Cargo Coverage

Overview

Generate detailed, per-file coverage reports using cargo llvm-cov with JSON output formatted into readable tables. Provides function, line, and region coverage percentages.

Quick Start

1. Run tests with coverage enabled first (see cargo-test skill).
2. Execute skills/cargo-coverage/scripts/run_coverage.sh from repository root.
3. Review the coverage table output showing per-file statistics.

Workflow

1. Generate coverage data: Run tests with cargo llvm-cov nextest or cargo llvm-cov test.
2. Extract report: cargo llvm-cov report --json --summary-only outputs coverage in JSON format.
3. Format output: Pipe through jq and xan to create readable tables.
4. Environment:
   • NO_COLOR=1 - Disables color output
   • CARGO_TERM_COLOR=never - Ensures cargo output is colorless

Coverage Metrics

• Functions: Percentage of functions executed during tests
• Lines: Percentage of code lines covered
• Regions: Percentage of code regions (branches, conditionals) covered

Best Practices

• Run tests first: Coverage data is generated by running tests with instrumentation.
• Use colorless output: Keep NO_COLOR=1 for CI/log parsing.
• Workspace scope: Report includes all workspace members.
• Run from root: Execute from repository root for correct file path resolution.
• Rerun after changes: Regenerate coverage after modifying tests or source code.

Common Commands

Generate and view coverage report

cargo llvm-cov nextest --workspace --all-features
cargo llvm-cov report --json --summary-only | jq

Coverage with specific format

cargo llvm-cov report --lcov --output-path coverage.lcov
cargo llvm-cov report --html

Failure Handling

• No coverage data: Run tests with coverage enabled first.
• Missing tools: Install jq and xan via package manager or cargo.
• Path errors: Ensure running from repository root directory.