rust-unit-tests

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:

Choosing a framework

1. stdlib #[test] is the default - built into the language, no Cargo.toml change needed.
2. rstest when the spec has 4+ input/output case pairs or setup shared across 3+ tests - #[rstest] + #[case] runs each pair as a named test, discovered by cargo test natively → references/rstest.md. Match an existing rstest convention (rstest in [dev-dependencies] AND #[rstest] usage in tests) rather than introducing it ad hoc.
3. Mocking trait boundaries → mockall, references/rust-mocking.md.
4. Property-based invariants → proptest-testing (qa-property-based plugin).

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);
    }
}

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

#[cfg(test)] keeps the module out of release builds.

Step 2 - Assertion macros

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

assert_eq! / assert_ne! print BOTH left and right on failure; bare assert!(x == y) only reports false (rust-test (opens in new window)). For diff-rich struct comparisons, the pretty_assertions crate colorizes the output.

Step 3 - #[should_panic] and Result returns

#[test]
#[should_panic(expected = "negative")]
fn specific_panic_message() {
    sqrt(-1.0);
}

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

The Result return allows ? in test bodies - a failing ? fails the test with the real error instead of "called unwrap on None".

Step 4 - Integration tests

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

Each file in tests/ compiles to its own binary - slower but better-isolated; only the crate's public API is visible.

Step 5 - 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. The example IS the test, so docs can't drift from the implementation.

Step 6 - Runner flags and #[ignore]

cargo test -- --test-threads=1           # serial
cargo test -- --nocapture                # show println! output
cargo test -- --ignored                  # only #[ignore]-marked tests
cargo test -- --include-ignored          # ignored + normal
cargo test some_pattern -- --exact       # exact name match

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

Always include the = "reason" or ignored tests get forgotten.

Step 7 - Coverage and benchmarks

Coverage needs an extra crate - cargo-llvm-cov (cross-platform, recommended) or cargo-tarpaulin (Linux-only):

cargo install cargo-llvm-cov
cargo llvm-cov --html
cargo llvm-cov --lcov --output-path coverage.lcov
cargo llvm-cov --fail-under-lines 80     # gate at 80%

Benchmarks on stable use Criterion (stdlib #[bench] is nightly-only and breaks CI):

[dev-dependencies]
criterion = "0.5"

[[bench]]
name = "math_bench"
harness = false

// benches/math_bench.rs
use criterion::{black_box, criterion_group, criterion_main, Criterion};
use my_crate::math::add;

fn bench_add(c: &mut Criterion) {
    c.bench_function("add 1 2", |b| b.iter(|| add(black_box(1), black_box(2))));
}

criterion_group!(benches, bench_add);
criterion_main!(benches);

Run cargo bench (bheisler.github.io/criterion.rs).

Step 8 - CI integration

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

Authoring conventions

When authoring a new unit test in an existing project:

1. Detect the framework: stdlib #[test] unless rstest is in [dev-dependencies] AND existing tests use #[rstest]. Conflicting signals → stop and ask.
2. Placement: the conventional unit-test idiom is an inline #[cfg(test)] mod tests block at the end of the source file; use a separate tests/<name>.rs only for public-API integration scenarios (doc.rust-lang.org/cargo/guide/tests).
3. One spec → one new test function; never modify existing tests, never fabricate symbols the module does not declare, no assert!(true) smoke asserts when the spec names a concrete value.
4. Async needs a runtime: a bare #[test] fn calling .await does not compile - use #[tokio::test] (when the project depends on Tokio) or rstest's #[future] injection (references/rstest.md).
5. Refuse universally-quantified specs ("any non-negative price") - property-based scope → proptest-testing (qa-property-based).

Anti-patterns

Limitations

• No fixture concept beyond mod tests shared state (rstest adds fixtures - references/rstest.md).
• Doc tests compile slowly (each is a separate doctest binary).
• No mocking in stdlib - mockall is the community standard (references/rust-mocking.md).
• No parametrize beyond hand-rolled loops or rstest.

References

• rust-test (opens in new window) - Rust Book Chapter 11 (testing)
• doc.rust-lang.org/cargo/commands/cargo-test.html - cargo test reference
• crates.io/crates/cargo-llvm-cov - coverage tool
• bheisler.github.io/criterion.rs - Criterion docs
• references/rstest.md - rstest parametrize + fixtures
• references/rust-mocking.md - mockall
• go-unit-tests - sister umbrella for Go
• proptest-testing (qa-property-based) - Rust property-based
• test-code-conventions (qa-test-review) - test code hygiene