We can create tests in one of the two ways: either by adding a tests directory with a lib.rs file or by simply adding a file with the tests for that module. As we are already using a directory structure, let's stay with that for the unit tests.

As previously discussed in Chapter 1, Introducing and Installing Rust, to add a unit test, we precede the code with the following:

#[test] 

Then to build, we need to do the following:

cargo test 

When we do this, though, we hit a problem. Our unit test file looks like this:

extern crate mathslib; 
use mathsLib::conversions::temperature; 
 
#[cfg(test)] 
mod temperature_tests 
{ 
    #[test] 
    fn test_kelvin_to_celcius_pass() 
    { 
        let calc = kelvin_to_celcius(14.5); 
        assert_eq!(calc.0, true); 
    } 
     
    #[test] 
    #[should_panic(expected = "assertion failed")] 
    fn test_kelvin_to_celcius_fail() 
    { 
        let calc = kelvin_to_celcius(-4f32); 
        assert_eq!(calc.0,true); 
    } 
} 

On the face of it, this should work, but it comes back with something that is somewhat perplexing:

This doesn't make sense; we know there is a module called Temperature, so why are we getting this message? The answer is that it's all down to the privacy of the module and functions.