Tracking issue for dyn-star

@nikomatsakis

This issue tracks known issues or missing functionality with support for dyn* Trait objects. These are an experimental feature that are meant to support async function in dyn traits.

In addition to the issues linked here, you can also find things that need to be done by greping the code for // FIXME(dyn-star) or looking at the F-dyn_star label.

Associated PRs

Initial implementation of dyn* #101212
(list is incomplete)

About this issue

This is a lang-team experiment -- there hasn't been an RFC for this functionality, we're exploring it both as a potential implementation detail and (more speculatively) a possible language feature in the future. You can read more about dyn* at this blog post.

Lang-team champion: @nikomatsakis

@rustbot

@rustbot label +C-tracking-issue

Rollup merge of rust-lang#104557 - eholk:dyn-star-in-traits, r=compil…

Auto merge of rust-lang#106349 - LeSeulArtichaut:dyn-star-tracking-is…

Currently the dynamic type hidden by dyn* is required to match usize in size and alignment. In principle it seems possible to allow smaller alignment, and possibly even smaller size (though in the latter case care has to be taken that the vtable always ends up at the same offset -- this would be easier if the vtable was the first component of the pair, not the 2nd). Are there plans to allow that?

Right now you can actually place smaller size & align types with dyn* by stuffing it inside a usize with a little bit of unsafe. Due to the existing alignment requirements for the vtable ptr, it will always end up at the same offset as long as size_of::<T>() <= size_of::<usize>() && align_of::<T>() <= align_of::<usize>().

Due to the existing alignment requirements for the vtable ptr, it will always end up at the same offset as long as size_of::() <= size_of::() && align_of::() <= align_of::().

This is not true for size_of::<T>() == 0. Also, this assumes that for a usize (and pointers), size == align.

This is not true for size_of::<T>() == 0. Also, this assumes that for a usize (and pointers), size == align.

Ah, right. I guess a better way to describe the requirements is something like

union DynStarCompatible<T> {
  true_value: T,
  size_align: usize,
}

where DynStarCompatible<T> matches the size and align of usize. (of course, this still has the same assumption about usize and ptrs, but this could be trivially replaced with *const ())

Currently the dynamic type hidden by dyn* is required to match usize in size and alignment. In principle it seems possible to allow smaller alignment, and possibly even smaller size (though in the latter case care has to be taken that the vtable always ends up at the same offset -- this would be easier if the vtable was the first component of the pair, not the 2nd). Are there plans to allow that?

Yes, this is something I would like to support.

From the initial PR:

Note that this is never intended to become stable surface syntax for Rust, but rather dyn* is planned to be used as an implementation detail for async functions in dyn traits.

Honestly that seems like a shame -- this could be useful way beyond async fn I think. (But then there are other proposals that are even more general so indeed stabilizing this particular one might be the wrong call.)

Unrolled build for rust-lang#134601

Rollup merge of rust-lang#134642 - kpreid:pointerlike-cell, r=compile…

Unrolled build for rust-lang#134642

Rollup merge of rust-lang#134642 - kpreid:pointerlike-cell, r=compile…

Rollup merge of rust-lang#143036 - compiler-errors:no-dyn-star, r=oli…

Auto merge of #143036 - compiler-errors:no-dyn-star, r=oli-obk

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Tracking issue for dyn-star #102425

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Tracking issue for dyn-star #102425

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Rageking8 commented on Sep 29, 2022

RalfJung commented on Feb 6, 2023

ds84182 commented on Feb 6, 2023

RalfJung commented on Feb 6, 2023

ds84182 commented on Feb 6, 2023

eholk commented on Feb 7, 2023

RalfJung commented on Feb 10, 2023

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