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 implement the Termination trait, for example:

  • ()
  • Result<T, E> where T: Termination, E: Debug
  • !

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

The test harness calls the returned value’s report method, and classifies the test as passed or failed depending on whether the resulting ExitCode represents successful termination. In particular:

  • 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 return ExitCode::SUCCESS pass, and tests that return ExitCode::FAILURE fail.
  • Tests that do not terminate neither pass nor fail.
#![allow(unused)]
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)]
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)]
fn main() {
#[test]
#[should_panic(expected = "values don't match")]
fn mytest() {
    assert_eq!(1, 2, "values don't match");
}
}