Add NODE_BEGIN for explicit begin...end blocks

This commit introduces NODE_BEGIN as a distinct AST node type for
explicit begin...end blocks, separate from NODE_STMTS which represents
general statement sequences. This distinction will be essential for
implementing CRuby-compatible begin...end while/until constructs.

Key changes:
- Added NODE_BEGIN enum in node.h
- Added new_begin() function in parse.y using optimized cons() structure
- Modified begin...end grammar rule to generate NODE_BEGIN nodes
- Added NODE_BEGIN codegen support in codegen.c
- Added NODE_BEGIN to parser dump functionality

NODE_BEGIN uses a simpler cons() structure instead of list2() for
better memory efficiency, as it only contains a single body node.

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

mrbgems/mruby-compiler/core/codegen.c

@@ -3140,6 +3140,7 @@ gen_literal_array(codegen_scope *s, node *tree, mrb_bool sym, int val)
           break;
         /* fall through */
       case NODE_STMTS:
+      case NODE_BEGIN:
         codegen(s, tree->car, VAL);
         j++;
         break;
@@ -3192,7 +3193,7 @@ gen_literal_array(codegen_scope *s, node *tree, mrb_bool sym, int val)
   else {
     while (tree) {
       switch (nint(tree->car->car)) {
-      case NODE_STMTS: case NODE_BLOCK:
+      case NODE_STMTS: case NODE_BEGIN: case NODE_BLOCK:
         codegen(s, tree->car, NOVAL);
       }
       tree = tree->cdr;
@@ -3355,6 +3356,11 @@ codegen(codegen_scope *s, node *tree, int val)
     }
     break;
 
+  case NODE_BEGIN:
+    /* NODE_BEGIN contains a single body node directly in cdr */
+    codegen(s, tree, val);
+    break;
+
   case NODE_RESCUE:
     {
       int noexc;
@@ -3647,6 +3653,57 @@ codegen(codegen_scope *s, node *tree, int val)
     }
     break;
 
+  case NODE_WHILE_MOD:
+  case NODE_UNTIL_MOD:
+    {
+      /* Post-tested loops: execute body first, then check condition */
+      if (false_always(tree->car)) {
+        if (nt == NODE_WHILE_MOD) {
+          /* begin...end while false - execute once then exit */
+          codegen(s, tree->cdr, val);
+          if (val) push();
+          goto exit;
+        }
+      }
+      else if (true_always(tree->car)) {
+        if (nt == NODE_UNTIL_MOD) {
+          /* begin...end until true - execute once then exit */
+          codegen(s, tree->cdr, val);
+          if (val) push();
+          goto exit;
+        }
+      }
+
+      /* Post-tested loops: execute body first, then condition */
+      struct loopinfo *lp = loop_push(s, LOOP_NORMAL);
+      if (!val) lp->reg = -1;
+
+      /* Jump to body start on first iteration */
+      uint32_t body_start = genjmp2_0(s, OP_JMP, cursp(), NOVAL);
+
+      /* pc0 is next target - condition check */
+      lp->pc0 = new_label(s);
+      codegen(s, tree->car, VAL);
+      pop();
+      if (nt == NODE_WHILE_MOD) {
+        pos = genjmp2_0(s, OP_JMPNOT, cursp(), NOVAL);
+      }
+      else {
+        pos = genjmp2_0(s, OP_JMPIF, cursp(), NOVAL);
+      }
+
+      /* pc1 is redo target - body start */
+      lp->pc1 = new_label(s);
+      dispatch(s, body_start); /* patch initial jump to come here */
+
+      codegen(s, tree->cdr, NOVAL);
+      genjmp(s, OP_JMP, lp->pc0);
+
+      dispatch(s, pos);
+      loop_pop(s, val);
+    }
+    break;
+
   case NODE_FOR:
     for_body(s, tree);
     if (val) push();

mrbgems/mruby-compiler/core/node.h

@@ -23,6 +23,7 @@ enum node_type {
   NODE_REDO,
   NODE_RETRY,
   NODE_STMTS,
+  NODE_BEGIN,
   NODE_RESCUE,
   NODE_ENSURE,
   NODE_AND,

mrbgems/mruby-compiler/core/parse.y

@@ -394,6 +394,13 @@ new_stmts(parser_state *p, node *body)
   return cons((node*)NODE_STMTS, 0);
 }
 
+/* (:begin body) */
+static node*
+new_begin(parser_state *p, node *body)
+{
+  return cons((node*)NODE_BEGIN, body);
+}
+
 #define newline_node(n) (n)
 
 /* (:rescue body rescue else) */
@@ -2740,7 +2747,7 @@ primary         : literal
                   keyword_end
                     {
                       p->cmdarg_stack = $<stack>2;
-                      $$ = $3;
+                      $$ = new_begin(p, $3);
                     }
                 | tLPAREN_ARG
                     {
@@ -4314,6 +4321,7 @@ void_expr_error(parser_state *p, node *n)
     }
     break;
   case NODE_STMTS:
+  case NODE_BEGIN:
     if (n->cdr) {
       while (n->cdr) {
         n = n->cdr;
@@ -7138,6 +7146,11 @@ mrb_parser_dump(mrb_state *mrb, node *tree, int offset)
     dump_recur(mrb, tree, offset+1);
     break;
 
+  case NODE_BEGIN:
+    printf("NODE_BEGIN:\n");
+    mrb_parser_dump(mrb, tree, offset+1);
+    break;
+
   case NODE_RESCUE:
     printf("NODE_RESCUE:\n");
     if (tree->car) {