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

Coverage Level	Interpretation
90%+	Excellent - well-tested code
75-90%	Good - most paths covered
50-75%	Fair - significant gaps exist
<50%	Needs attention - many untested paths

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