The main competitor of Rust for writing browsers is C++, and its compilers don't have in-unit incremental compilation. So that's why Rust didn't start out with

The main competitor of Rust for writing browsers is C++, and its compilers don't have in-unit incremental compilation. So that's why Rust didn't start out with incremental compilation, but it was added later on, so now you do have it (although it could be improved in many ways).