[steveklabnik]

> it is conceptually limited to the equivalent of reference counting with a maximum reference count of 1.

This is a thing people say, but I think it's misleading. Reference counting can increase the lifetime of an object, but borrowing cannot. I've seen this really trip up beginners.

> This inherent limitation makes a lot of things hard

It can make them different, which can be hard, but these things are already hard. And some people think it can make them easier or even better; see Bodil Stokke's work on persistent data structures in Rust.

[bunderbunder]

> This is a thing people say, but I think it's misleading. Reference counting can increase the lifetime of an object, but borrowing cannot. I've seen this really trip up beginners.

I'm not sure I follow.

The only reference-counted language I've used is (pre-ARC) Objective-C. There, it was a very common idiom to "borrow" objects - so common that it didn't even have a name. There was just objects you "retained" (that is, staked a claim on), and ones you didn't.

Maybe there's a pitfall to how the "automatic" part of automatic reference counting is typically implemented?

[steveklabnik]

It has been years since I've written objective-c, so I'll write out some psuedo-code. This may be wrong, please correct me! (It should map to C++ pretty directly, and certainly does in unsafe Rust.)

* You have an object. You call retain on it. You have a count of one.

* You also have a pointer to that object. The "borrow" in your analogy.

* You return this pointer, and stash it somewhere. The object still has a count of one, so it's still live, so this is okay.

* Later in your program, you use that pointer to call release.

Here, we've only ever had a reference count of one, but our object has lived across arbitrary inner scopes. In Rust, this would not work, unless you dropped into unsafe.

Obviously, with Arc and autoretain this kind of code doesn't get written anymore, I would hope. And even without, it wouldn't be guaranteed, so you'd want the "borrow" to actually bump the refcount. But Rust is about guaranteeing that it can't.

[bunderbunder]

Ah, I think I follow.

So, it sounds to me like it's not necessarily that Rust's model is fundamentally different from "ref counting with a limit of 1", at least in terms of how you should be managing your memory, so much as that the language doesn't let you some things that you really shouldn't be doing in the first place.

Sometimes it felt like Objective C wouldn't just let you point a gun at your foot, it would actively cheer you on while you did it.

[rbehrends]

> This is a thing people say, but I think it's misleading. Reference counting can increase the lifetime of an object, but borrowing cannot. I've seen this really trip up beginners.

"With a maximum reference count of 1." As the reference count becomes 1 upon object creation, it cannot really be increased further. Hence, only operations that keep the (virtual) reference count at 1 or reduce it to 0 are allowed.

My point here is that you inherently cannot do things where you cannot prove that this virtual reference count can be capped at 1.

[steveklabnik]

That doesn't change much; the point is that (in many languages), variables going into or out of scopes don't fiddle with the ref count[1], and so people assume that something will live until they make the count go down explicitly.

It also only refers to ownership, not borrowing, and both are equally important.

Beyond that, what I'm saying is something more meta: It doesn't really matter if this analogy is spot-on or not; it's got enough wiggle room in it that I've seen it trip up beginners. Maybe that's because they misunderstand the analogy, but given that its point is to convey understanding, that means that it isn't a great analogy, in my experience. YMMV.

1: directly, of course; this also depends on the language.

[rbehrends]

> It also only refers to ownership, not borrowing, and both are equally important.

I addressed borrowing above. Borrowing is proving lifetime subset properties and that you therefore can avoid increasing the virtual reference count.

And this is not about whether this is useful for beginners. It is to illustrate inherent limitations of the approach.

[steveklabnik]

> I addressed borrowing above. Borrowing is proving lifetime subset properties and that

Right, so what I'm saying is, the description of borrowing doesn't really fit in with the reference counting aspect of the analogy, so it ends up being separate from it.

> And this is not about whether this is useful for beginners.

Right, that was my point. :)

I mean, in the end, do what you'd like. All I'm saying is that I've seen this analogy lead to tons of confusion. YMMV.