No (well, there is still work to be done on custom allocators, but it's planned). But given that Rust uses jemalloc by default, not the system allocator, it is

No (well, there is still work to be done on custom allocators, but it's planned). But given that Rust uses jemalloc by default, not the system allocator, it is substantially less likely that you will want to replace it with your own. You certainly can't do it without copious amounts of unsafe code (which is really rather unfortunate).