Code coverage for Rust using LLVM instrumentation with support for multiple output formats and CI integration. Use when measuring test coverage, generating coverage reports, enforcing coverage thresholds, or integrating with codecov/coveralls. Trigger terms: coverage, llvm-cov, code coverage, test coverage, coverage report, codecov, coveralls, branch coverage.
/plugin marketplace add laurigates/claude-plugins/plugin install rust-plugin@lgates-claude-pluginsThis skill inherits all available tools. When active, it can use any tool Claude has access to.
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.
# 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
# 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
# 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
# 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
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 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
# rust-toolchain.toml
[toolchain]
channel = "nightly"
components = ["llvm-tools-preview"]
# Allows using `cargo llvm-cov --branch` without +nightly
# 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
# .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 }}
# .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
# 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.*'
# 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 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/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 previous coverage data
cargo llvm-cov clean
# Clean and run fresh coverage
cargo llvm-cov clean && cargo llvm-cov --html
# 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
| 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 |
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
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
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