I spent a long time implementing precompiled C++ headers. The main problem was converting the pointers in the complex data structures to/from file offsets. I never wanted to go through that again.

D has a simple and effective module system. It even treats unmodified .c and .cpp files as modules (though the C and C++ files are unaware of that!). D's builtin C compiler can even import other C files as modules.

Both C and C++ should have just copied it.