I suspect it's actually possible to prove such an algorithm can't exist.
By definition, a ZST hold no runtime data. It does hold some compile-time data based on its existence, but after compiling, that has been type erased away.
Since a ZST holds zero bits of state, there can only be one valid instance of it. You can't have multiple different versions of the same ZST representing different things.
So if you have one, you automatically know it's going to be equal to all other instances of the same ZST type. And not equal to any other ZSTs. There is no point doing a pointer comparison, as that gives you no extra information, type ownership is enough.
Donald Rumsfeld forgot the zero-sized unknown: you know its type, you know its complete value, but you don't know which instance it is.
https://en.wikipedia.org/wiki/There_are_unknown_unknowns
COM would ask the object for its canonical IUnknown pointer. Rust answers that the question has been optimized away.
A ZST has one possible value, not one possible occurrence. That doesn't prove that no identity-dependent algorithm can exist; it proves that a ZST cannot carry the identity such an algorithm requires. If identity matters, it must be represented somewhere -- and then it is no longer zero-overhead.
COM is essentially a language-neutral ABI built from C++-style vtables, and QueryInterface is essentially a type-safe dynamic_cast.
In Rumsfeldian terms, IUnknown provides a known way to interrogate an otherwise unknown object. QueryInterface resolves known unknowns by asking about specific interface IDs; it cannot ask about unknown unknowns.
https://news.ycombinator.com/item?id=12975257
> COM is essentially a formal way of using C++ vtables [1] from C and other languages, so you can create and consume components in any language, and call back and forth between them. It's a way of expressing a rational subset of how C++ classes work and format in memory, in a way that can be implemented in other languages.
> It was the outcome of the C / C++ / Visual Basic language wars at Microsoft.