# Phase 74: BindingId Infrastructure - Implementation Summary

**Date**: 2025-12-13
**Status**: ✅ Complete
**Test Results**: All acceptance criteria met

---

## Implementation Deliverables

### Files Created
1. **`src/mir/binding_id.rs`** (130 lines)
   - `BindingId` type definition
   - Overflow protection with `debug_assert!`
   - 5 unit tests (creation, next, display, ordering, overflow)

2. **`docs/development/current/main/phase74-bindingid-infrastructure.md`** (~300 lines)
   - Complete architecture documentation
   - Implementation details
   - Test strategy
   - Migration roadmap (Phase 75-77)

### Files Modified
1. **`src/mir/mod.rs`** (+2 lines)
   - Added `pub mod binding_id;`
   - Re-exported `BindingId` in public API

2. **`src/mir/builder.rs`** (+85 lines)
   - Added `next_binding_id: u32` field
   - Added `binding_map: BTreeMap<String, BindingId>` field
   - Implemented `allocate_binding_id()` method
   - Added 4 unit tests (initialization, sequential, shadowing, parallel)

3. **`src/mir/builder/vars/lexical_scope.rs`** (+30 lines)
   - Extended `LexicalScopeFrame` with `restore_binding` field
   - Modified `pop_lexical_scope()` to restore BindingId mappings
   - Modified `declare_local_in_current_scope()` to allocate BindingIds

---

## Test Results

### Unit Tests (9 total)
```
src/mir/binding_id.rs:
  ✅ test_binding_id_creation
  ✅ test_binding_id_next
  ✅ test_binding_id_display
  ✅ test_binding_id_ordering
  ✅ test_binding_id_overflow (debug only)

src/mir/builder.rs:
  ✅ test_binding_map_initialization
  ✅ test_binding_allocation_sequential
  ✅ test_shadowing_binding_restore
  ✅ test_valueid_binding_parallel_allocation
```

### Regression Tests
```
cargo test --release --lib:     958 passed ✅
cargo test --features normalized_dev --test normalized_joinir_min:  54 passed ✅
cargo build --lib:              Success ✅
```

---

## Key Design Decisions

### 1. Parallel Allocation
- `ValueId` and `BindingId` allocate independently
- `next_value_id()` → `value_gen.next()`
- `allocate_binding_id()` → `next_binding_id++`

### 2. BTreeMap for Determinism
- `binding_map: BTreeMap<String, BindingId>` (not HashMap)
- Consistent with Phase 25.1 determinism strategy

### 3. Symmetric Restoration
- `pop_lexical_scope()` restores both `variable_map` and `binding_map`
- Prevents asymmetric bugs

### 4. Overflow Protection
- `debug_assert!` in `allocate_binding_id()` and `BindingId::next()`
- Test only runs in debug builds (`#[cfg(debug_assertions)]`)

---

## What's Next: Phase 75

### Pilot Integration Plan
1. **Identify 1-2 files** for isolated BindingId usage
2. **Add logging** (e.g., `NYASH_BINDING_TRACE=1`)
3. **Validate behavior** unchanged via smoke tests
4. **Document findings** for Phase 76 promotion

### Candidate Components
- `src/mir/builder/vars/` - Local variable tracking
- `src/mir/builder/stmts.rs` - Statement lowering (shadowing-heavy)

---

## Acceptance Criteria Status

| Criterion | Status | Notes |
|-----------|--------|-------|
| `cargo build --lib` succeeds | ✅ | No compilation errors |
| `cargo test --release --lib` all pass | ✅ | 958 tests passed |
| `cargo test --features normalized_dev --test normalized_joinir_min` passes | ✅ | 54 tests passed |
| New unit tests pass | ✅ | 9 tests added, all passing |
| Zero production impact | ✅ | Infrastructure-only, no behavior changes |

---

## Conclusion

Phase 74 successfully establishes the **BindingId infrastructure** with:
- ✅ Complete implementation (4 files, 547 lines)
- ✅ Full test coverage (9 unit tests + existing smoke tests)
- ✅ Zero regressions (958 + 54 tests pass)
- ✅ Production-ready (no behavior changes)

**Ready for Phase 75 pilot integration.**
