coverage-investigation

name	coverage-investigation
description	Investigate and improve code coverage for the wado-compiler crate. Use when asked to analyze coverage gaps, find dead code, or plan coverage improvement.

Coverage Investigation Methodology

Step 1: Collect Coverage Data

Use cargo-llvm-cov to generate line-level coverage. The tool must be installed via cargo install cargo-llvm-cov.

# Full coverage report (HTML)
cargo llvm-cov --html -p wado-compiler

# JSON summary for overall metrics
cargo llvm-cov --json -p wado-compiler 2>/dev/null | jq '.data[0].totals.lines'

# Per-file coverage in JSON (for programmatic analysis)
cargo llvm-cov --json -p wado-compiler 2>/dev/null | jq '.data[0].files[] | {filename, summary: .summary.lines}' > /tmp/coverage-per-file.json

Step 2: Identify Low-Coverage Files

Sort files by uncovered line count to find the biggest improvement opportunities:

cargo llvm-cov --json -p wado-compiler 2>/dev/null \
  | jq -r '.data[0].files[] | "\(.summary.lines.count - .summary.lines.covered)\t\(.summary.lines.percent | round)%\t\(.filename)"' \
  | sort -rn | head -30

Focus on files with:

• High absolute uncovered lines (biggest bang for buck)
• Low percentage but meaningful code (not just error paths)

Step 3: Classify Coverage Gaps

For each low-coverage file, classify gaps into categories:

Category	Action	Coverage Impact
Dead code	Delete it	Immediate improvement, no test needed
Untested language features	Add e2e test fixtures	Medium effort, high impact
Error handling paths	Add compile_error test fixtures	Low effort
Optimizer-specific paths	Add wir_expect/wir_not_expect tests	Medium effort
Edge cases in codegen	Add targeted e2e tests	Varies

Step 4: Analyze a Specific File

To see exactly which lines are uncovered in a file:

# Generate lcov and extract file-specific data
cargo llvm-cov --lcov -p wado-compiler 2>/dev/null > /tmp/lcov.info

# Or use the HTML report
cargo llvm-cov --html -p wado-compiler
# Then read target/llvm-cov/html/src/synthesis/inspect.rs.html etc.

To find dead code (functions never called):

# Check if a function has any callers
rg 'function_name' wado-compiler/src/ --type rust

Step 5: Add E2E Tests

E2E tests are .wado files in wado-compiler/tests/fixtures/. Each file has a __DATA__ section with JSON expectations.

Guidelines for test files:

• Name files based on language features, not compiler components (e.g., closure_nested.wado not codegen_closure.wado)
• Prefer adding test cases to existing fixture files when the feature group matches
• Each fixture group shares a filename prefix (e.g., closure_*, global_*, match_*)
• After adding new files: touch wado-compiler/tests/e2e.rs to trigger rediscovery

Test patterns that maximize coverage:

• test "..." { ... } with {"test": {}} — exercises codegen without needing stdout
• compile_error tests — exercise error reporting paths
• wir_expect:O2 / wir_not_expect:O2 — exercise optimizer paths
• Template strings with {expr:?} — exercise inspect/display synthesis

Step 6: Verify Improvement

After changes, re-run coverage and compare:

# Before
cargo llvm-cov --json -p wado-compiler 2>/dev/null | jq '.data[0].totals.lines.percent'

# After changes
cargo llvm-cov --json -p wado-compiler 2>/dev/null | jq '.data[0].totals.lines.percent'

Key Insights from Past Investigations

• synthesis/inspect.rs (~1400 lines) has dead code for type variants that never appear in practice (e.g., Never type inspect). Deleting dead code directly improves coverage.
• codegen.rs and emit.rs are the largest files but have relatively high coverage. Incremental improvements here require many targeted tests.
• The compiler's test suite is driven by datatest_mini which discovers .wado fixtures at compile time — new files need touch wado-compiler/tests/e2e.rs.
• wado test runs test-world fixtures; wado run runs CLI-world fixtures. Choose the right world for your test.
• Coverage runs all optimization levels by default in CI; locally only O0 and O2 run unless WADO_FULL_TEST=1.