Off hand question for Rust: if memory is borrowed, I gather Rust does not need --- perhaps resists --- C++ style allocators? Here I give emphasis to owner of the (heap) data rather than efficiency. Both languages might use an allocator for fast re-use, less fragmentation, MT support etc.. On the the hand STL strings and containers are replete with allocators in their constructors leading to such questions like if two non-empty std::vector A,B vectors are equal (same size, specialized both on T, resp. elements equal) is A==B true if A has a different allocator from B?
Now granted allocator aware containers is a STL thing not per se a C++ thing. But the STL should leverage C++ design to the good ... so it's not too wrong or off base to think about the language in terms of a primary exemplar library.