#testing #async #futures


std::future::Future and Result-based testing trait for managing the lifecycle of stateful, asynchronous tests

2 releases

0.1.1 Jan 26, 2021
0.1.0 Aug 15, 2020

#127 in Asynchronous

MIT license



A std::future::Future and Result-based testing trait for managing the lifecycle of stateful, asynchronous tests.

Why spekt:

Most Rust unit tests terminate based on panics (generally triggered by assert!), with resource clean-up implemented manually through the Drop trait. Working synchronously with a stateful resource like a database might look like this:

use postgres::{Client, NoTls, Row, error::Error as PostgresError};

struct PostgresTest {
    client: Client

impl PostgresTest {
    fn new() -> Self  {
        let mut client = Client::connect("host=localhost user=postgres", NoTls).expect("Error connecting to database");

        Self { client }

    fn add_test_table(&self) -> Result<Row, PostgresError> {
        self.client.batch_execute("CREATE TABLE my_test_table ()")

impl Drop for PostgresTest {
    fn drop(&mut self) {
        self.client.batch_execute("DROP TABLE my_test_table").expect("Error cleaning up test table");

fn adds_queryable_test_table() {
    let client = TestClient::new();
    let create_response = client.adds_test_table();

    assert!(create_response.is_ok(), "Error creating test table");

    let query_response = client.query("SELECT FROM my_test_table");

    assert!(query_response.is_ok(), "Error creating test table");

While this works for many cases, there are a couple of issues with this recommendation:

  1. Technically, Rust doesn't guarantee that Drop will be run, and one shouldn't rely on Drop to be run in all cases.
  2. Drop also cannot be asynchronous! There has been much discussion around Asynchronous destructors, but no reliable destructor trait has yet materialized for async functions.
  3. panic-based assertions (and their associated unwinding) also behave in ways that might be unpredictable across runtimes. This is, specifically, an issue in tests for which there is no good universal solution.
  4. In addition, while new and Drop make sense for resources, those conventions make less sense for the more abstract idea of a "Test". In most testing frameworks, the idea of a "test" is the combination of a some stateful test context initialized before the actual test, a test case that can mutate its own context, and some clean-up to be run after the actual test.

spekt avoids all of these issues by providing a Test trait that encompasses the before -> test -> after lifecycle of stateful async tests that use Result to drive assertions.

How to use:

spekt::Test can be implemented for any Send + Sync test state, enabling a test() method that returns a std::future::Future. The returned Future is runtime-agnostic, and can be evaluated synchronously with .wait(), through a per-suite custom runtime (e.g. tokio::runtime::Runtime), or through an async test-runner like tokio::test.

Rewriting the example above with spekt::Test:

use tokio_postgres::{Client, NoTls, Row, error::Error as PostgresError};
use spekt::Test;

struct PostgresTest {
    client: Client

// spekt optionally re-exports async_trait
impl Test for PostgresTest {
    type Error = anyhow::Error; // any Error will do, but anyhow is recommended

    async fn before() -> Result<Self, Self::Error> {
        let mut client = Client::connect("host=localhost user=postgres", NoTls).await?;

        client.batch_execute("CREATE TABLE my_test_table ()").await?;

        Ok(Self { client })

    async fn after(&self) -> Result<(), Self::Error> {
        self.client.batch_execute("DROP TABLE my_test_table")?;


// any executor will do, but tokio::test is recommended
async fn adds_queryable_test_table() {
    // PostgresTest::test runs before() first, passes the output of before() to test() as context,
    // and finally runs after() regardless of the result of the test run itself,
    // bubbling all Self::Errors to top-level test failures
    PostgresTest::test(|context| async move {
        context.client.query("SELECT FROM my_test_table").await?;




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