docs: Add field visibility analysis and MIR BoxCall documentation

## Field Visibility Analysis Results
- Confirmed init{} fields are **public** in current Nyash implementation
- No access modifier (private/public) system currently implemented
- All fields accessible via me.fieldname syntax
- Documented findings for future reference

## MIR Documentation Enhancements
- Created comprehensive mir-dumper-guide.md for reading MIR dumps
- Enhanced mir-26-specification.md with BoxCall vs regular Call examples
- Added clear identification patterns:
  * BoxCall: `call %value.method(args)` (plugins/builtins)
  * Regular Call: `call %func(%me, args)` (user-defined boxes)

## VM Backend BoxRef Handling Improvements
- Fixed BoxRef method dispatch using share_box() instead of clone_box()
- Prevents unintended constructor calls during method resolution
- Maintains proper instance identity throughout VM execution

## MIR Builder User-Defined Box Tracking
- Added user_defined_boxes HashSet to track declared user boxes
- Improved method lowering decisions for user-defined vs builtin boxes
- Enhanced AST→MIR conversion accuracy for method calls

## Plugin Tester Lifecycle Enhancements
- Added comprehensive FileBox lifecycle testing (open/write/close)
- Enhanced cloneSelf() and copyFrom() testing with proper Handle parsing
- Added TLV encoding helpers for strings and bytes
- Improved error reporting and step-by-step validation

🤖 Generated with [Claude Code](https://claude.ai/code)

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

docs/reference/execution-backend/mir-26-specification.md

@@ -88,6 +88,21 @@ This document specifies the official 26-instruction set for Nyash MIR (Machine I
     ```
     Effect: context-dependent
     
+    **重要な識別方法：**
+    - BoxCall形式: `call %値.メソッド名(引数)` - 値（%7など）に対してメソッドを直接呼ぶ
+    - 通常のCall形式: `call %関数値(%me, 引数)` - 関数値を呼び、第1引数にmeを渡す
+    
+    例：
+    ```mir
+    ; BoxCall（プラグイン/ビルトインBoxのメソッド）
+    %17 = call %7.open(%14, %16)
+    %22 = call %7.write(%21)
+    
+    ; 通常のCall（ユーザー定義Boxのメソッド）
+    %func = const "UserBox.method/2"
+    %result = call %func(%me, %arg1)
+    ```
+
 13. **ExternCall** - Call external function
     ```mir
     %dst = extern_call "interface.method"(%arg1, %arg2, ...)

docs/reference/mir-dumper-guide.md

@@ -0,0 +1,59 @@
+# MIR Dumper Output Guide
+
+MIRダンプ出力を正しく読み解くためのガイドです。
+
+## BoxCall vs 通常のCall の見分け方
+
+### BoxCall形式（プラグイン/ビルトインBoxのメソッド）
+```mir
+%8 = call %7.cloneSelf()
+%17 = call %7.open(%14, %16)
+%22 = call %7.write(%21)
+%23 = call %8.copyFrom(%7)
+```
+
+**特徴：**
+- `call %値.メソッド名(引数)` の形式
+- 値（%7, %8など）に対して直接メソッドを呼ぶ
+- プラグインBoxやビルトインBoxで使用される
+
+### 通常のCall形式（ユーザー定義Boxのメソッド）
+```mir
+%func = const "UserBox.calculate/2"
+%result = call %func(%me, %arg1)
+```
+
+**特徴：**
+- 事前に `const "クラス名.メソッド名/引数数"` で関数値を取得
+- `call %関数値(%me, 引数...)` の形式で呼び出し
+- 第1引数は常に `%me`（self相当）
+
+## 実例での比較
+
+### plugin_boxref_return.nyashのMIRダンプ
+```mir
+11: %7 = new FileBox()
+12: call %7.birth()
+13: %8 = call %7.cloneSelf()        ← BoxCall（プラグインメソッド）
+26: %17 = call %7.open(%14, %16)    ← BoxCall（プラグインメソッド）
+33: %22 = call %7.write(%21)        ← BoxCall（プラグインメソッド）
+34: %23 = call %8.copyFrom(%7)      ← BoxCall（プラグインメソッド）
+```
+
+これらはすべてBoxCall形式で、プラグインのFileBoxメソッドを直接呼び出しています。
+
+### ユーザー定義Boxの場合（仮想例）
+```mir
+; ユーザー定義Box "Calculator" のメソッド呼び出し
+%calc_func = const "Calculator.add/2"
+%result = call %calc_func(%me, %10, %20)
+```
+
+この場合は、MIR関数として事前にlower済みのメソッドを呼び出しています。
+
+## まとめ
+
+- **`call %値.メソッド()`** → BoxCall（プラグイン/ビルトイン）
+- **`call %関数値(%me, ...)`** → 通常のCall（ユーザー定義Box）
+
+MIRダンプを見る際は、この形式の違いに注目することで、どのタイプのメソッド呼び出しかを判断できます。

src/backend/vm.rs

@@ -90,8 +90,8 @@ impl VMValue {
             VMValue::String(s) => Box::new(StringBox::new(s)),
             VMValue::Future(f) => Box::new(f.clone()),
             VMValue::Void => Box::new(VoidBox::new()),
-            // Phase 9.78a: BoxRef returns cloned Box
+            // BoxRef returns a shared handle (do NOT birth a new instance)
-            VMValue::BoxRef(arc_box) => arc_box.clone_box(),
+            VMValue::BoxRef(arc_box) => arc_box.share_box(),
         }
     }
     
@@ -504,7 +504,8 @@ impl VM {
                 
                 // Handle BoxRef for proper method dispatch
                 let box_nyash = match &box_vm_value {
-                    VMValue::BoxRef(arc_box) => arc_box.clone_box(),
+                    // Use shared handle to avoid unintended constructor calls
+                    VMValue::BoxRef(arc_box) => arc_box.share_box(),
                     _ => box_vm_value.to_nyash_box(),
                 };
                 
@@ -607,8 +608,8 @@ impl VM {
                 };
                 match created {
                     Ok(b) => {
-                        // Register for scope-based finalization (clone for registration)
+                        // Register for scope-based finalization (share; keep same instance)
-                        let reg_arc = std::sync::Arc::from(b.clone_box());
+                        let reg_arc = std::sync::Arc::from(b.share_box());
                         self.scope_tracker.register_box(reg_arc);
                         // Store value in VM
                         self.set_value(*dst, VMValue::from_nyash_box(b));

src/mir/builder.rs

@@ -11,6 +11,7 @@ use super::{
 };
 use crate::ast::{ASTNode, LiteralValue, BinaryOperator};
 use std::collections::HashMap;
+use std::collections::HashSet;
 
 /// MIR builder for converting AST to SSA form
 pub struct MirBuilder {
@@ -39,6 +40,9 @@ pub struct MirBuilder {
     /// Origin tracking for simple optimizations (e.g., object.method after new)
     /// Maps a ValueId to the class name if it was produced by NewBox of that class
     pub(super) value_origin_newbox: HashMap<ValueId, String>,
+
+    /// Names of user-defined boxes declared in the current module
+    pub(super) user_defined_boxes: HashSet<String>,
 }
 
 impl MirBuilder {
@@ -53,6 +57,7 @@ impl MirBuilder {
             variable_map: HashMap::new(),
             pending_phis: Vec::new(),
             value_origin_newbox: HashMap::new(),
+            user_defined_boxes: HashSet::new(),
         }
     }
 
@@ -299,6 +304,8 @@ impl MirBuilder {
                     self.build_static_main_box(methods.clone())
                 } else {
                     // Support user-defined boxes - handle as statement, return void
+                    // Track as user-defined (eligible for method lowering)
+                    self.user_defined_boxes.insert(name.clone());
                     self.build_box_declaration(name.clone(), methods.clone(), fields.clone())?;
 
                     // Phase 2: Lower constructors (birth/N) into MIR functions
@@ -993,7 +1000,8 @@ impl MirBuilder {
         if let ASTNode::New { class, .. } = object {
             // Build function name and only lower to Call if the function exists (user-defined)
             let func_name = format!("{}.{}{}", class, method, format!("/{}", arg_values.len()));
-            let can_lower = if let Some(ref module) = self.current_module { module.functions.contains_key(&func_name) } else { false };
+            let can_lower = self.user_defined_boxes.contains(&class)
+                && if let Some(ref module) = self.current_module { module.functions.contains_key(&func_name) } else { false };
             if can_lower {
                 let func_val = self.value_gen.next();
                 self.emit_instruction(MirInstruction::Const { dst: func_val, value: ConstValue::String(func_name) })?;
@@ -1013,7 +1021,8 @@ impl MirBuilder {
             // If the object originates from a NewBox in this function, we can lower to Call as well
             if let Some(class_name) = self.value_origin_newbox.get(&object_value).cloned() {
                 let func_name = format!("{}.{}{}", class_name, method, format!("/{}", arg_values.len()));
-                let can_lower = if let Some(ref module) = self.current_module { module.functions.contains_key(&func_name) } else { false };
+                let can_lower = self.user_defined_boxes.contains(&class_name)
+                    && if let Some(ref module) = self.current_module { module.functions.contains_key(&func_name) } else { false };
                 if can_lower {
                     let func_val = self.value_gen.next();
                     self.emit_instruction(MirInstruction::Const { dst: func_val, value: ConstValue::String(func_name) })?;

tools/plugin-tester/src/main.rs

@@ -100,6 +100,33 @@ fn tlv_encode_one_handle(type_id: u32, instance_id: u32) -> Vec<u8> {
     buf
 }
 
+fn tlv_encode_two_strings(a: &str, b: &str) -> Vec<u8> {
+    let ab = a.as_bytes();
+    let bb = b.as_bytes();
+    let mut buf = Vec::with_capacity(4 + 2 * (4 + ab.len().min(u16::MAX as usize)));
+    buf.extend_from_slice(&1u16.to_le_bytes()); // ver
+    buf.extend_from_slice(&2u16.to_le_bytes()); // argc=2
+    // first string
+    buf.push(6u8); buf.push(0u8);
+    buf.extend_from_slice(&((ab.len().min(u16::MAX as usize) as u16).to_le_bytes()));
+    buf.extend_from_slice(ab);
+    // second string
+    buf.push(6u8); buf.push(0u8);
+    buf.extend_from_slice(&((bb.len().min(u16::MAX as usize) as u16).to_le_bytes()));
+    buf.extend_from_slice(bb);
+    buf
+}
+
+fn tlv_encode_bytes(data: &[u8]) -> Vec<u8> {
+    let mut buf = Vec::with_capacity(4 + 4 + data.len());
+    buf.extend_from_slice(&1u16.to_le_bytes()); // ver
+    buf.extend_from_slice(&1u16.to_le_bytes()); // argc=1
+    buf.push(7u8); buf.push(0u8);
+    buf.extend_from_slice(&((data.len().min(u16::MAX as usize) as u16).to_le_bytes()));
+    buf.extend_from_slice(data);
+    buf
+}
+
 fn tlv_decode_u32(data: &[u8]) -> Result<u32, String> {
     if data.len() >= 4 {
         Ok(u32::from_le_bytes([data[0], data[1], data[2], data[3]]))
@@ -330,50 +357,73 @@ fn test_lifecycle_v2(config_path: &PathBuf, box_type: &str) {
         
         println!("{}: Birth successful, instance_id = {}", "✓".green(), instance_id);
 
-        // Optional: If method 'copyFrom' exists, create another instance and pass it as Box arg
+        // 1b. Open the first instance (if open exists)
-        if box_config.methods.contains_key("copyFrom") {
+        if let Some(open_def) = box_config.methods.get("open") {
-            println!("\n{}", "1b. Testing method with Box arg: copyFrom(other) ...".cyan());
+            println!("\n{}", "1b. Opening src FileBox (id=instance_id) ...".cyan());
+            let args_open = tlv_encode_two_strings("test_lifecycle.txt", "w");
+            let mut out = vec![0u8; 1024];
+            let mut out_len = out.len();
+            let rc = invoke_fn(box_config.type_id, open_def.method_id, instance_id, args_open.as_ptr(), args_open.len(), out.as_mut_ptr(), &mut out_len);
+            if rc == 0 { println!("{}: open ok", "✓".green()); } else { eprintln!("{}: open rc={}", "WARN".yellow(), rc); }
+        }
 
-            // Birth another instance to serve as argument handle
+        // 1c. Write some bytes (if write exists)
-            let args2 = tlv_encode_empty();
+        if let Some(write_def) = box_config.methods.get("write") {
-            let mut out2 = vec![0u8; 1024];
+            println!("\n{}", "1c. Writing to src FileBox ...".cyan());
-            let mut out2_len = out2.len();
+            let args_write = tlv_encode_bytes(b"hello nyash");
-            let rc2 = invoke_fn(
+            let mut out = vec![0u8; 1024];
-                box_config.type_id,
+            let mut out_len = out.len();
-                0,
+            let rc = invoke_fn(box_config.type_id, write_def.method_id, instance_id, args_write.as_ptr(), args_write.len(), out.as_mut_ptr(), &mut out_len);
-                0,
+            if rc == 0 { println!("{}: write ok", "✓".green()); } else { eprintln!("{}: write rc={}", "WARN".yellow(), rc); }
-                args2.as_ptr(),
-                args2.len(),
-                out2.as_mut_ptr(),
-                &mut out2_len,
-            );
-            if rc2 == 0 {
-                if let Ok(other_id) = tlv_decode_u32(&out2[..out2_len]) {
-                    // Encode one Box handle as argument
-                    let arg_buf = tlv_encode_one_handle(box_config.type_id, other_id);
-                    let mut ret = vec![0u8; 1024];
-                    let mut ret_len = ret.len();
-                    let method_id = box_config.methods.get("copyFrom").unwrap().method_id;
-                    let rc_call = invoke_fn(
-                        box_config.type_id,
-                        method_id,
-                        instance_id,
-                        arg_buf.as_ptr(),
-                        arg_buf.len(),
-                        ret.as_mut_ptr(),
-                        &mut ret_len,
-                    );
-                    if rc_call == 0 {
-                        println!("{}: copyFrom call succeeded (arg=BoxRef)", "✓".green());
-                    } else {
-                        eprintln!("{}: copyFrom call failed (rc={})", "WARN".yellow(), rc_call);
         }
-                } else {
+
-                    eprintln!("{}: Failed to decode other instance_id", "WARN".yellow());
+        // 1d. Create destination instance via cloneSelf() if available; else birth
+        let mut dst_id = None;
+        if let Some(clone_def) = box_config.methods.get("cloneSelf") {
+            println!("\n{}", "1d. Cloning via cloneSelf() ...".cyan());
+            let args0 = tlv_encode_empty();
+            let mut out = vec![0u8; 1024];
+            let mut out_len = out.len();
+            let rc = invoke_fn(box_config.type_id, clone_def.method_id, instance_id, args0.as_ptr(), args0.len(), out.as_mut_ptr(), &mut out_len);
+            if rc == 0 && out_len >= 16 && out[4] == 8 { // Handle
+                // parse handle payload at bytes 8..16
+                let t = u32::from_le_bytes([out[8],out[9],out[10],out[11]]);
+                let i = u32::from_le_bytes([out[12],out[13],out[14],out[15]]);
+                if t == box_config.type_id { dst_id = Some(i); println!("{}: cloneSelf returned id={}", "✓".green(), i); }
+            } else { eprintln!("{}: cloneSelf rc={}", "WARN".yellow(), rc); }
         }
-            } else {
+        if dst_id.is_none() {
-                eprintln!("{}: Failed to create other instance for copyFrom (rc={})", "WARN".yellow(), rc2);
+            println!("\n{}", "1d. Cloning fallback via birth() ...".cyan());
+            let args0 = tlv_encode_empty();
+            let mut out = vec![0u8; 1024];
+            let mut out_len = out.len();
+            let rc = invoke_fn(box_config.type_id, 0, 0, args0.as_ptr(), args0.len(), out.as_mut_ptr(), &mut out_len);
+            if rc == 0 { dst_id = tlv_decode_u32(&out[..out_len]).ok(); }
+            if let Some(i) = dst_id { println!("{}: birth dst id={}", "✓".green(), i); } else { eprintln!("{}: birth dst failed rc={}", "WARN".yellow(), rc); }
         }
+
+        // 1e. copyFrom(dst <- src)
+        if let (Some(copy_def), Some(dst)) = (box_config.methods.get("copyFrom"), dst_id) {
+            println!("\n{}", "1e. Testing copyFrom(dst <- src) ...".cyan());
+            let arg_buf = tlv_encode_one_handle(box_config.type_id, instance_id);
+            let mut out = vec![0u8; 1024];
+            let mut out_len = out.len();
+            let rc = invoke_fn(box_config.type_id, copy_def.method_id, dst, arg_buf.as_ptr(), arg_buf.len(), out.as_mut_ptr(), &mut out_len);
+            if rc == 0 { println!("{}: copyFrom ok", "✓".green()); } else { eprintln!("{}: copyFrom rc={}", "WARN".yellow(), rc); }
+        }
+
+        // 1f. close both
+        if let Some(close_def) = box_config.methods.get("close") {
+            println!("\n{}", "1f. Closing both instances ...".cyan());
+            let args0 = tlv_encode_empty();
+            let mut out = vec![0u8; 64];
+            let mut out_len = out.len();
+            let _ = invoke_fn(box_config.type_id, close_def.method_id, instance_id, args0.as_ptr(), args0.len(), out.as_mut_ptr(), &mut out_len);
+            if let Some(dst) = dst_id {
+                out_len = out.len();
+                let _ = invoke_fn(box_config.type_id, close_def.method_id, dst, args0.as_ptr(), args0.len(), out.as_mut_ptr(), &mut out_len);
+            }
+            println!("{}: close done", "✓".green());
         }
 
         // Optional: If method 'cloneSelf' exists, call it and verify Handle return