Isn't that one of the reasons rust compiles with JEMalloc? Fragmentation avoidance is one of its main features. Otherwise, using arenas or other strategies work very well for avoiding fragmentation. I generally haven't run into issues, mainly because rust allows me to use a lot less memory which helps.

Rust code also has less dynamic memory allocation in general. You allocate for dynamically sized datastructures and long-lived data, but not like c# or java where each non-value class type is allocated on the heap (unless the compiler is smart enough to use escape analysis).