CODEWITHSUNDEEP
rust
Derokorian
Derokorian

Reputation: 1450

How to suppress "function is never used" warning for a function used by tests?

I'm writing a program in Rust and I have some tests for it. I wrote a helper function for these tests, but whenever I build using cargo build it warns me that the function is never used:

warning: function is never used: ... #[warn(dead_code)] on by default

How I can mark this function as used so as not to get the warnings?

Upvotes: 90

Views: 50061

Answers (8)

Poseidon
Poseidon

Reputation: 165

For some reason I found that setting the main function to public:

pub fn main()

and then copying my main.rs file to lib.rs

cp src/main.rs src/lib.rs

then recompiling fixed this.

No need to use a macro for it, although the macro should work for non main functions.

Upvotes: 0

mpavel
mpavel

Reputation: 1201

You can disable specific lints for a whole project in Rust by going into your main.rs file and adding the following at the very top of the file:

#![allow(
dead_code,
unused_imports
)]

Upvotes: 5

sylvain
sylvain

Reputation: 771

You max prefix the unused function name with an underscore:

fn _dummy() {}

fn main() {}

See: https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#dead-code

Upvotes: 0

Timmmm
Timmmm

Reputation: 97058

There is another situation where this can occur. If you have several helper functions in a module, e.g. in tests/utils/mod.rs and then several integration tests (tests/a.rs, tests/b.rs) each of which does

mod utils;
use utils::...;

then you will get dead code warnings if you do not use all of the code from all of the tests. For example if test a.rs only uses utils::foo and b.rs only uses utils::bar then you will get dead code warnings for both.

That is because each test is compiled as an independent crate. Here is the bug report for it. It looks difficult to solve so I wouldn't hold my breath.

Upvotes: 15

Michael Eliot
Michael Eliot

Reputation: 1196

For people getting this warning while making a rust library, you may get this if you don't have your modules set to pub in your lib.rs.

pub mod foo;

Upvotes: 49

chills42
chills42

Reputation: 14533

Specific question

How I can mark this function as used so as not to get the warnings?

The Rust compiler runs many lints to warn you about possible issues in your code and the dead_code lint is one of them. It can be very useful in pointing out mistakes when code is complete, but may also be a nuisance at earlier stages. Often, this can be solved by either deleting unused code, or by marking a public method. However, all lints can be turned off by allowing them, and your error message (#[warn(dead_code)] on by default) contains the name of the lint you could disable.

#[allow(dead_code)]
fn my_unused_function() {}

Alternative for testing

I wrote a helper function for these tests, but whenever I build using cargo build it warns me that the function is never used.

This happens to be a special case, which is that code that is only used for testing isn't needed in the real executable and should probably not be included.

In order to optionally disable compilation of test code, you can mark it accordingly using the cfg attribute with the test profile.

#[cfg(test)]
fn my_test_specific_function() {}

When marked in this way, the compiler knows to ignore the method during compilation. This is similar to commonly used ifdef usage in other languages like C or C++, where you are telling a preprocessor to ignore the enclosed code unless TESTING is defined.

#ifdef TESTING
...
#endif

Upvotes: 106

Chris Morgan
Chris Morgan

Reputation: 90882

If something is only used in tests, it should be omitted altogether. This can be done with the #[cfg(test)] attribute.

Upvotes: 21

DK.
DK.

Reputation: 59155

dead_code is a lint, which means you can allow it on the thing that's causing it to trigger.

#[allow(dead_code)]
fn dummy() {}

fn main() {}

Upvotes: 8

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