Unfortunately, doing that at a particularly interesting scale - so I could have, for example, automatic compilation as I type into my editor, meaning hitting "run" would be near-instant - means rearchitecting the compiler to behave closer to Roslyn, which is a huge task.

Oh, sure, it's kinda a pipe dream to get something like this for C++ (use ccache instead). But Rust is still forming its story here (we have experimental incremental compilation which does this!) and I see no reason to constrain it by tying it to files :)