It doesn't actually use it, this is just referencing it in the dependency graph (Cargo.lock). bs58 is an optional (feature-gated) transitive dependency of another crate, which is never referenced at all in the top-level crate. The build.rs doesn't run or anything.

I believe there's (several) open issues about changing this behavior of the lock file, but I have moved on from Rust so don't know the status of things.

That said, this is confusing behavior. I remember I first noticed it when a bunch of crates were showing up in my lock file for every OS under the sun, even though I was specifically using a "wayland" (linux-only) feature.

Not to defend Rust's crazy culture of dependencies, mind you.