If you have a single source file that's big enough for the compiler to bog down on it, then either that compiler is beyond ridiculously slow, or you need to refactor.
What makes this incremental compilation different from that of, say, Go, is that it works inside a package. Before incremental compilation, Rust and Go had the exact same model: package-at-a-time compilation. With incremental compilation, Rust is more fine-grained than that: Rust can compile specific parts of a package while leaving the other parts untouched.
