undeclared crate or module inside `if cfg!(test)` even if defined in dev-dependencies

Question

Here is the error:

let tmp = tempdir::TempDir::new("cargo_test").expect("failed to create tempdir");
          ^^^^^^^ use of undeclared crate or module `tempdir`

My cargo.toml looks like this:

#...
[dev-dependencies]
tempdir = "0.3.7

And in the code:

let path = if cfg!(test) {
            let tmp = tempdir::TempDir::new("cargo_test").expect("failed to create tempdir");
            tmp.path().to_str().map(|x| x.to_string()).unwrap_or(path)
        } else {
            path
        };

I do not understand why it fails, when i run cargo test. :(

cafce25 · Accepted Answer · 2023-03-22T15:30:25.267

2

The cfg! macro evaluates to a constant true or false depending on the contents, so on non-test builds it evaluates to

let path = if false {
    let tmp = tempdir::TempDir::new("cargo_test").expect("failed to create tempdir");
    tmp.path().to_str().map(|x| x.to_string()).unwrap_or(path)
} else {
    path
};

which still has the dependency in it, and will type check everything and so it sees the missing crate. Instead you have to remove the whole first block from compilation:

#[cfg(test)]
let path = {
    let tmp = tempdir::TempDir::new("cargo_test").expect("failed to create tempdir");
    tmp.path().to_str().map(|x| x.to_string()).unwrap_or(path)
};

edited Mar 22 '23 at 15:30

answered Mar 22 '23 at 14:41

cafce25

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1

In case you need an `else` branch (which you don't need here), you can use `#[cfg(not(test))]` – Sven Marnach Mar 22 '23 at 14:54
@SvenMarnach it is exactly what i did, after reading the answer! Thanks! – Jitterbug Mar 22 '23 at 15:05
There is also [`cfg_if`](https://docs.rs/cfg-if/latest/cfg_if/) if your 'conditions' are more complex. – cafce25 Mar 22 '23 at 15:07

undeclared crate or module inside `if cfg!(test)` even if defined in dev-dependencies

1 Answers1