Seems like I've misread the recent Rust version announcement:

> This release brings about the stabilization of the core parts of allocator_api, enabling library authors to define custom allocators that can be used together with certain standard library types - currently Vec and Box. These are enabled through the newly-stabilized Allocator trait.

That made it sound like it was rather limited currently.

I asked an LLM (because it's outside my expertise) and it said:

> It is fair to say that the heaviest architectural lifting is already done: adding a defaulted type parameter (A: Allocator = Global) preserves backward compatibility for existing code, and the core data structures already store and thread the allocator internally. However, plumbing that generic parameter through String, HashMap's std wrapper, and hundreds of standard library methods still represents substantial retrofitting work rather than a completed state.

Looks like The patch notes lead me a bit astray. But it's also not at an 'already all there' state either.