cargo-test

Overview

Per doc.rust-lang.org/book/ch11-00-testing.html (opens in new window):

Rust's testing is built into Cargo. The #[test] attribute marks test functions; cargo test discovers and runs them.

Three test categories per the Rust Book (opens in new window):

How to use

1. Put unit tests in a #[cfg(test)] mod tests { ... } block next to the code; put cross-crate scenarios in tests/ (Steps 1, 5).
2. Assert with assert_eq! / assert_ne! / assert!; return Result<(), E> and use ? instead of unwrap() (Steps 2, 4).
3. Cover error paths with #[should_panic(expected = "...")], and mark slow/networked tests #[ignore = "reason"] (Steps 3, 8).
4. Run cargo test --all-targets --workspace locally; filter by name pattern and add -- --nocapture to see println! output (Steps 1, 7).
5. Keep documented examples honest with doc tests in /// comments, verified by cargo test --doc (Step 6).
6. Gate coverage and track performance with cargo llvm-cov and Criterion (see references/coverage-and-benchmarks.md).
7. Wire the same commands into CI so every push runs tests, doc tests, and the coverage gate (Step 10).

Step 1 - Unit tests

// src/math.rs
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn adds_two_numbers() {
        assert_eq!(add(1, 2), 3);
    }

    #[test]
    fn handles_zero() {
        assert_eq!(add(0, 0), 0);
    }
}

Run:

cargo test                 # all tests
cargo test add             # filter by name pattern
cargo test --lib           # only unit tests in lib
cargo test --bins          # binary tests
cargo test --all-targets   # everything
cargo test --workspace     # multi-crate workspace

Step 2 - Assertion macros

assert!(condition);
assert!(condition, "format message: {}", value);
assert_eq!(actual, expected);
assert_eq!(actual, expected, "with message");
assert_ne!(actual, unexpected);

// Custom error message:
assert!(actual > 0, "Expected positive, got {}", actual);

For richer assertions, the pretty_assertions crate provides diff-rich macros:

use pretty_assertions::assert_eq;
assert_eq!(actual_complex_struct, expected_complex_struct);
// shows colorized diff on failure

Step 3 - #[should_panic]

#[test]
#[should_panic]
fn panics_on_negative() {
    sqrt(-1.0);
}

#[test]
#[should_panic(expected = "negative")]
fn specific_panic_message() {
    sqrt(-1.0);
    // panics with message containing "negative"
}

Step 4 - Result return for ?-style propagation

#[test]
fn parses_config() -> Result<(), Box<dyn Error>> {
    let cfg = Config::from_file("test/fixtures/config.toml")?;
    assert_eq!(cfg.port, 8080);
    Ok(())
}

Test passes if Ok(()); fails if Err(...). Avoids unwrap() boilerplate; allows ? operator in test bodies.

Step 5 - Integration tests

my-crate/
  src/
    lib.rs
  tests/
    integration_test.rs    # automatically discovered
    common/
      mod.rs               # shared helpers (note: NO mod.rs in tests/ root)

// tests/integration_test.rs
use my_crate::Calculator;

#[test]
fn end_to_end() {
    let c = Calculator::new();
    assert_eq!(c.add(1, 2), 3);
}

mod common;   // shared test helpers from tests/common/mod.rs

Each file in tests/ compiles to its own binary - slower but better-isolated than unit tests.

Step 6 - Doc tests

/// Adds two numbers.
///
/// # Examples
///
/// ```
/// use my_crate::math::add;
/// assert_eq!(add(1, 2), 3);
/// ```
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

cargo test --doc runs only doc tests; cargo test runs everything including docs. The example IS the test - drift between docs + implementation is caught.

# Examples heading is convention; required for cargo doc HTML rendering to show the example.

Step 7 - Common test runner flags

cargo test -- --test-threads=1           # serial (single-threaded)
cargo test -- --nocapture                 # show stdout (println in tests)
cargo test -- --show-output               # show stdout for passed tests too
cargo test -- --ignored                   # only run #[ignore]-marked tests
cargo test -- --include-ignored           # run both ignored + normal
cargo test -- --list                      # list tests without running
cargo test some_pattern -- --exact        # exact match (no substring)

Step 8 - #[ignore] for slow tests

#[test]
#[ignore = "Requires network access"]
fn integration_with_external_api() {
    // ...
}

Run via cargo test -- --include-ignored.

Step 9 - Coverage and benchmarks

Both need an extra crate installed (cargo-llvm-cov / cargo-tarpaulin for coverage; Criterion for benchmarks on stable). Full commands, the Cargo.toml bench wiring, and the Criterion example are in references/coverage-and-benchmarks.md.

Step 10 - CI integration

- run: cargo test --all-targets --workspace
- run: cargo test --doc                # explicit doc tests
- run: cargo install cargo-llvm-cov
- run: cargo llvm-cov --lcov --output-path coverage.lcov
- uses: codecov/codecov-action@v4
  with: { files: coverage.lcov }

Worked example

A crate exposes add(a, b) in src/math.rs and you need it tested before release.

1. Add a #[cfg(test)] mod tests block with adds_two_numbers and handles_zero, each using assert_eq! (Step 1).
2. Add a doc test in the /// comment on add so the documented example is executed (Step 6).
3. Run cargo test --all-targets: the unit block prints test result: ok. 2 passed, then Doc-tests my_crate ... 1 passed.
4. Add cargo llvm-cov --fail-under-lines 80; it reports add fully covered and exits 0 (see references/coverage-and-benchmarks.md).
5. CI runs cargo test --all-targets --workspace plus the coverage gate on every push (Step 10).

Result: unit, doc, and coverage checks are all green - the function is release-ready.

Anti-patterns

Limitations

• No fixture concept beyond mod tests shared state.
• Doc tests slow to compile (each runs as separate doctest binary).
• No mocking library in stdlib - mockall is the community standard for trait mocking.
• No parametrize beyond hand-rolled loops or rstest (see rstest-tests).

References

• rust-test (opens in new window) - Rust Book Chapter 11 (testing)
• doc.rust-lang.org/cargo/commands/cargo-test.html - cargo test reference
• bheisler.github.io/criterion.rs - Criterion docs
• crates.io/crates/cargo-llvm-cov - coverage tool
• go-test, ginkgo-tests, rstest-tests - sister tools
• proptest-testing (qa-property-based) - Rust property-based
• test-code-conventions (qa-test-review)