[pierreyoda]

  > In the Python world, you can install an "extra" along with a    package. So you can make the deliberate decision to omit Unicode case folding from your CommonMark parser. Maybe something like that is possible with a crate?

This is definitely possible in Rust with Cargo crate features! [0]

For instance in a game library like quicksilver you can opt in to WebAssembly support, images, fonts, audio handling, etc. Then the image or audio library themselves can put formats and codecs behind feature flags for instance. Each crate then uses conditional compilation to branch out these features at compilation time. More commonly as an end-user, test modules use a special conditional compilation macro [1].

[0] https://doc.rust-lang.org/cargo/reference/manifest.html#the-... [1] https://doc.rust-lang.org/stable/rust-by-example/testing/uni...