If you literally want one and only one allocator instead of the Rust's built-in one, that's been supported for years[1]. Almost 8 years, to be precise. It's very easy to implement and works with the entire standard library and all its abstractions, and almost every 3rd party library will support it out of the box too (except the ones that go out of their way not to support it for some reason). Borrow checking and all the other safety features are still fully supported.

Now, mixing different allocators is a different beast, and much less supported. But it sounds like you are very much not interested in this use case, right?

[1] https://doc.rust-lang.org/std/alloc/index.html