To me (a C++ daily driver and Rust hobbyist) this "DAG of strongly-connected" tiered graph system is sane and maps well to practice --- tightly couple your impl

To me (a C++ daily driver and Rust hobbyist) this "DAG of strongly-connected" tiered graph system is sane and maps well to practice --- tightly couple your impl details however you want, then expose a clean interface at the crate level. It encourages you to split your code to separate files as you write _without immediately forcing you to untangle the dependency_, and then as "islands" of code organically form, which pieces belong to a separate crate becomes naturally evident. Contrast this with C++ where the DAG relationship is enforced at much lower levels (e.g. Bazel cc_library, CMake target, which in normal cases correspond to a cc/h pair), shifting the friction (of having to deal with deps) much earlier in the dev cycle, which then discourages you from (incrementally) splitting a file at all. I've seen too many times this resulting in 5000+ LoC cc file.