cargo-llvm-cov

cargo-llvm-cov - Code Coverage with LLVM

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Install skill "cargo-llvm-cov" with this command: npx skills add laurigates/claude-plugins/laurigates-claude-plugins-cargo-llvm-cov

cargo-llvm-cov - Code Coverage with LLVM

cargo-llvm-cov provides accurate code coverage for Rust using LLVM's instrumentation-based coverage. It supports multiple output formats and integrates seamlessly with CI platforms.

Installation

Install cargo-llvm-cov

cargo install cargo-llvm-cov

Verify installation

cargo llvm-cov --version

Install llvm-tools-preview component (required)

rustup component add llvm-tools-preview

Basic Usage

Run tests and generate coverage

cargo llvm-cov

Generate HTML report

cargo llvm-cov --html open target/llvm-cov/html/index.html

Generate LCOV report

cargo llvm-cov --lcov --output-path target/llvm-cov/lcov.info

Generate JSON report

cargo llvm-cov --json --output-path target/llvm-cov/coverage.json

Show coverage as text summary

cargo llvm-cov --text

Show coverage for specific files

cargo llvm-cov --text -- --show-instantiations

Output Formats

HTML report (interactive, line-by-line)

cargo llvm-cov --html --open

LCOV format (compatible with many tools)

cargo llvm-cov --lcov --output-path lcov.info

JSON format (programmatic analysis)

cargo llvm-cov --json --output-path coverage.json

Cobertura XML (for Jenkins, GitLab)

cargo llvm-cov --cobertura --output-path cobertura.xml

Text summary (terminal output)

cargo llvm-cov --text

Multiple formats simultaneously

cargo llvm-cov --html --lcov --output-path lcov.info

Coverage Thresholds for CI

Fail if coverage is below threshold

cargo llvm-cov --fail-under-lines 80

Multiple threshold types

cargo llvm-cov --fail-under-lines 80 --fail-under-functions 75

Available threshold types

cargo llvm-cov --fail-under-lines 80 # Line coverage cargo llvm-cov --fail-under-regions 80 # Region coverage cargo llvm-cov --fail-under-functions 75 # Function coverage

Threshold Configuration

Create a script or Makefile for consistent thresholds:

Makefile

.PHONY: coverage coverage-ci

coverage: cargo llvm-cov --html --open

coverage-ci: cargo llvm-cov
--fail-under-lines 80
--fail-under-functions 75
--lcov --output-path lcov.info

Or use a shell script:

#!/usr/bin/env bash

scripts/coverage.sh

set -euo pipefail

COVERAGE_THRESHOLD="${COVERAGE_THRESHOLD:-80}"

cargo llvm-cov
--fail-under-lines "$COVERAGE_THRESHOLD"
--lcov --output-path target/llvm-cov/lcov.info
--html

echo "Coverage threshold: $COVERAGE_THRESHOLD% (lines)"

Branch Coverage (Nightly)

Branch coverage requires Rust nightly:

Install nightly toolchain

rustup toolchain install nightly rustup component add llvm-tools-preview --toolchain nightly

Run with branch coverage

cargo +nightly llvm-cov --branch --html

Branch coverage with thresholds

cargo +nightly llvm-cov
--branch
--fail-under-lines 80
--fail-under-branches 70

Branch Coverage Configuration

rust-toolchain.toml

[toolchain] channel = "nightly" components = ["llvm-tools-preview"]

Allows using cargo llvm-cov --branch without +nightly

Integration with cargo-nextest

Use nextest as test runner

cargo llvm-cov nextest --html

With nextest profile

cargo llvm-cov nextest --profile ci --lcov --output-path lcov.info

All nextest options work

cargo llvm-cov nextest -E 'not test(slow_)' --html

Codecov Integration

.github/workflows/coverage.yml

name: Coverage

on: [push, pull_request]

jobs: coverage: runs-on: ubuntu-latest steps: - uses: actions/checkout@v4

  - uses: dtolnay/rust-toolchain@stable
    with:
      components: llvm-tools-preview

  - name: Install cargo-llvm-cov
    uses: taiki-e/install-action@v2
    with:
      tool: cargo-llvm-cov

  - name: Generate coverage
    run: cargo llvm-cov --all-features --lcov --output-path lcov.info

  - name: Upload to codecov
    uses: codecov/codecov-action@v4
    with:
      files: lcov.info
      fail_ci_if_error: true
      token: ${{ secrets.CODECOV_TOKEN }}

Coveralls Integration

.github/workflows/coverage.yml

name: Coverage

on: [push, pull_request]

jobs: coverage: runs-on: ubuntu-latest steps: - uses: actions/checkout@v4

  - uses: dtolnay/rust-toolchain@stable
    with:
      components: llvm-tools-preview

  - name: Install cargo-llvm-cov
    uses: taiki-e/install-action@v2
    with:
      tool: cargo-llvm-cov

  - name: Generate coverage
    run: cargo llvm-cov --all-features --lcov --output-path lcov.info

  - name: Upload to Coveralls
    uses: coverallsapp/github-action@v2
    with:
      github-token: ${{ secrets.GITHUB_TOKEN }}
      path-to-lcov: lcov.info

Advanced Configuration

Exclude Files from Coverage

Exclude generated code

cargo llvm-cov --ignore-filename-regex '.generated.'

Exclude test files

cargo llvm-cov --ignore-filename-regex '.test.'

Multiple patterns

cargo llvm-cov
--ignore-filename-regex '.generated.'
--ignore-filename-regex '.mock.'

Workspace Coverage

Coverage for all workspace members

cargo llvm-cov --workspace --html

Coverage for specific packages

cargo llvm-cov -p my_lib -p my_app --html

Exclude specific packages

cargo llvm-cov --workspace --exclude integration_tests --html

Coverage with Feature Flags

Coverage with all features

cargo llvm-cov --all-features --html

Coverage with specific features

cargo llvm-cov --features async,tls --html

Coverage without default features

cargo llvm-cov --no-default-features --html

GitHub Actions: Complete Example

.github/workflows/coverage.yml

name: Coverage

on: push: branches: [main] pull_request:

env: CARGO_TERM_COLOR: always COVERAGE_THRESHOLD: 80

jobs: coverage: runs-on: ubuntu-latest steps: - uses: actions/checkout@v4

  - uses: dtolnay/rust-toolchain@stable
    with:
      components: llvm-tools-preview

  - uses: Swatinem/rust-cache@v2

  - name: Install cargo-llvm-cov
    uses: taiki-e/install-action@v2
    with:
      tool: cargo-llvm-cov

  - name: Install cargo-nextest
    uses: taiki-e/install-action@v2
    with:
      tool: nextest

  - name: Generate coverage
    run: |
      cargo llvm-cov nextest \
        --all-features \
        --fail-under-lines $COVERAGE_THRESHOLD \
        --lcov --output-path lcov.info \
        --html

  - name: Upload coverage to Codecov
    uses: codecov/codecov-action@v4
    with:
      files: lcov.info
      fail_ci_if_error: true
      token: ${{ secrets.CODECOV_TOKEN }}

  - name: Upload HTML report
    uses: actions/upload-artifact@v4
    if: always()
    with:
      name: coverage-report
      path: target/llvm-cov/html/

Clean Coverage Data

Clean previous coverage data

cargo llvm-cov clean

Clean and run fresh coverage

cargo llvm-cov clean && cargo llvm-cov --html

Doctests Coverage

Include doctests in coverage

cargo llvm-cov --doc --html

Doctests with workspace

cargo llvm-cov --workspace --doc --html

Note: doctests run in separate context, may not integrate perfectly

Comparison with tarpaulin

Feature cargo-llvm-cov cargo-tarpaulin

Backend LLVM (compiler) ptrace (runtime)

Accuracy Excellent Good

Performance Fast Slower

Branch coverage Yes (nightly) Yes

Stability Stable Sometimes unstable

Platform support All Rust targets Linux, limited macOS

Setup complexity Simple Simple

Best Practices

Use with nextest for faster execution:

cargo llvm-cov nextest --all-features --html

Enforce coverage thresholds in CI:

cargo llvm-cov --fail-under-lines 80

Generate multiple formats for different tools:

cargo llvm-cov --html --lcov --output-path lcov.info

Exclude generated code from coverage:

cargo llvm-cov --ignore-filename-regex '.generated.'

Cache coverage dependencies in CI:

  • uses: Swatinem/rust-cache@v2

Use branch coverage on nightly for critical code:

cargo +nightly llvm-cov --branch --fail-under-branches 70

Clean coverage data between runs:

cargo llvm-cov clean

Troubleshooting

Missing llvm-tools-preview:

rustup component add llvm-tools-preview

Coverage seems inaccurate:

Clean and regenerate

cargo llvm-cov clean cargo clean cargo llvm-cov --html

Nightly features not working:

rustup toolchain install nightly rustup component add llvm-tools-preview --toolchain nightly cargo +nightly llvm-cov --branch --html

Slow coverage generation:

Use nextest for parallel execution

cargo llvm-cov nextest --html

Doctests not included:

Run with --doc flag

cargo llvm-cov --doc --html

References

  • cargo-llvm-cov documentation

  • LLVM coverage mapping

  • Codecov Rust

  • coveralls-api crate

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