Phase 4.2 Final Implementation Plan Summary

STATUS: FINAL - Ready for Implementation
TIMELINE: 5.5-6 hours (parallel execution)
TESTS: 19 new tests → 94 total (100% pass rate target)

KEY FEATURES (User Decisions Incorporated):

1. Pattern Guards
   - Runtime conditions using nested if statements (safe, debuggable)
   - Supports both 'if' and 'where' keywords (Rust + Swift syntax)
   - Guards ignored for exhaustiveness (simple, safe)

2. Swift Syntax Support
   - Dual syntax: switch/case .Variant(let x) vs match Variant(x) =>
   - Config-driven (match.syntax = "rust" or "swift")
   - Both normalize to same markers (plugin sees no difference)

3. Tuple Destructuring
   - Match multiple values: (Ok(x), Ok(y)) => handleBoth(x, y)
   - Max 6 elements (64 pattern limit, balanced performance)
   - Wildcard catch-all semantics: (_, _) makes match exhaustive

4. Enhanced Error Messages
   - Always-on rustc-style diagnostics (consistent DX)
   - Source snippets with carets ^^^^
   - Actionable suggestions for all error types

IMPLEMENTATION ROADMAP (Parallel Batches):

Batch 1 (1h, 2 parallel): Guards parsing + Enhanced errors foundation
Batch 2 (1.5h, 2 parallel): Swift preprocessor + Guard plugin
Batch 3 (1.5h, collaborate): Tuple preprocessor + plugin
Batch 4 (1h, 2 parallel): 12 golden tests
Batch 5 (0.5h, sequential): Integration, docs, verification

RISK MITIGATION:

- Tuple limit prevents exponential complexity (max 64 patterns)
- Nested if strategy safer than goto labels (simpler AST)
- Dual guard keywords minimal complexity (single regex alternation)
- Enhanced errors gracefully degrade if file read fails

PERFORMANCE TARGETS:

- Total overhead: <20ms per file (+5ms from Phase 4.1)
- Tuple exhaustiveness: <1ms for 6-element tuples
- Enhanced errors: <10ms per error
- Runtime: Zero overhead (compiles to native Go)

BACKWARD COMPATIBILITY:

- All Phase 4.1 tests MUST pass (57/57)
- No breaking config changes
- No changes to existing golden outputs
- Swift syntax opt-in via config

SUCCESS CRITERIA:

✅ 94 tests passing (57 Phase 4.1 + 19 Phase 4.2 + integration)
✅ <20ms compile overhead
✅ Clean, idiomatic Go output
✅ 4 feature guides written
✅ >80% code coverage

DELIVERABLES:

- 4 new files (~650 lines): SwiftMatchProcessor, EnhancedError + tests
- 4 modified files (+470 lines): RustMatchProcessor, PatternMatchPlugin, generator, docs
- 12 golden test suites (24 files: .dingo + .go.golden)
- 3 config files (Swift syntax tests)
- 4 feature documentation files

READY TO PROCEED: No open questions, all decisions finalized.
