[steveklabnik]

C++ cannot because it does not have the necessary information present in its syntax. It’s really that simple. C++ could add such syntax, but outside of what Circle is doing, I’m not aware of any real proposal to add it.

Also, Google (more specifically, the Chrome folks) tried to make it work via templates, but found that it was not possible. There’s a limit to template magic, even.

[arc619]

Although it's not as extensive as Rust's lifetime management, Nim manages to infer lifetimes without specific syntax, so is it really a syntax issue? As you say, though, C++ template magic definitely has its limits.

[steveklabnik]

Nim has a garbage collector.

That said, you're right on some level that it's truly semantics that matter, not syntax, but you need syntax to control the semantics.

[arc619]

Nim is stack allocated unless you specifically mark a type as a reference, and "does not use classical GC algorithms anymore but is based on destructors and move semantics": https://nim-lang.org/docs/destructors.html

Where Rust won't compile when a lifetime can't be determined, IIRC Nim's static analysis will make a copy (and tell you), so it's more as a performance optimisation than for correctness.

Regardless of the details and extent of the borrow checking, however, it shows that it's possible in principle to infer lifetimes without explicit annotation. So, perhaps C++ could support it.

As you say, it's the semantics of the syntax that matter. I'm not familiar with C++'s compiler internals though so it could be impractical.

[steveklabnik]

I did not hear that Nim made ORC the default, thanks for that!

I still think that my overall point stands: sure, you can treat this as an optimization pass, but that kind of overhead isn't acceptable in the C++/Rust world. And syntax is how you communicate programmer intent, to resolve the sorts of ambiguous cases described in some other comments here.

I am again reminded of escape analysis https://steveklabnik.com/writing/borrow-checking-escape-anal...

[aw1621107]

> Where Rust won't compile when a lifetime can't be determined, IIRC Nim's static analysis will make a copy (and tell you), so it's more as a performance optimisation than for correctness.

Wait, how does that work? For example, take the following Rust function with insufficient lifetime specifiers:

    pub fn lt(x: &i32, y: &i32) -> &i32 {
        if x < y { x } else { y }
    }

You're saying Nim will change one/all of those references to copies and will also emit warnings saying it did that?

[j-james]

It will not emit warnings saying it did that. The static analysis is not very transparent. (If you can get the right incantation of flags working to do so and it works, let me know! The last time I did that it was quite bugged.)

Writing an equivalent program is a bit weird because: 1) Nim does not distinguish between owned and borrowed types in the parameters (except wrt. lent which is bugged and only for optimizations), 2) Nim copies all structures smaller than $THRESHOLD regardless (the threshold is only slightly larger than a pointer but definitely includes all integer types - it's somewhere in the manual) and 3) similarly, not having a way to explicitly return borrows cuts out much of the complexity of lifetimes regardless, since it'll just fall back on reference counting. The TL;DR here though is no, unless I'm mistaken, Nim will fall back on reference counting here (were points 1 and 2 changed).

For clarity as to Nim's memory model: it can be thought of as ownership-optimized reference counting. It's basically the same model as Koka (a research language from Microsoft). If you want to learn more about it, because it is very neat and an exceptionally good tradeoff between performance/ease of use/determinism IMO, I would suggest reading the papers on Perseus as the Nim implementation is not very well-documented. (IIRC the main difference between Koka and Nim's implementation is that Nim frees at the end of scope while Koka frees at the point of last use.)

[aw1621107]

Oh, that's interesting. I think not distinguishing between owned and borrowed types clears things up for me; it makes a lot more sense for copying to be an optimization here if reference-ness is not (directly?) exposed to the programmer.

Thanks for the explanation and the reading suggestions! I'll see about taking a look.

[arc619]

> It will not emit warnings saying it did that.

You're right. I was sure I read that it would announce when it does a copy over a sink but now I look for it I can't find it!

> The static analysis is not very transparent.

There is '--expandArc' which shows the compile time transformations performed but that's a bit more in depth.

[gpderetta]

I'm pretty sure you could embed a language with lifetimes in a dsl built with c++ templates. You wouldn't want to use it beyond toy programs though.

[steveklabnik]

Maybe, but nobody has demonstrated that it's actually possible. And even then, toys are fun, but still, at the end of the day, not good enough.

[gpderetta]

Of course, it would be completely impractical. Nobody has demonstrated it because they were interested in a practical solution.