I personally don't think that compilation units should be driven by files. Too often have I been forced to keep something in the "wrong" place in C++ because of the compilation model. (This happens in rust too, due to other reasons)
Instead, the compiler should figure out what needs to be recompiled and do just that. This can mean benefits like not needing to recompile everything because you added a comment to a header file.
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 :)
Instead, the compiler should figure out what needs to be recompiled and do just that. This can mean benefits like not needing to recompile everything because you added a comment to a header file.