Cargo complaining on unused super::* import on tests module

Question

I'm new to Rust and I'm trying to write some tests to my module, but cargo keeps complaining on my use super::*; import, which doesn't make sense for me. Look at this MCVE:

src/ga/world2.rs:

use crate::ga::individual::*;

#[derive(Debug, Clone)]
pub struct World {
    pub mutation_rate: f64,
    pub crossover_rate: f64,
    pub elitism: usize,
    pub max_generations: usize,
    pub population: Vec<Individual>,
}

impl World {
    pub fn new(
        population_size: usize,
        mutation_rate: f64,
        crossover_rate: f64,
        elitism: usize,
        max_generations: usize,
    ) -> Self {
        World {
            mutation_rate,
            crossover_rate,
            elitism,
            max_generations,
            population: vec![Individual::new(10); population_size],
        }
    }
}

mod tests {
    use super::*;
    #[test]
    fn test_world_new() {
        let world = World::new(4, 0.5, 1., 1, 1000);
        assert_eq!(world.population.len(), 4);
    }
}

If I run cargo check (or just save file in VSCode with Rust Analyzer), I get this warning:

warning: unused import: `super::*`
  --> src\ga\world2.rs:31:9
   |
31 |     use super::*;
   |         ^^^^^^^^

If I, however, drop this use statement, I get an error (which makes absolute sense for me):

mod tests {
    #[test]
    fn test_world_new() {
        let world = World::new(4, 0.5, 1., 1, 1000);
        assert_eq!(world.population.len(), 4);
    }
}

Gives:

error[E0433]: failed to resolve: use of undeclared type `World`
  --> src\ga\world2.rs:33:21
   |
33 |         let world = World::new(4, 0.5, 1., 1, 1000);
   |                     ^^^^^ use of undeclared type `World`
   |
help: consider importing one of these items
   |
31 +     use crate::ga::world2::World;
   |

So accepting the Cargo's suggestion (who am I to decline it?):

mod tests {
    use crate::ga::world2::World;

    #[test]
    fn test_world_new() {
        let world = World::new(4, 0.5, 1., 1, 1000);
        assert_eq!(world.population.len(), 4);
    }
}

I get the same warning as before:

warning: unused import: `crate::ga::world2::World`
  --> src\ga\world2.rs:31:9
   |
31 |     use crate::ga::world2::World;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^

Besides the warning, the code compiles and the tests run succesfully. But this is annoying. Am I missing something? Or maybe this is a bug?

Answer 1

It complains because the import is only used when the tests get compiled which isn't the case during a normal compilation so the import remains unused as far as cargo can tell. If you annotate your tests module with the #[cfg(test)] attribute, it'll stop complaining, because now the import statement and the tests are only compiled together. I.e.:

#[cfg(test)]
mod tests {
    use super::*;
    // ...
}

See also The Book for more details.