Comment by ketzu
15 hours ago
> In Rust, that question is answered by the borrow-checker at compile time:
(About zero sized objects being the same)
But why does that mean the programmer never has to check? (or if they want that information from the borrow checker how would they get it?) It's not motivated as the intro above for c++ was just "In C++, we might do this to check if two pointers refer to the same object".
So the borrow checker knows already, why does that stop the programmer from wanting to know or separate these cases?
> if they want that information from the borrow checker how would they get it?
If you want to compare if two pointers point to the same place, you use https://doc.rust-lang.org/stable/std/ptr/fn.eq.html
Rust just defaults to value equality over reference equality. This is true for everything, not just ZSTs.
(I find the post's framing of "it's stored in the borrow checker" to be a bit odd, but I can't put my finger on exactly what it is. The borrow checker doesn't determine these sorts of semantics, it checks for liveliness and aliasing, so "do these pointers alias" isn't inherently not the borrow checker's job, it just strikes me as an odd way to put it. Maybe it's because you don't "ask the borrow checker for that information" really.)
As the article itself discusses, pointers to zero sized objects are not necessarily different (they write it is only the case in debug mode).
> I find the post's framing of "it's stored in the borrow checker" to be a bit odd
That's exactly what I wanted to say as well.
I feel like it would have been better to just skip the borrow checker mention and just go "in rust this can not be done reliably ..(section about pointers being the same)"
This can be done reliably, though. You can just do
if a == b { ...
Just that the check will be replaced at compiletime with a constant, since the borrow-checker tracks all objects lifetimes and can use that information to optimize the check away.
The objects themselves don't make it into the compiled binary, since they have no size, but all the required information about them will make it in. So you can treat them like ordinary objects and do all the usual operations on it, without wasting any memory during runtime.
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I'm not sure if this quite answers your question, but the difference in philosophy here is that C++ objects with pointers always have identity, whereas a Rust object only has identity if it has a non-zero size. It's an application of the zero-overhead principle, "you only pay for what you use".
> whereas a Rust object only has identity if it has a non-zero size.
Rust also has the complication that function pointers are not guaranteed to have an unique identity; If multiple functions compile to the same code, the compiler is allowed de-duplicate them.
The documentation [0] also warns it's also possible for the compiler to create multiple versions of the same function. And while I've absolutely seen the compiler to create multiple optimised versions of functions in disassembled code (partial inlining based on the caller), I'm not sure it's possible to get pointers to more than one version.
[0] https://doc.rust-lang.org/std/ptr/fn.fn_addr_eq.html
"zero sized objects don't have identity" actually would answer my question and is a really interesting factoid, thanks!
This is why C++ doesn't have ZSTs, it wants all objects to have identities, the obvious way to distinguish them is by where they are in memory, but ZSTs don't have distinct addresses in memory.
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I've tried (admittedly for just a few minutes) to come up with a case where you'd need to compare two pointers to ZSTs in any real algorithm, and failed. Can you think of one?
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.
It's not obvious to me that the borrow checker actually knows, so much as it can prove there are no illegal conflicts between mut and non-mut references to the same object.
If you somehow need this property in your programs, I think you can just add a 1-byte member and use pointer equality. (I'm not a Rust expert.)
I assumed it's a figure of speech, but, yeah, the borrow checker only enforces function-local invariants, with no interprocedural analysis or memorization. Parameter aliasing is just a logical deduction from assuming local invariants are maintained at all call sites. That's why the borrow checker imposes very minimal cost (esp when it was block based), and doesn't even require static compilation. Traits and other aspects of the type system are where the complexity and slow compile times come from.
I guess maybe the semantics of ZSTs complicate the aliasing story a tad?