The Import Relationship Between Translation Units

There is a gigantic shift that happens when it comes to the adoption of C++ modules in the tooling ecosystem. The order at which you translated individual units was irrelevant until now. With build module interfaces, you have this new import relationship where in order to translate this particular unit, you need to have translated the module being imported beforehand. Now, you need the build system to topologically sort all the translation units in order to build them in the right order because otherwise they just can't build.