Since crates are the compilation boundary, and you can make projects out of multiple crates, larger Rust projects tend to be split up, even internally.

Take Servo, for instance: https://github.com/servo/servo/tree/master/components or rustc itself: https://github.com/rust-lang/rust/tree/master/src

Each of these subdirectories (basically, a few aren't) is a crate, so you'll only be recompiling stuff in the subdirectory you're editing. Incremental compilation will still help with projects like this, of course.