Testing

Testing async code

Standard unit tests (#[test]) cannot test async code. To test async code, you need an async runtime like tokio or async-std.

Using tokio

[dev-dependencies]
tokio = { version = "1", features = ["macros", "rt"] }

Use #[tokio::test] instead of #[test].

Using async-std

[dev-dependencies]
async-std = { version = "1", features = ["attributes"] }

Use #[async_std::test] instead of #[test].

Mocking dependencies

Use mockall crate https://crates.io/crates/mockall.

use mockall::automock;

#[automock]
pub trait Database {
    async fn get_user(&self, id: u32) -> u32;
}

automock gives you a mock implementation with the prefix Mock (in the above example, it is MockDatabase). It generates methods on the mock that can be used to setup expectations and verify the expectations

#[tokio::test]
async fn test_with_mock() {
    let mut mock = MockDatabase::new();

    // Setup expectations
    mock.expect_get_user()
        .with(mockall::predicate::eq(5))
        .returning(|_| 42); // Returns a value, not a future!

    // Actual call that exercises the mock
    let result = mock.get_user(5).await;
    assert_eq!(result, 42);
}

Mocking HTTP calls

Mocking reqwest is difficult due to the fact that you cannot create the reqwest::Response or reqwest::Client objects. There are two options

1. Mock the network server by starting a fake local server if you care about the HTTP details.
2. Mock the logic (hide the reqwest behind a trait) if you don’t care about the HTTP details.
   1. Do not return reqwest::Response in your trait. Return your domain objects like User (e.g., Result<User, Error>)

Use wiremock to spin up a local server.

Hide the dependency logic behind a trait and mock the trait for tests using mockall.

Code coverage

Generate code coverage data by using cargo tarpaulin https://crates.io/crates/cargo-tarpaulin

# This command takes about 2m
cargo install cargo-tarpaulin

# On-demand
cargo tarpaulin --out Lcov

# Auto-generation
cargo watch -x "tarpaulin --out Lcov"

View the coverage data in vscode using Coverage Gutters by ryanluker. Coverage Gutters requires an LCOV file.

Show coverage by using the command >Coverage Gutters: Display Coverage.

Code coverage using nextest and cargo-llvm-cov

cargo install cargo-nextest --locked
cargo +stable install cargo-llvm-cov --locked

https://nattrio.medium.com/visualizing-rust-code-coverage-in-vs-code-781aaf334f11

Generate the coverage report

cargo llvm-cov nextest
# Generate coverage report in lcov format for vscode
cargo llvm-cov nextest --lcov --output-path ./target/lcov.info
cargo watch -x 'llvm-cov nextest --lcov --output-path ./target/lcov.info' -w src

Open the report in HTML format

cargo llvm-cov nextest --open