Tracking issue for dyn upcasting coercion

[alexreg]

This is a tracking issue for RFC3324. Corresponding MCP is here.
The feature gate for the issue is #![feature(trait_upcasting)].

STATUS UPDATE

Stabilized in Rust 1.86 via #134367.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the experimental feature behind a feature gate.
• Define the semantics explicitly.
• Gathering design docs into initiative repo: https://github.com/rust-lang/dyn-upcasting-coercion-initiative
• Adjust documentation ([see instructions on rustc-dev-guide][doc-guide]) Add trait_upcasting related languages changes reference#1259
• Gather data about memory usage? add feature gate to permit measuring memory usage of dyn upcasting #112355
• Stabilization PR ([see instructions on rustc-dev-guide][stabilization-guide]) Stablize trait_upcasting feature #101718, Stabilize RFC3324 dyn upcasting coercion #118133
• Fix soundness problems.
  • Make casts of pointers to trait objects stricter #120248
• Restabilize.
  • Stabilize feature(trait_upcasting) #134367

Previous discussions

• RFC401: Type conversions rfcs#401
• RFC982: DST custom coercions. rfcs#982
• RFC-issue2765: Objects should be upcastable to supertraits rfcs#2765
• MCP: Trait upcasting coercion lang-team#98 (Current lang-team MCP)
• Initiative: Dyn upcasting coercion initiative
• RFC: Support upcasting of dyn Trait values rfcs#3324

Unresolved Questions

• Currently CoerceUnsized trait cannot express this case: a smart pointers that wrap a raw pointer and don't guarantee via a custom invariant that it is valid. Maybe a separate CoerceUnsizedUnsafe trait is needed. (see Trait upcasting coercion (part4) #88010 (comment)).
• Should we make upcasting opt-in in some form to limit vtable size by default? The current inclination of the lang-team is "no", but it would be useful to gather data on how much supporting upcasting contributors to overall binary size.
• Before stabilizing it we should check that libs-api is ok with upcasting for all dyn-allowed traits in the library, since those we can't change. (addressed by Tracking issue for dyn upcasting coercion #65991 (comment))
• Should we add an opt-out mechanism, and extend library stabilization checklist with "do we want to opt-out for now"?

Implementation history

• Initial attempt: Trait upcasting #60900
• Second attempt:
  • Preparation refactoring: Refactor vtable codegen #86291
  • Unify vtable building: Change vtable memory representation to use tcx allocated allocations. #86475
  • Update vtable format: Refactor vtable format for upcoming trait_upcasting feature. #86461
  • Feature gate implementation: Trait upcasting coercion (part1) #86264
  • Pointer conversion adjustments: Trait upcasting coercion (part2) #87515
  • Support multiple occurrence of a target trait with different substsdefs: Trait upcasting coercion (part 3) #88135
  • Unsafety checking for raw dyn pointers: Trait upcasting coercion (part4) #88010 (not prefered approach, likely will need to do nothing here, but need a design meeting discussion with lang-team) Solution: https://rust-lang.zulipchat.com/#narrow/stream/326132-t-types.2Fmeetings/topic/meeting.202022-09-02.20planning/near/296826711, Commit to safety rules for dyn trait upcasting #101336
  • Lint for easing migration: Tracking issue for future-incompatibility lint deref_into_dyn_supertrait #89460
  • Reference changes
    • Add trait_upcasting related languages changes reference#1259
    • Add trait_upcasting related languages changes reference#1622
  • Mark this feature no longer incomplete and stablize: Stablize trait_upcasting feature #101718
  • Make trait upcasting work with supertraits with fewer associated types: Rework upcasting confirmation to support upcasting to fewer projections in target bounds #114036