Error E0433 - Use of undeclared type or module when accessing a module from a test

Question

As a first project in Rust, I'm trying to create a run length encoding library.

This is my src/lib.rs (initially created by cargo):

#[cfg(test)]

mod rle;
mod tests {

    #[test]

    fn it_works() {
        let rle1 = rle::Rle {
            lengths: vec![1, 4, 2],
            values: vec![1, 2, 1],
        };
    }

}

This is my src/rle.rs:

pub struct Rle {
    lengths: Vec<i32>,
    values: Vec<i32>,
}

Using this layout, I get an error[E0433]: failed to resolve. Use of undeclared type or module 'rle'

I have tried to follow the guidelines in the Rust Docs called Crates and Modules.

What am I doing wrong?

In case the layout was not obvious:

$ ls src
lib.rs rle.rs

Answer 1

You have an error in the use of #[cfg(test)].

Rust has two ways of applying attributes:

• #[...] applies to the next item
• #![...] applies to the enclosing item

This means that here, #[cfg(test)] is applying to the next item (mod rle;) which will thus only be compiled in test mode. On the other hand, your tests module will always be compiled.

As a result, when NOT running in test mode, you have:

• no rle module
• a tests module referring to a rle module

The fix is simple: move the attribute so it applies to mod tests, not mod rle.

---

Note:

• as mentioned by Chris, you ALSO need to use super::rle to refer to the rle module.
• as mentioned by ljedrz, you ALSO need to declare the fields of Rle public to be able to name them from outside the file (a problem you should not have if the tests are in the same file).