Testing - The Rust Reference

# Testing attributes

The following attributes are used for specifying functions for performing tests. Compiling a crate in "test" mode enables building the test functions along with a test harness for executing the tests. Enabling the test mode also enables the `test` conditional compilation option.

## The `test` attribute

The `test` attribute marks a function to be executed as a test. These functions are only compiled when in test mode. Test functions must be free, monomorphic functions that take no arguments, and the return type must be one of the following:

- `()`
- `Result<(), E> where E: Error`

> Note: The implementation of which return types are allowed is determined by the unstable `Termination` trait.

> Note: The test mode is enabled by passing the `--test` argument to `rustc` or using `cargo test`.

Tests that return `()` pass as long as they terminate and do not panic. Tests that return a `Result<(), E>` pass as long as they return `Ok(())`. Tests that do not terminate neither pass nor fail.

    #![allow(unused_variables)]
    fn main() {
    use std::io;
    fn setup_the_thing() -> io::Result<i32> { Ok(1) }
    fn do_the_thing(s: &i32) -> io::Result<()> { Ok(()) }
    #[test]
    fn test_the_thing() -> io::Result<()> {
        let state = setup_the_thing()?; // expected to succeed
        do_the_thing(&state)?;          // expected to succeed
        Ok(())
    }
    }

## The `ignore` attribute

A function annotated with the `test` attribute can also be annotated with the `ignore` attribute. The `ignore` attribute tells the test harness to not execute that function as a test. It will still be compiled when in test mode.

The `ignore` attribute may optionally be written with the MetaNameValueStr syntax to specify a reason why the test is ignored.

    #![allow(unused_variables)]
    fn main() {
    #[test]
    #[ignore = "not yet implemented"]
    fn mytest() {
        // …
    }
    }

> Note: The `rustc` test harness supports the `--include-ignored` flag to force ignored tests to be run.

## The `should_panic` attribute

A function annotated with the `test` attribute that returns `()` can also be annotated with the `should_panic` attribute. The `should_panic` attribute makes the test only pass if it actually panics.

The `should_panic` attribute may optionally take an input string that must appear within the panic message. If the string is not found in the message, then the test will fail. The string may be passed using the MetaNameValueStr syntax or the MetaListNameValueStr syntax with an `expected` field.

    #![allow(unused_variables)]
    fn main() {
    #[test]
    #[should_panic(expected = "values don't match")]
    fn mytest() {
        assert_eq!(1, 2, "values don't match");
    }
    }