What's the best way to avoid using GC in D? Is there a way to use classes that doesn't involve their memory being managed, or do you have to use pointers to malloc'd structs like you would in C and C++?
It was decided a long time ago that classes need to be reference types because of the slicing problem. On the other hand, D is a systems language. Therefore, using classes with manual memory management is ugly but do-able.
In D2 + Phobos, you can (unsafely) allocate a class instance on the stack using std.typecons.scoped(). You can (again, unsafely) allocate a class in any arbitrary memory block by using std.conv.emplace(). The block of memory you allocate the class in can be created, for example, by using core.stdc.malloc(). However, note that you will have to call GC.addRange() if the class could possibly contain pointers into GC-allocated memory.