In the corner of rust I have experience in (web servers, ffi for cli's) you just use the system allocator. I know it's not that hard to switch that default allocator to something like jemalloc, but I've never needed to do that. Rust is currently refactoring the standard library to be genetic over the allocator like c++ so you you can have two vectors with different allocators, but I don't really understand the point outside of stuff like kernel development.