ABI stability funding [LWN.net]

ABI stability funding

Posted Nov 30, 2025 17:11 UTC (Sun) by NYKevin (subscriber, #129325)
In reply to: ABI stability funding by khim
Parent article: APT Rust requirement raises questions

dyn Trait can never reach parity with impl Trait. impl Trait is Sized, and that implies a lot of API flexibility that dyn Trait is physically incapable of expressing under Rust's current data model.

Alternative data models, and why they do not solve this problem:

* dyn Trait in a Sized context becomes syntactic sugar for Box<dyn Trait> (autoboxing) - There is no guarantee that we even have a heap in the first place. It would create a bizarre inconsistency where T is not the pointee of &T. And there are several problems (see below) that it doesn't actually solve.
* Methods with a self receiver implicitly specialize to accept Box<dyn Trait> as the receiver - Does nothing for non-receiver Self arguments, since we cannot prove that they have the same concrete type as the receiver. In the general case, it is possible to impl Trait for Box<dyn Trait> (or write an inherent impl for dyn Trait), and then this is ambiguous. And it does nothing for associated types and non-method associated functions, which would remain dyn-incompatible anyway.
* Introduce &owned references that "confer ownership" and can be moved out of (unlike &mut, which can be swapped but not moved-from) - Similar to the previous bullet, this fixes the self receiver and does not help with much else. On top of that, it is yet another type of reference we would need to think about.
* Steal dynamic_cast from C++ - This has already been done, see std::any::Any (or you could reinvent your own with unsafe code, if for some reason Any is too inflexible for you). But that forces these APIs to become either fallible or unsafe when dispatched from dyn Trait, and that's much less ergonomic than impl Trait.

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ABI stability funding

Posted Nov 30, 2025 17:32 UTC (Sun) by khim (subscriber, #9252) [Link] (6 responses)

> impl Trait is Sized, and that implies a lot of API flexibility that dyn Trait is physically incapable of expressing under Rust's current data model

Where have you read the idea that it wouldn't change the “current data model”? It would, of course. The same way Swift did: by permitting !Sized types on stack and so on. Large work, sure, but nothing impossible.

> Alternative data models, and why they do not solve this problem:

Have you excluded not just obvious, but already implemented (in Swift) solution on purpose? Introduce dynamic, parametrised types — and that's it. Yes, it would change language in a subtle ways: str would no longer be one, single, type, but would become parametrised type with length fixed at runtime… so pointer or reference would need to include reference and also length… oh, right, that's how things already word, isn't it? That would actually simplify the language instead of making it more complex. Would remove lots of corner cases related to !Sized types.

The big question is not whether that can be done, but how hard would it be. Probably lots of simple, tedious work… but nothing really crazy.

Backward compatibility could be problematic, though.

The worst thing that would happen: some panics that currently happen at compile-time would start happening at runtime… consider function that removes one element from [u8] (slice, not array!) and returns the result… what should it do if slice is already empty?

Currently all that zoo is kept out of stable, but there are lots of struggles in attempting to make it all work… struggles that, ironically enough, become trivial if you introduce runtime-parametrised types and make dyn Trait identical to impl Trait.

It's more of a political decision than technical decision: now one would fund such work without promise of it being included in the compiler — and no one would give such a promise if only ideas on paper exist and there are no working code. But maybe someone can do that as Rust fork?

The biggest question is with functions that may return unknown type (known type, unknown parameters: think about function that removes duplicates from a slice). As first step these may be forbidden outright: every type that leaves function should be describable in terms of input (but then the aforementioned function that removes one element from slice is also impossible) — a bit line lifetimes are handled today.

ABI stability funding

Posted Dec 1, 2025 23:12 UTC (Mon) by NYKevin (subscriber, #129325) [Link] (5 responses)

> The same way Swift did: by permitting !Sized types on stack and so on. Large work, sure, but nothing impossible.

I refer the honorable gentleperson to the answer I gave some moments ago:

> Similar to the previous bullet, this fixes the self receiver and does not help with [associated types, associated functions, and various other dyn-incompatible things besides the self receiver].

ABI stability funding

Posted Dec 1, 2025 23:14 UTC (Mon) by NYKevin (subscriber, #129325) [Link] (4 responses)

(And yes, I can see that you wrote a bunch of stuff that boils down to "let's make method dispatch fallible." But that is so obviously absurd that it is undeserving of a serious response.)

ABI stability funding

Posted Dec 1, 2025 23:41 UTC (Mon) by khim (subscriber, #9252) [Link] (3 responses)

One may face random compilation error today with impl even if all formal trait-level checks succeed. Naturally if we want parity between impl and dyn we couldn't skip that quirk, too. And because with dyn they couldn't be done at compile time they have to be done at runtime.

As I have said: that's not a big deal, in practice, the majority of existing software is written in languages that have that “defect” and it's rarely a problem, in practice. Whether Rust team wants to deliver something usable in realistic timeframe or would prefer to punish users with what they have now is up to them, ultimately: using extern "C" doesn't solve that problem, in fact it makes it worse.

ABI stability funding

Posted Dec 2, 2025 8:44 UTC (Tue) by taladar (subscriber, #68407) [Link] (2 responses)

As a Rust user anything that causes more failures at runtime that could instead be compile time failures would be something I would definitely strongly oppose.

ABI stability funding

Posted Dec 2, 2025 10:21 UTC (Tue) by khim (subscriber, #9252) [Link] (1 responses)

We are talking about Rust, not Haskell, here. Uses of extern "C" definitely generates more problems at runtime that higher-level interface ever could — only they generate random crashes, not predictable panics.

In the past for things like integer overflow or array access Rust usually picked up panics in place of UB, but with stable ABI approach is the opposite.

That looks a bit illogical to me, but then, people are not always rational.

ABI stability funding

Posted Dec 9, 2025 0:37 UTC (Tue) by gmatht (subscriber, #58961) [Link]

The current convention for static linking avoids this. If dynamic linking became more common in Rust, making a missing library a compile time error may not be possible, but it would be nice to at least be a link time error.