Well, keeping in mind that I don't know much of the specifics of Rust, and am just making a guess at what it's like to use, this is what I mean:
Actually predicting where data is really going to go involves solving the halting problem. So by necessity any static analysis of ownership is going to be conservative, in the sense that it has to err on the side of safety.
So there's a process of structuring things so that it's not just the programmer who understands, but the compiler who understands. Structuring the code in alternative ways so that ownership is made clear and/or ambiguous cases are resolved. Sometimes this could be a small amount of work, but sometimes it could be a very large amount of work (analogous to the simpler situation in C++ where you are using const everywhere but need to change something deep down in the call tree and now either everyone has to lose their consts or you have to do unsavory things).
At points, it might be possible to structure things so that the compiler would understand them and let you do it, but it would take a large amount of refactoring that one doesn't want to do right now (especially if one has had a few experiences of starting that refactor and having it fail), so instead one might punt and just say "this parameter is owned, problem solved". And that's great, you can stop refactoring, but you just took a performance hit.
Now, in some cases it is probably the case that this is in reality an ambiguous and dangerous ownership situation and the language just did you a favor. But there are also going to be cases where it's not a favor, it's just the understanding of ownership being conservative (because it has to be), and therefore diverging from reality. But I want to get work done today so I make the parameter owned, now the compiler is happy, but there is a performance cost there. If I were not so eager on getting work done today, I might be able to avoid this performance hit by wrestling with the factoring. But I might deem the cost of that prohibitive.
That's all I mean. But like I said, I have never written a large program in Rust so I am not speaking from experience.
(And really my point is that I perceive there is an ambient pressure toward copying function parameters in general in order to minimize refactoring ... which is what I mean by there being an overall performance impact).
I don't know if I agree, per se, but I will say that Rust is still in such early stages that we'll be seeing how it shakes out as more people work with Rust. I haven't found this personally to be an issue, but I've also been doing Rust so long that it's hard to see things from a fresh perspective, you know?
I think there is, honestly—the JSON API is a legitimate example of that—but I think it's no worse in Rust than in C, where equivalent pressure already exists.
Invoking The Halting Problem is a pretty big sledge hammer. Especially for a language that you claim you don't know much (of the specifics) about. And your argument is so broad and without specifics that it just boils down to "expressing things in such a way that the compiler believes you", with no mention of Rust except that it might cost some performance if you are not able to communicate this. But how hard will it really be to communicate? All semantic properties are not created equal across languages -- some languages make a set of them easy to check in all realistic cases, others very hard or impractical.
For instance, there is definitely a concrete conversation to be had about the ownership model and implementing data structures -- you even have to use `unsafe {}` for implementing something as simple as a doubly linked list[1]. So that's a concrete example of how single-ownership makes something conceptually simple and safe hard to express in safe code. But in this case, there is so much vagueness about the supposed "massive performance implications" (HP + laziness = massively inefficient) that it comes off as trying a bit hard to... let's just say "to be negative".
> (And really my point is that I perceive there is an ambient pressure toward copying function parameters in general in order to minimize refactoring ... which is what I mean by there being an overall performance impact).
Well admittedly this argument is more concrete.
[1] But that's just a burden for the implementer, since you can expose a safe interface to the data structure.
Actually predicting where data is really going to go involves solving the halting problem. So by necessity any static analysis of ownership is going to be conservative, in the sense that it has to err on the side of safety.
So there's a process of structuring things so that it's not just the programmer who understands, but the compiler who understands. Structuring the code in alternative ways so that ownership is made clear and/or ambiguous cases are resolved. Sometimes this could be a small amount of work, but sometimes it could be a very large amount of work (analogous to the simpler situation in C++ where you are using const everywhere but need to change something deep down in the call tree and now either everyone has to lose their consts or you have to do unsavory things).
At points, it might be possible to structure things so that the compiler would understand them and let you do it, but it would take a large amount of refactoring that one doesn't want to do right now (especially if one has had a few experiences of starting that refactor and having it fail), so instead one might punt and just say "this parameter is owned, problem solved". And that's great, you can stop refactoring, but you just took a performance hit.
Now, in some cases it is probably the case that this is in reality an ambiguous and dangerous ownership situation and the language just did you a favor. But there are also going to be cases where it's not a favor, it's just the understanding of ownership being conservative (because it has to be), and therefore diverging from reality. But I want to get work done today so I make the parameter owned, now the compiler is happy, but there is a performance cost there. If I were not so eager on getting work done today, I might be able to avoid this performance hit by wrestling with the factoring. But I might deem the cost of that prohibitive.
That's all I mean. But like I said, I have never written a large program in Rust so I am not speaking from experience.