How to use a crate only for a given platform?

Question

I would like to use the nix crate in a project.

However, this project also has an acceptable alternative implementation for OSX and Windows, where I would like to use a different crate.

What is the current way of expressing that I only want nix in Linux platforms?

Answer 1

There's two steps you need to make a dependency completely target-specific.

First, you need to specify this in your Cargo.toml, like so:

[target.'cfg(target_os = "linux")'.dependencies]
nix = "0.5"

This will make Cargo only include the dependency when that configuration is active. However, this means you'll get a compile error on your extern crate when you try to build on other platforms! To remedy this, annotate it with a cfg attribute, like so:

#[cfg(target_os = "linux")]
extern crate nix;

Of course, you then have to ensure that you only use the nix crate in code that's also annotated with the same cfg attribute.