Tracking issue for dyn-star

[eholk]

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.

• dyn* ICE primitive read not possible for type: [i32; 4] #102173
• &dyn* causes an ICE #102172
• Broken LLVM function when pushing dyn* item to Vec. #102141
• Boxes can not be cast to dyn* #102427
• ICE involving references to dyn* #102429
• Bad MIR when casting a reference to dyn* #102430
• rustfmt eats dyn_star syntax rustfmt#5542
• Support pretty-printing dyn* trait objects #134601
• dyn* does not support trait upcasting #104800
• dyn and dyn* should (probably?) have their TyKind representation split from one another -- Create a new TyKind::DynStar variant #107908 was an attempt at this

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

[Rageking8]

@rustbot label +C-tracking-issue

[RalfJung]

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?