mruby-compiler: simplify mrb_ast_array_node from flat array to cons list

Replace complex flat array packing with simple cons-list storage to reduce
memory overhead and code complexity. This continues the compiler simplification
work by reverting array nodes to the original memory-efficient approach.

- Remove len/flags fields from mrb_ast_array_node structure
- Eliminate complex two-pass processing (count + copy) in new_array()
- Replace array indexing with cons-list iteration in gen_array_var()
- Reduce parser code from 30+ lines to 4 lines for array creation
- Maintain full functionality with zero test regressions

Following the same successful pattern used for mrb_ast_case_node upgrade,
this change proves that flat array packing provides no memory benefit
since cons lists aren't recycled, while adding unnecessary complexity.

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

mrbgems/mruby-compiler/core/codegen.c

@@ -3707,16 +3707,14 @@ static void
 gen_array_var(codegen_scope *s, node *varnode, int val)
 {
   struct mrb_ast_array_node *array = array_node(varnode);
-  int len = ARRAY_NODE_LEN(array);
-  struct mrb_ast_node **elements = ARRAY_NODE_ELEMENTS(array);
-  int i;
+  node *elements = ARRAY_NODE_ELEMENTS(array);
   int regular_elements = 0;
   int first = 1;
   int slimit = GEN_VAL_STACK_MAX;
 
   if (!val) return;
 
-  if (len == 0) {
+  if (!elements) {
     genop_2(s, OP_ARRAY, cursp(), 0);
     push();
     return;
@@ -3724,9 +3722,10 @@ gen_array_var(codegen_scope *s, node *varnode, int val)
 
   if (cursp() >= GEN_LIT_ARY_MAX) slimit = INT16_MAX;
 
-  /* Process each element, handling splats */
-  for (i = 0; i < len; i++) {
-    struct mrb_ast_node *element = elements[i];
+  /* Process each element using cons-list iteration, handling splats */
+  node *current = elements;
+  while (current) {
+    struct mrb_ast_node *element = current->car;
     int is_splat = is_splat_node(element);
 
     if (is_splat || cursp() >= slimit) { /* flush accumulated elements */
@@ -3764,6 +3763,8 @@ gen_array_var(codegen_scope *s, node *varnode, int val)
     else {
       regular_elements++;
     }
+
+    current = current->cdr;
   }
 
   /* Handle any remaining regular elements */

mrbgems/mruby-compiler/core/node.h

@@ -219,12 +219,10 @@ struct mrb_ast_call_node {
   struct mrb_ast_node *args;         /* Arguments Information */
 };
 
-/* Variable-sized array node with inline element storage */
+/* Variable-sized array node - revert to original cons list approach */
 struct mrb_ast_array_node {
-  struct mrb_ast_var_header header;  /* 8 bytes */
-  uint16_t len;                      /* Number of elements */
-  uint16_t flags;                    /* Array-specific flags */
-  struct mrb_ast_node *elements[];   /* Flexible array for elements */
+  struct mrb_ast_var_header header;    /* 8 bytes */
+  struct mrb_ast_node *elements;       /* Original elements list */
 };
 
 /* Variable-sized hash node with inline key-value storage */
@@ -447,7 +445,6 @@ struct mrb_ast_super_node {
 #define CALL_NODE_HAS_BLOCK(n) (call_node(n)->has_block)
 #define CALL_NODE_SAFE(n) (call_node(n)->safe_call)
 
-#define ARRAY_NODE_LEN(n) (array_node(n)->len)
 #define ARRAY_NODE_ELEMENTS(n) (array_node(n)->elements)
 
 #define HASH_NODE_LEN(n) (hash_node(n)->len)

mrbgems/mruby-compiler/core/parse.y

@@ -989,34 +989,10 @@ new_or(parser_state *p, node *a, node *b)
 static node*
 new_array(parser_state *p, node *a)
 {
-  /* Count array elements */
-  uint16_t len = 0;
-  node *elem_iter = a;
-  while (elem_iter) {
-    len++;
-    elem_iter = elem_iter->cdr;
-  }
+  struct mrb_ast_array_node *n = (struct mrb_ast_array_node*)parser_alloc_var(p, sizeof(struct mrb_ast_array_node), SIZE_CLASS_MEDIUM);
 
-  /* Calculate total size needed */
-  size_t base_size = sizeof(struct mrb_ast_array_node);
-  size_t elems_size = len * sizeof(struct mrb_ast_node*);
-  size_t total_size = base_size + elems_size;
-
-  enum mrb_ast_size_class size_class = size_to_class(total_size);
-
-  struct mrb_ast_array_node *n = (struct mrb_ast_array_node*)
-    parser_alloc_var(p, total_size, size_class);
-
-  init_var_header(&n->header, p, NODE_ARRAY, size_class);
-  n->len = len;
-  n->flags = 0;
-
-  /* Copy elements into flexible array */
-  elem_iter = a;
-  for (int i = 0; i < len; i++) {
-    n->elements[i] = elem_iter->car;
-    elem_iter = elem_iter->cdr;
-  }
+  init_var_header(&n->header, p, NODE_ARRAY, SIZE_CLASS_MEDIUM);
+  n->elements = a;
 
   return cons_head((node*)NODE_VARIABLE, (node*)n);
 }