY,

mrbgems/mruby-compiler/core/parse.y

@@ -1401,15 +1401,15 @@ new_args_tail(parser_state *p, node *kws, mrb_sym kwrest, mrb_sym blk)
   /* allocate register for keywords arguments */
   /* order is for Proc#parameters */
   for (k = kws; k; k = k->cdr) {
-    if (!k->car->cdr->cdr->car) { /* allocate required keywords */
-      local_add_f(p, node_to_sym(k->car->cdr->car));
+    if (!k->car->cdr) { /* allocate required keywords - simplified structure: (key . NULL) */
+      local_add_f(p, node_to_sym(k->car->car));
     }
   }
   for (k = kws; k; k = k->cdr) {
-    if (k->car->cdr->cdr->car) { /* allocate keywords with default */
-      local_add_lv(p, k->car->cdr->cdr->car->cdr);
-      k->car->cdr->cdr->car = k->car->cdr->cdr->car->car;
-      local_add_f(p, node_to_sym(k->car->cdr->car));
+    if (k->car->cdr) { /* allocate keywords with default - simplified structure: (key . value) */
+      local_add_lv(p, k->car->cdr->cdr);  /* value->cdr for default args */
+      k->car->cdr = k->car->cdr->car;    /* value->car for default args */
+      local_add_f(p, node_to_sym(k->car->car));
     }
   }
 
@@ -1417,12 +1417,12 @@ new_args_tail(parser_state *p, node *kws, mrb_sym kwrest, mrb_sym blk)
   return cons(kws, cons(sym_to_node(kwrest), sym_to_node(blk)));
 }
 
-/* (:kw_arg kw_sym def_arg) */
+/* (kw_sym . def_arg) - simplified from NODE_KW_ARG wrapper */
 static node*
 new_kw_arg(parser_state *p, mrb_sym kw, node *def_arg)
 {
   mrb_assert(kw);
-  return list3((node*)NODE_KW_ARG, sym_to_node(kw), def_arg);
+  return cons(sym_to_node(kw), def_arg);
 }
 
 /* (:kw_rest_args . a) */
@@ -8583,12 +8583,6 @@ mrb_parser_dump(mrb_state *mrb, node *tree, int offset)
     dump_recur(mrb, ((parser_heredoc_info*)tree)->doc, offset+1);
     break;
 
-
-  case NODE_KW_ARG:
-    printf("NODE_KW_ARG %s:\n", mrb_sym_name(mrb, node_to_sym(tree->car)));
-    mrb_parser_dump(mrb, tree->cdr->car, offset + 1);
-    break;
-
   case NODE_KW_REST_ARGS:
     if (tree)
       printf("NODE_KW_REST_ARGS %s\n", mrb_sym_name(mrb, node_to_sym(tree)));

mrbgems/mruby-compiler/core/y.tab.c

@@ -1463,15 +1463,15 @@ new_args_tail(parser_state *p, node *kws, mrb_sym kwrest, mrb_sym blk)
   /* allocate register for keywords arguments */
   /* order is for Proc#parameters */
   for (k = kws; k; k = k->cdr) {
-    if (!k->car->cdr->cdr->car) { /* allocate required keywords */
-      local_add_f(p, node_to_sym(k->car->cdr->car));
+    if (!k->car->cdr) { /* allocate required keywords - simplified structure: (key . NULL) */
+      local_add_f(p, node_to_sym(k->car->car));
     }
   }
   for (k = kws; k; k = k->cdr) {
-    if (k->car->cdr->cdr->car) { /* allocate keywords with default */
-      local_add_lv(p, k->car->cdr->cdr->car->cdr);
-      k->car->cdr->cdr->car = k->car->cdr->cdr->car->car;
-      local_add_f(p, node_to_sym(k->car->cdr->car));
+    if (k->car->cdr) { /* allocate keywords with default - simplified structure: (key . value) */
+      local_add_lv(p, k->car->cdr->cdr);  /* value->cdr for default args */
+      k->car->cdr = k->car->cdr->car;    /* value->car for default args */
+      local_add_f(p, node_to_sym(k->car->car));
     }
   }
 
@@ -1479,12 +1479,12 @@ new_args_tail(parser_state *p, node *kws, mrb_sym kwrest, mrb_sym blk)
   return cons(kws, cons(sym_to_node(kwrest), sym_to_node(blk)));
 }
 
-/* (:kw_arg kw_sym def_arg) */
+/* (kw_sym . def_arg) - simplified from NODE_KW_ARG wrapper */
 static node*
 new_kw_arg(parser_state *p, mrb_sym kw, node *def_arg)
 {
   mrb_assert(kw);
-  return list3((node*)NODE_KW_ARG, sym_to_node(kw), def_arg);
+  return cons(sym_to_node(kw), def_arg);
 }
 
 /* (:kw_rest_args . a) */
@@ -15412,12 +15412,6 @@ mrb_parser_dump(mrb_state *mrb, node *tree, int offset)
     dump_recur(mrb, ((parser_heredoc_info*)tree)->doc, offset+1);
     break;
 
-
-  case NODE_KW_ARG:
-    printf("NODE_KW_ARG %s:\n", mrb_sym_name(mrb, node_to_sym(tree->car)));
-    mrb_parser_dump(mrb, tree->cdr->car, offset + 1);
-    break;
-
   case NODE_KW_REST_ARGS:
     if (tree)
       printf("NODE_KW_REST_ARGS %s\n", mrb_sym_name(mrb, node_to_sym(tree)));