That would all come undone with separate compilation. You'd have to recompile the entire program every time.

You'd also have to keep around a massive amount of data at compile time. A variable that's a pointer to a struct, can't just be represented as a pointer to a struct. It'd have to have a map of which pointers in the struct are managed for every pointer to that struct.

The way the D compiler is implemented now, a struct S has only one instance, as every pointer to a struct's type points to that same instance. And still sometimes people compile programs so large the compiler runs out of memory.