How do I use conditional compilation with `cfg` and Cargo?

rust rust-cargo

22,725

Solution 1

You have to introduce the existing features in your Cargo.toml.

I was able to conditionally compile by doing the following:

In Cargo.toml, create a features section and introduce a certain feature name:
```
[features]

customfeature = [] # feature has no explicit dependencies
```
If you want your feature to have specific dependencies check the examples in the documentation.
In your code, use #[cfg(feature="customfeature")]
Run cargo build --features customfeature

Since your steps 2 & 3 seem to be fine, there must probably be a problem with your Cargo.toml.

Solution 2

As stated in other answers, you can use features for this. I would like to add that features do not only allow you to conditionally compile parts of your code but also to conditionally include dependencies that may be part of that code. Consider the following snippets:

You can activate the conditional code using a feature flag as already described in other anwsers:

cargo build --features customfeature

You need to mark your conditional code to exist only when your customfeature is enabled:

#[cfg(feature = "customfeature")]
fn my_func() {
    my_optional_dependency::do_something(); 
}

// This includes dependencies only when customfeature is enabled
#[cfg(feature = "customfeature")] 
extern crate my_optional_dependency;
....

#[cfg(feature = "customfeature")]
use my_optional_dependency::*;
....

Your Cargo.toml needs to have the following sections:

[dependencies.my_optional_dependency]
version = "1.2.3"
optional = true

[features]
customfeature = ["my_optional_dependency"]

This allows you to activate certain parts of your code along with their dependencies only if a feature is enabled.

22,725

Author by

盛安安

Updated on March 30, 2020

Comments

盛安安 about 4 years
I want to conditionally compile my source code using cfg with Cargo, after Googling for a while, it seems that the solution is to use cargo --features.

http://doc.crates.io/manifest.html

I tried adding a few
```
#[cfg(feature = "foo")]
```
in the source code and
```
cargo build --features foo
```
, but it says
```
Package `xxx v0.0.1 (file:///C:/yyy/xxx)` does not have these features: `foo`
```
How can I let cargo identify the features? Do I have to add something in Cargo.toml?

Here's the version of rustc and cargo I am using:
```
C:\>rustc --version
rustc 0.13.0-nightly (42deaa5e4 2014-12-16 17:51:23 +0000)

C:\>cargo --version
cargo 0.0.1-pre-nightly (5af754d 2014-12-18 01:50:48 +0000)
```
Bulat M. over 6 years

is it possible to test for feature in "if"? Thanks.
Pauan over 6 years

@BulatM. Yes, you can use if cfg!(feature = "customfeature") { ... }
Just a learner over 3 years

Thanks Alex! After reading your answer I think I finally fully understood what is Rust features.