cargo-nextest - Next-Generation Test Runner

cargo-nextest is a faster, more reliable test runner for Rust that executes each test in its own process for better isolation and parallel performance.

When to Use This Skill

Use this skill when...	Use sibling skill instead when...
Running tests in parallel with process isolation	Measuring code coverage -- use `cargo-llvm-cov`
Filtering tests by package, name, or pattern	Linting test code style -- use `clippy-advanced`
Setting up flaky-test retries in CI	Auditing unused test deps -- use `cargo-machete`
Optimizing test runtime via partitioning	Authoring tests / async patterns -- use `rust-development`

Installation

# Install cargo-nextest
cargo install cargo-nextest --locked

# Verify installation
cargo nextest --version

Basic Usage

# Run all tests with nextest
cargo nextest run

# Run specific test
cargo nextest run test_name

# Run tests in specific package
cargo nextest run -p package_name

# Run with verbose output
cargo nextest run --verbose

# Run ignored tests
cargo nextest run -- --ignored

# Run all tests including ignored
cargo nextest run -- --include-ignored

Configuration File

Create .config/nextest.toml in your project root:

[profile.default]
# Number of retries for flaky tests
retries = 0

# Test threads (default: number of logical CPUs)
test-threads = 8

# Fail fast - stop on first failure
fail-fast = false

# Show output for passing tests
success-output = "never"  # never, immediate, final, immediate-final

# Show output for failing tests
failure-output = "immediate"  # never, immediate, final, immediate-final

[profile.ci]
# CI-specific configuration
retries = 2
fail-fast = true
success-output = "never"
failure-output = "immediate-final"

# Slow test timeout
slow-timeout = { period = "60s", terminate-after = 2 }

[profile.ci.junit]
# JUnit XML output for CI
path = "target/nextest/ci/junit.xml"

Test Filtering with Expression Language

# Run tests matching pattern
cargo nextest run -E 'test(auth)'

# Run tests in specific binary
cargo nextest run -E 'binary(my_app)'

# Run tests in specific package
cargo nextest run -E 'package(my_crate)'

# Complex expressions with operators
cargo nextest run -E 'test(auth) and not test(slow)'

# Run all integration tests
cargo nextest run -E 'kind(test)'

# Run only unit tests in library
cargo nextest run -E 'kind(lib) and test(/)'

Expression Operators

test(pattern) - Match test name (regex)
binary(pattern) - Match binary name
package(pattern) - Match package name
kind(type) - Match test kind (lib, bin, test, bench, example)
platform(os) - Match target platform
not expr - Logical NOT
expr and expr - Logical AND
expr or expr - Logical OR

Parallel Execution

Nextest runs each test in its own process by default, providing:

Better isolation - Tests cannot interfere with each other
True parallelism - No global state conflicts
Fault isolation - One test crash doesn't affect others
Better resource management - Each test has clean environment

# Control parallelism
cargo nextest run --test-threads 4

# Single-threaded execution
cargo nextest run --test-threads 1

# Use all available cores
cargo nextest run --test-threads 0

Output Formats

# Human-readable output (default)
cargo nextest run

# JSON output for programmatic parsing
cargo nextest run --message-format json

# JSON with test output
cargo nextest run --message-format json-pretty

# Generate JUnit XML for CI
cargo nextest run --profile ci

# Combine with cargo-llvm-cov for coverage
cargo llvm-cov nextest --html

Flaky Test Management

# In .config/nextest.toml
[profile.default]
# Retry flaky tests automatically
retries = 3

# Detect tests that pass on retry
[profile.default.overrides]
filter = 'test(flaky_)'
retries = 5

# Run with retries
cargo nextest run --retries 3

# Show retry statistics
cargo nextest run --retries 3 --failure-output immediate-final

GitHub Actions Integration

name: Test

on: [push, pull_request]

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

      - uses: dtolnay/rust-toolchain@stable

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

      - name: Run tests
        run: cargo nextest run --profile ci --all-features

      - name: Upload test results
        if: always()
        uses: actions/upload-artifact@v4
        with:
          name: test-results
          path: target/nextest/ci/junit.xml

      - name: Publish test results
        uses: EnricoMi/publish-unit-test-result-action@v2
        if: always()
        with:
          files: target/nextest/ci/junit.xml

Advanced Configuration

# .config/nextest.toml
[profile.default]
# Timeout for slow tests
slow-timeout = { period = "30s", terminate-after = 3 }

# Test groups for resource management
[test-groups.database]
max-threads = 1  # Serialize database tests

[profile.default.overrides]
filter = 'test(db_)'
test-group = 'database'

# Platform-specific overrides
[[profile.default.overrides]]
platform = 'x86_64-pc-windows-msvc'
slow-timeout = { period = "60s" }

Doctests Limitation

Important: nextest does not support doctests. Use cargo test for doctests:

# Run doctests separately
cargo test --doc

# Run both nextest and doctests in CI
cargo nextest run && cargo test --doc

Workspace Configuration

# In workspace root .config/nextest.toml
[profile.default]
default-filter = 'all()'

# Per-package overrides
[[profile.default.overrides]]
filter = 'package(slow_tests)'
test-threads = 1
slow-timeout = { period = "120s" }

Comparison with cargo test

Feature	cargo test	cargo nextest
Execution model	In-process	Per-test process
Parallelism	Thread-based	Process-based
Test isolation	Shared state	Complete isolation
Output format	Limited	Rich (JSON, JUnit)
Flaky test handling	Manual	Built-in retries
Doctests	Supported	Not supported
Performance	Good	Excellent

Best Practices

Use profiles for different environments:
- default for local development
- ci for continuous integration
- coverage for coverage analysis
Configure flaky test detection:
```
[profile.default]
retries = 2
```

Group resource-intensive tests:

[test-groups.expensive]
max-threads = 2

Set appropriate timeouts:

[profile.default]
slow-timeout = { period = "60s", terminate-after = 2 }

Use expression filters effectively:

# Run fast tests during development
cargo nextest run -E 'not test(slow_)'

Always run doctests separately in CI:

- run: cargo nextest run --all-features
- run: cargo test --doc --all-features

Troubleshooting

Tests timeout too quickly:

[profile.default]
slow-timeout = { period = "120s", terminate-after = 3 }

Too much parallel execution:

cargo nextest run --test-threads 4

Need test output for debugging:

cargo nextest run --success-output immediate --failure-output immediate

Flaky tests in CI:

[profile.ci]
retries = 3
failure-output = "immediate-final"

cargo-nextest