A test in Rust is a function that’s annotated with the test attribute. To change a function into a test function, add #[test] on the line before fn. And then use cargo test to run the tests, in which Rust builds a test runner binary of the annotated functions and reports.

• assert!(condition) - panic when condition is not true
• assert_eq!(left, right) - panic when left is not equal to right
• assert_en!(left, right) - panic when left is equal to right

We can also add custom error message in assert! macro by passing the message next to the condition.

If we know that a function should panic, we can add the attribute #[should_panic] in the line before fn and after #[test].

We can also make the tested function return a Result<T, E>. And if the function returns an error type, the test fails. Otherwise, the test is passed.

Control How Tests are Run

By default, Rust would parallelize the tests when running. To make sure tests are running only by one thread, do cargo test -- --test-threads=1

• Show the stdout from functions: cargo test -- --show-output
• Running a subset of tests by name: cargo test <pattern>
• Ignore a test case by adding #[ignore]
• Run the ignored tests only: cargo test -- --ignored