rust-coverage-report

name: rust-coverage-report description: Generate comprehensive code coverage reports for Rust workspaces using cargo-llvm-cov and nextest. Produces formatted markdown tables with per-crate and per-file coverage breakdowns, including collapsible detail sections. Use when the user asks for test coverage, coverage reports, or wants to analyze which parts of their Rust codebase lack tests. compatibility: Requires cargo-llvm-cov, cargo-nextest, and Python 3. Works with Cargo workspaces. metadata: author: tgies version: "1.0"

Rust Coverage Report Skill

This skill generates detailed code coverage reports for Rust projects using cargo-llvm-cov and cargo-nextest.

Prerequisites

Ensure these tools are installed:

cargo install cargo-llvm-cov
cargo install cargo-nextest

Workflow

1. Run coverage collection

Generate coverage data in JSON format:

cargo llvm-cov nextest --workspace --all-features --json --output-path coverage.json

Options:

• Use -p <crate> to limit to a specific crate
• Remove --all-features if you want default feature coverage only
• Add --ignore-filename-regex <pattern> to exclude files

2. Generate the report

Run the included Python script to parse the JSON and produce a markdown report:

python3 scripts/generate_coverage_report.py coverage.json

This outputs:

• A summary table showing coverage by crate
• Detailed tables for each crate showing per-file coverage

3. Format the output

When presenting results to the user, use this structure:

Summary by Crate (always visible)

| Crate | Covered Lines | Total Lines | Percentage |
| :--- | :--- | :--- | :--- |
| crate-name | 1387 | 1397 | **99.28%** |

Per-Crate Details (use collapsible sections)

<details>
<summary><b>crate-name (99.28%)</b></summary>

| File | Covered Lines | Total Lines | Percentage |
| :--- | :--- | :--- | :--- |
| src/lib.rs | 263 | 265 | 99.25% |
| src/utils.rs | 234 | 234 | 100.00% |
</details>

Interpreting Results

Common Low-Coverage Patterns

• 0% on a file: Often indicates placeholder/stub code or entirely untested modules
• CLI command handlers: Often lower coverage due to integration test limitations
• Error paths: Easy to miss; consider adding error injection tests

Optional: Running alongside /test workflow

If you have a /test workflow, run coverage after confirming all tests pass:

# First verify tests pass
cargo fmt --all -- --check
cargo clippy --workspace --all-targets --all-features -- -D warnings
cargo nextest run --workspace --all-features

# Then collect coverage
cargo llvm-cov nextest --workspace --all-features --json --output-path coverage.json
python3 scripts/generate_coverage_report.py coverage.json

Cleanup

Remove generated files when done:

rm -f coverage.json