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

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.