Module named test without cfgtest

What it does

Whenever you have a mod test block without a #[cfg(test)] it is probably an error.

I had code which had a package as a dev dependency and only being used in the tests. Running the tests worked but not building or running the binary. The error given was very confusing.

error[E0432]: unresolved import `approx`
  --> src/main.rs:57:9
   |
57 |     use approx::assert_relative_eq;
   |         ^^^^^^ use of undeclared crate or module `approx`

Lint Name

mod test without #[cfg(test)]

Category

suspicious, cargo

Advantage

• Test module won't be compiled to binary/library by accident.
• Forgetting #[cfg(test)] from test module is just probably a mistake almost all of the time.

Drawbacks

None that I can think of.

Example

mod test {
    use super::*;
    use approx::assert_relative_eq;
}

Could be written as:

#[cfg(test)]
mod test {
    use super::*;
    use approx::assert_relative_eq;
}

Start by locating existing Clippy lints in the suspicious and cargo categories that inspect module declarations, then review their associated tests and registration points. Implement detection for an unconditionally compiled module named test and verify that the example without #[cfg(test)] is warned about while the guarded form is accepted.