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

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.

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.