Memory allocation is one of the areas where currently C/C++ has or had genuine advantages over Rust. Custom allocators took Rust years, and giving standard library constructs like a Vector a custom allocator that isn't the global allocator is still experimental (=opt-in nightly-only). Similarly while Rust gives you good control over where the data ends up being stored, there is no way to make sure it isn't also put on the stack during function execution. One of the implicit assumptions underlying the language seems to be that the stack is cheap and effectively infinite while the heap is expensive. So you have a lot of control over what touches the heap, but less control over what touches the stack.

Those are temporary pains that have remedies in the works. Rust is a fairly young language, and a lot of good-enough solutions get thrown out before ever getting beyond the experimental stage. But if you are writing software today then needing absolute control over where exactly your data touches is a good reason to prefer C/C++ today. Not that that's a very common need.