rstest-tests

rstest-tests

Overview

Per github.com/la10736/rstest:

rstest is a Rust crate for parametrize + fixture testing - features that Rust's stdlib #[test] lacks. Pairs with cargo-test (rstest tests are still discovered by cargo test).

When to use

• Same input pattern repeated across many tests (parametrize wins).
• Setup code shared across multiple tests (fixture wins over mod tests shared state).
• Async test patterns benefit from rstest's async-fixture support.
• Migration from pytest/JUnit5 patterns to Rust.

Step 1 - Install

Cargo.toml:

[dev-dependencies]
rstest = "0.21"

Step 2 - Parametrize with #[case]

use rstest::rstest;

#[rstest]
#[case(1, 2, 3)]
#[case(0, 0, 0)]
#[case(-1, 1, 0)]
#[case(100, 200, 300)]
fn add_cases(#[case] a: i32, #[case] b: i32, #[case] expected: i32) {
    assert_eq!(add(a, b), expected);
}

Each #[case] runs as a separate test - failures don't stop subsequent cases. Test names become add_cases::case_1, add_cases::case_2, etc.

Step 3 - Named cases

#[rstest]
#[case::positive(1, 2, 3)]
#[case::zero(0, 0, 0)]
#[case::negative(-1, 1, 0)]
fn add_named(#[case] a: i32, #[case] b: i32, #[case] expected: i32) {
    assert_eq!(add(a, b), expected);
}

Test names: add_named::positive, add_named::zero, etc. - much better for failure logs.

Step 4 - Fixtures

use rstest::{fixture, rstest};

#[fixture]
fn db() -> Database {
    Database::new_test_instance()
}

#[fixture]
fn user(db: Database) -> User {
    db.create_user("alice")
}

#[rstest]
fn test_user_id(user: User) {
    assert_eq!(user.id, 1);
}

#[rstest]
fn test_user_email(user: User, db: Database) {
    assert_eq!(db.find_user(user.id).email, "[email protected]");
}

Fixtures are functions with #[fixture]; tests with #[rstest] auto-receive them via parameter name matching. Fixture chaining works (a fixture can request other fixtures).

Step 5 - Matrix tests (cartesian product)

#[rstest]
fn test_matrix(
    #[values("alice", "bob", "charlie")] name: &str,
    #[values(0, 18, 65)] age: u32,
) {
    let user = User::new(name, age);
    assert!(user.is_valid());
}

Runs 3 × 3 = 9 tests with all combinations. Equivalent to nested #[case] for cartesian product.

Step 6 - Combined #[case] + #[values]

#[rstest]
#[case::happy("alice")]
#[case::edge("a")]
fn test_with_values(
    #[case] base_name: &str,
    #[values(0, 1, 100)] count: u32,
) {
    // happy × 3 values = 3 tests
    // edge × 3 values = 3 tests
}

Step 7 - Async tests

use rstest::rstest;

#[rstest]
#[case(1, 2, 3)]
#[case(0, 0, 0)]
#[tokio::test]
async fn async_add_cases(#[case] a: i32, #[case] b: i32, #[case] expected: i32) {
    let result = add_async(a, b).await;
    assert_eq!(result, expected);
}

For async fixtures, use #[future]:

use rstest::*;

#[fixture]
async fn db_async() -> Database {
    Database::connect_async().await.unwrap()
}

#[rstest]
#[tokio::test]
async fn test_async(#[future] db_async: Database) {
    let db = db_async.await;
    assert!(db.is_connected());
}

Step 8 - Indirect fixtures (parameterize fixture instances)

#[fixture]
fn user(#[default("alice")] name: &str) -> User {
    User::new(name)
}

#[rstest]
#[case::alice("alice")]
#[case::bob("bob")]
fn test_user(#[case] expected_name: &str, #[with(expected_name)] user: User) {
    assert_eq!(user.name, expected_name);
}

#[with(...)] injects custom args into the fixture for that test.

Step 9 - CI integration

Same as plain cargo test:

- run: cargo test --all-targets

rstest tests are discovered + run by cargo test natively - no separate runner.

Anti-patterns

Anti-pattern	Why it fails	Fix
Use rstest for single-case tests	Adds dependency for no benefit	Plain #[test] for single cases
Skip #[case::name]	Test names become case_1, case_2; debug-hostile	Always name cases (Step 3)
Heavy matrix (5 dims × 5 values = 3125 tests)	Combinatorial explosion	Strategic cases vs full matrix
Mix sync + async tests in same parametrize	Confusing	Separate #[rstest] blocks per sync/async

Limitations

• Adds a dev dependency for what's syntactic sugar over #[test].
• Test name generation can be opaque without explicit case names.
• Some IDE integrations don't show parametrized test cases as separate items.
• Async fixture support is solid but newer; pin recent rstest version.

References

• rs-gh - rstest repository + docs
• crates.io/crates/rstest - published crate
• docs.rs/rstest - API reference
• go-test, ginkgo-tests, cargo-test - sister tools
• proptest-testing - Rust property-based
• test-code-conventions