Comment by ninkendo | Hacker News Reader

Comment by ninkendo | Hacker News Reader

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I don’t think GP is moving the goalposts at all, rather I think a lot of people are willfully misrepresenting GP’s point.

Rust-to-rust code should be able to be dynamically linked with an ABI that has better safety guarantees than the C ABI. That’s the point. You can’t even express an Option<T> via the C ABI, let alone the myriad of other things rust has that are put together to make it a safe language.

You can look to Swift for prior art on how this can be done: https://faultlore.com/blah/swift-abi/

It would be very hard to accomplish. Apple was extremely motivated to make Swift have a resilient/stable ABI, because they wanted to author system frameworks in swift and have third parties use them in swift code (including globally updating said frameworks without any apps needing to recompile.) They wanted these frameworks to feel like idiomatic swift code too, not just be a bunch of pointers and manual allocation. There’s a good argument that (1) Rust doesn’t consider this an important enough feature and (2) they don’t have enough resources to accomplish it even if they did. But if you could wave a magic wand and make it “done”, it would be huge for rust adoption.

> You can look to Swift for prior art on how this can be done: https://faultlore.com/blah/swift-abi/

> It would be very hard to accomplish.

Since Rust cares very much about zero-overhead abstractions and performance, I would guess if something like this were to be implemented, it would have to be via some optional (crate/module/function?) attributes, and the default would remain the existing monomorphization style of code generation.

Swift’s approach still monomorphizes within a binary, and only has runtime costs when calling code across a dylib boundary. I think rust could do something like this as well.

> You can’t even express an Option<T> via the C ABI

But you can express Option<Foo> for a concrete Foo. Do you really need any more than that?

> But you can express Option<Foo> for a concrete Foo

I don’t think that’s true?

https://users.rust-lang.org/t/option-is-ffi-safe-or-not/2982...

You could maybe say that a pointer can be transmuted to an Option<&T> because there’s an Option-specific optimization that an Option<&T> uses null as the None value, but that’s not always guaranteed. And it doesn’t apply to non-references, for instance Option<bool>’s None value would be indistinguishable from false. You could get lucky if you launder your Option<T> through repr(C) and the compiler versions match and don’t mangle the internal representation, but there’s no guarantees here, since the ABI isn’t stable. (You even get a warning if you try to put a struct in your function signatures that doesn’t have a stable repr(C).)

You're right that there isn't a single standard convention for representing e.g. Option<bool>, but that's just as true of C. You'd just define a repr(C) compatible object that can be converted to or from Option<Foo>, and pass that through the ABI interface, while the conversion step would happen internally and transparently on both sides. That kind of marshaling is ubiquitous when using FFI.

> but that's just as true of C

Right, that's the whole point of this thread. The only stable ABI rust has is one where you can only use C's features at the boundaries. It would be really nice if that wasn't the case (ie. if you could express "real" rust types at a stable ABI boundary.)

As OP said, "I don't think deflecting by saying "but C is no safer" is super interesting". People seem intent on steering that conversation that way anyway, I guess.

> I don’t think GP is moving the goalposts at all

Thank you :-)

> It would be very hard to accomplish.

Yeah it's a super hard problem especially when you provide safety using the type system!

The work the Swift team did here is hella impressive.

> But if you could wave a magic wand and make it “done”, it would be huge for rust adoption.

Yeah!

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