Rust allows you to take interior pointers to things (important for performance), which precludes the ability to move objects within memory at random. But for "memory-constrained" applications like embedded devices/microcontrollers, fragmentation isn't a problem in the first place because you often don't have a heap. For long-running programs that do have heaps, picking a modern memory allocator (jemalloc, tcmalloc, et al) will go a long way towards reducing fragmentation. And if you really need compaction, you could probably design a Rust library to provide it for certain types (though the operations it could provide would likely be restricted).