But you are specifying source files, although indirectly, aren't you? That's what all those `mod blah` with a corresponding `blah.rs` file present in the correct location are.

Yes and no. You're right that the mod declarations are necessary, but I'd argue that this is actually more direct (it's happening in the code, not in some external build file), and that you still aren't actually choosing where the file is located as much as stating that it's present, since you don't have the ability to look for `mod foo` in an arbitrary location, but only the places that the tooling already expects to find it.