Content-sensitivity is actually required for this to be most useful with Rust. The file isn't a hard organisational boundary with Rust, incremental compilation

Content-sensitivity is actually required for this to be most useful with Rust. The file isn't a hard organisational boundary with Rust, incremental compilation would work either at the module level or the function level (i.e. the goal is to be incremental on the actual structures the compiler works with). In any case, it has so many more benefits than just being actually-correct (as you say), e.g. one can edit comments/whitespace without forcing rebuilds.