use anyhow::Result;treated as unused import if only tests use it

[SichangHe]

Summary

If I add

use anyhow::Result;

and use Result<T> only in my tests, clippy thinks this import is unused.

Lint Name

unused_imports

Reproducer

This is all the code you need to have this false warning:

use anyhow::Result;

fn main() {
    println!("Hello, world!");
}

#[test]
fn test() -> Result<()> {
    Ok(())
}

The warning:

$ cargo clippy
warning: unused import: `anyhow::Result`
 --> blah/src/main.rs:1:5
  |
1 | use anyhow::Result;
  |     ^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: `blah` generated 1 warning

To show that this import is used, remove it:

fn main() {
    println!("Hello, world!");
}

#[test]
fn test() -> Result<()> {
    Ok(())
}

If you run the test, it shows an error:

$ cargo t
   Compiling blah
error[E0107]: this enum takes 2 generic arguments but 1 generic argument was supplied
   --> blah/src/main.rs:6:14
    |
6   | fn test() -> Result<()> {
    |              ^^^^^^ -- supplied 1 generic argument
    |              |
    |              expected 2 generic arguments
    |
note: enum defined here, with 2 generic parameters: `T`, `E`
   --> ~/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/result.rs:504:10
    |
504 | pub enum Result<T, E> {
    |          ^^^^^^ -  -
help: add missing generic argument
    |
6   | fn test() -> Result<(), E> {
    |                       +++

For more information about this error, try `rustc --explain E0107`.
error: could not compile `blah` due to previous error

Version

rustc 1.62.1 (e092d0b6b 2022-07-16)
binary: rustc
commit-hash: e092d0b6b43f2de967af0887873151bb1c0b18d3
commit-date: 2022-07-16
host: aarch64-apple-darwin
release: 1.62.1
LLVM version: 14.0.5

Additional Labels

No response

[Alexendoo]

This warning is from rustc itself rather than Clippy. #[test] fns have #[cfg(test)] applied to them, so when not compiled in test mode the Result import is unused. The usual way to do tests is within a #[cfg(test)] module, with the imports also in the test module

fn main() {
    println!("Hello, world!");
}

#[cfg(test)]
mod tests {
    use anyhow::Result;

    #[test]
    fn test() -> Result<()> {
        Ok(())
    }
}

[SichangHe]

So, it is a bug from rustc I guess.

[Alexendoo]

It's not a bug, the program when not compiling tests is essentially

use anyhow::Result;

fn main() {
    println!("Hello, world!");
}

Where the anyhow::Result import is unused. Test only imports should only be enabled when the tests themselves are enabled

[SichangHe]

Good point. The import is unused from the point of building. It still looks wonky I guess.

Also, why are we not suggesting people to separate tests to a module? Is that too much restriction?

[Alexendoo]

It's a reasonable suggestion, we don't have a lint for it probably because of some combination of it being difficult (due to how clippy works with conditional compilation) and nobody taking the time to implement it