# Phase 73: Completion Summary

**Date**: 2025-12-13
**Status**: ✅ Complete (Design Phase)
**Scope**: JoinIR ScopeManager → BindingId-Based Design

---

## Deliverables

### 1. Design Document (SSOT)
**File**: `docs/development/current/main/phase73-scope-manager-design.md`

**Contents**:
- ✅ Current state analysis (MIR + JoinIR scope systems)
- ✅ Problem identification (name-based vs BindingId mismatch)
- ✅ Proposed architecture (Option A: Parallel BindingId Layer)
- ✅ Integration with MIR Builder (binding_map additions)
- ✅ Migration path (Phases 74-77 roadmap)
- ✅ Example scenarios (shadowing, promoted variables)

**Key Insights**:
- MIR builder uses **BindingId** for lexical scope tracking (Phase 68-69)
- JoinIR lowering uses **name-based** lookup (fragile for shadowing)
- Naming convention hacks (`is_digit_pos`, `is_ch_match`) can be replaced with BindingId maps
- Gradual migration strategy minimizes risk

---

### 2. Proof-of-Concept Implementation
**File**: `src/mir/join_ir/lowering/scope_manager_bindingid_poc/mod.rs`

**Status**: ✅ All tests passing (6/6)
**Feature Gate**: `#[cfg(feature = "normalized_dev")]` (dev-only)

**Implemented Structures**:
- `BindingId` type wrapper
- `ConditionEnvV2` (parallel name + BindingId lookup)
- `CarrierInfoV2` (BindingId-based promotion tracking)
- `ScopeManagerV2` trait + `Pattern2ScopeManagerV2` implementation

**Test Coverage**:
```
test mir::join_ir::lowering::scope_manager_bindingid_poc::tests::test_condition_env_v2_basic ... ok
test mir::join_ir::lowering::scope_manager_bindingid_poc::tests::test_shadowing_simulation ... ok
test mir::join_ir::lowering::scope_manager_bindingid_poc::tests::test_promoted_binding_resolution ... ok
test mir::join_ir::lowering::scope_manager_bindingid_poc::tests::test_scope_manager_v2_binding_lookup ... ok
test mir::join_ir::lowering::scope_manager_bindingid_poc::tests::test_scope_manager_v2_promoted_lookup ... ok
test mir::join_ir::lowering::scope_manager_bindingid_poc::tests::test_unified_lookup_fallback ... ok
```

**Key Validations**:
- ✅ Parallel lookup (BindingId + name fallback) works
- ✅ Shadowing simulation (multiple bindings for same name)
- ✅ Promoted variable resolution (BindingId → BindingId mapping)
- ✅ Unified lookup with graceful fallback

---

## Design Highlights

### Problem Statement
**Before (Current)**:
```rust
// JoinIR lowering (name-based)
env.get("digit_pos") → searches for "is_digit_pos" via naming convention
                    → fragile, breaks if naming convention changes
```

**After (Phase 76+)**:
```rust
// JoinIR lowering (BindingId-based)
env.get_by_binding(BindingId(5)) → resolves promoted_bindings[BindingId(5)] = BindingId(10)
                                 → type-safe, no string matching
```

---

### Proposed Architecture (Option A)

**Gradual Migration Strategy**:
1. **Phase 74**: Add BindingId infrastructure (binding_map, binding_to_join)
2. **Phase 75**: Migrate Pattern 1 (simple, no carriers)
3. **Phase 76**: Migrate Pattern 2 (carrier promotion)
4. **Phase 77**: Migrate Pattern 3-4, remove legacy code

**Backward Compatibility**:
- New fields added alongside existing name-based maps
- Legacy code continues to work during transition
- Fallback mechanism ensures no breakage

---

### Integration Points

#### MirBuilder Changes (Phase 74)
```rust
pub struct MirBuilder {
    pub variable_map: HashMap<String, ValueId>,  // Existing (SSA conversion)
    pub binding_map: HashMap<String, BindingId>, // NEW (lexical scope)
    next_binding_id: u32,                        // NEW (allocator)
}
```

#### ConditionEnv Changes (Phase 74)
```rust
pub struct ConditionEnv {
    name_to_join: BTreeMap<String, ValueId>,      // Legacy (keep)
    binding_to_join: BTreeMap<BindingId, ValueId>, // NEW (Phase 74+)
    name_to_binding: BTreeMap<String, BindingId>,  // NEW (shadowing)
}
```

#### CarrierInfo Changes (Phase 76)
```rust
pub struct CarrierInfo {
    promoted_loopbodylocals: Vec<String>,           // Legacy (Phase 224)
    promoted_bindings: BTreeMap<BindingId, BindingId>, // NEW (Phase 76+)
}
```

---

## No Production Code Changes

**Confirmation**:
- ✅ No changes to `src/mir/builder.rs`
- ✅ No changes to `src/mir/join_ir/lowering/*.rs` (except mod.rs for PoC)
- ✅ PoC is feature-gated (`normalized_dev` only)
- ✅ All existing tests still pass

**Modified Files**:
1. `docs/development/current/main/phase73-scope-manager-design.md` (new)
2. `docs/development/current/main/phase73-completion-summary.md` (new)
3. `src/mir/join_ir/lowering/scope_manager_bindingid_poc/mod.rs` (new, dev-only)
4. `src/mir/join_ir/lowering/mod.rs` (1 line added for PoC module)

---

## Migration Roadmap

### Phase 74: Infrastructure (Estimated 2-3 hours)
**Goal**: Add BindingId tracking without breaking existing code

**Tasks**:
- [ ] Add `binding_map` to `MirBuilder`
- [ ] Add `binding_to_join` to `ConditionEnv`
- [ ] Update `declare_local_in_current_scope` to return `BindingId`
- [ ] Add BindingId allocator tests

**Acceptance**: All existing tests pass, BindingId populated

---

### Phase 75: Pattern 1 Pilot (Estimated 1-2 hours)
**Goal**: Prove BindingId integration with simplest pattern

**Tasks**:
- [ ] Update `CarrierInfo::from_variable_map` to accept `binding_map`
- [ ] Migrate Pattern 1 lowering to use BindingId
- [ ] Add E2E test with BindingId

**Acceptance**: Pattern 1 uses BindingId, legacy fallback works

---

### Phase 76: Pattern 2 Carrier Promotion (Estimated 2-3 hours)
**Goal**: Eliminate naming convention hacks

**Tasks**:
- [ ] Add `promoted_bindings: BTreeMap<BindingId, BindingId>` to `CarrierInfo`
- [ ] Update `resolve_promoted_join_id` to use BindingId
- [ ] Migrate Pattern 2 lowering

**Acceptance**: DigitPos pattern works without string matching

---

### Phase 77: Pattern 3-4 + Cleanup (Estimated 2-3 hours)
**Goal**: Complete migration, remove legacy code

**Tasks**:
- [ ] Migrate Pattern 3 (multi-carrier)
- [ ] Migrate Pattern 4 (generic case A)
- [ ] Remove `name_to_join`, `promoted_loopbodylocals` (legacy fields)

**Acceptance**: All patterns BindingId-only, full test suite passes

---

## Open Questions (for Future Phases)

### Q1: BindingId Scope (Per-Function vs Global)
**Current Assumption**: Per-function (like ValueId)

**Reasoning**:
- Each function has independent binding scope
- No cross-function binding references
- Simpler allocation (no global state)

**Alternative**: Global BindingId pool (for Phase 63 ownership analysis integration)

---

### Q2: Captured Variable Handling
**Proposed**: Add `binding_id` to `CapturedVar`

```rust
pub struct CapturedVar {
    name: String,
    host_id: ValueId,
    host_binding: BindingId,  // Phase 73+ (NEW)
    is_immutable: bool,
}
```

**Impact**: Requires updating `function_scope_capture` module

---

### Q3: Performance Impact
**Concern**: Dual maps (`binding_to_join` + `name_to_join`) double memory

**Mitigation**:
- Phase 74-76: Both maps active (transition period)
- Phase 77: Remove `name_to_join` after migration
- BTreeMap overhead minimal (<10 variables per loop typically)

**Measurement**: Profile after Phase 74 implementation

---

## Success Criteria (Phase 73)

### Design Document ✅
- [x] Current state analysis (MIR + JoinIR)
- [x] Proposed architecture (Option A)
- [x] Integration points (MirBuilder changes)
- [x] Migration path (Phases 74-77)
- [x] Example scenarios

### Proof-of-Concept ✅
- [x] BindingId type + structures
- [x] Parallel lookup (BindingId + name)
- [x] Shadowing simulation test
- [x] Promoted variable resolution test
- [x] All tests passing (6/6)

### No Production Impact ✅
- [x] Feature-gated (`normalized_dev`)
- [x] No production code changes
- [x] Existing tests unaffected

### Documentation ✅
- [x] SSOT design document
- [x] Completion summary (this document)
- [x] PoC code comments

---

## References

### Related Phases
- **Phase 68-69**: MIR lexical scope + shadowing (existing)
- **Phase 63**: Ownership analysis (dev-only, uses BindingId)
- **Phase 231**: ScopeManager trait (current implementation)
- **Phase 224**: Promoted LoopBodyLocal (naming convention hacks)

### Key Files
- `docs/development/current/main/phase73-scope-manager-design.md` (SSOT)
- `src/mir/join_ir/lowering/scope_manager_bindingid_poc/mod.rs` (PoC)
- `src/mir/builder/vars/lexical_scope.rs` (MIR lexical scope)
- `src/mir/join_ir/lowering/scope_manager.rs` (current ScopeManager)
- `src/mir/join_ir/lowering/carrier_info.rs` (current CarrierInfo)

---

## Estimated Total Effort (Phases 74-77)

| Phase | Task | Hours |
|-------|------|-------|
| 74 | Infrastructure | 2-3 |
| 75 | Pattern 1 Pilot | 1-2 |
| 76 | Pattern 2 Promotion | 2-3 |
| 77 | Pattern 3-4 + Cleanup | 2-3 |
| **Total** | **Full Migration** | **8-12** |

**Risk Level**: Low (gradual migration, backward compatible)

---

## Next Steps

1. **User Review**: Confirm design makes sense
2. **Phase 74 Start**: Implement BindingId infrastructure
3. **Iterative Migration**: Phases 75-77 (one pattern at a time)

---

## Conclusion

**Phase 73 Success**: ✅ Design + PoC Complete

**Key Achievements**:
- Comprehensive design document (SSOT for BindingId migration)
- Working proof-of-concept (6 tests passing)
- Clear migration path (Phases 74-77 roadmap)
- No production code impact (feature-gated)

**Ready for Phase 74**: Infrastructure implementation can begin immediately.