Or it can just live in the type system, as it does in Rust. This is basically always going to be cheaper, and for the case where you just want a single global default allocator it's free instead of two pointers.

I think with Rust 1.100 next month you could write an OdinStyleAllocator type which impl std::alloc::Allocator and has the two pointers inside it, and do the thing Odin does here if you wanted that but I expect you'd snap out of it.