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Tracking issue for dyn upcasting coercion #65991

Closed
#134367
#118133
Closed
Tracking issue for dyn upcasting coercion#65991
#134367
#118133
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crlf0710
Labels
A-trait-systemArea: Trait systemC-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCF-trait_upcasting`#![feature(trait_upcasting)]`S-tracking-ready-to-stabilizeStatus: This is ready to stabilize; it may need a stabilization report and a PRT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-langRelevant to the language teamT-typesRelevant to the types team, which will review and decide on the PR/issue.disposition-mergeThis issue / PR is in PFCP or FCP with a disposition to merge it.finished-final-comment-periodThe final comment period is finished for this PR / Issue.relnotesMarks issues that should be documented in the release notes of the next release.
Milestone
1.86.0

Description

@alexreg
alexreg
opened on Oct 31, 2019

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

Previous discussions

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

Activity

  1. Centril
    added
    T-langRelevant to the language team
    F-trait_upcasting`#![feature(trait_upcasting)]`
    on Oct 31, 2019
  2. bors
    added a commit that references this issue on Jun 16, 2021

    Auto merge of rust-lang#86291 - crlf0710:trait_vtbl_refactor, r=bjorn3

    2336406
  3. bjorn3
    added a commit that references this issue on Jul 7, 2021

    Auto merge of rust-lang#86291 - crlf0710:trait_vtbl_refactor, r=bjorn3

    9be7ca0
  4. crlf0710
    changed the title [-]Tracking issue for trait upcasting[/-] [+]Tracking issue for trait upcasting coercion[/+] on Jul 8, 2021

  5. nikomatsakis commented on Dec 13, 2021

    @nikomatsakis
    nikomatsakis
    on Dec 13, 2021
    Contributor

    This is currently blocked on resolving the safety conditions for upcasting. The following updated document describes my current thinking, looking for feedback:

    https://rust-lang.github.io/dyn-upcasting-coercion-initiative/design-discussions/upcast-safety-2.html

  6. dimpolo commented on Feb 4, 2022

    @dimpolo
    dimpolo
    on Feb 4, 2022
    Contributor

    I was playing around with this and discovered that the feature gate (and its incomplete_features warning) can be bypassed using #![feature(unsize)] instead.

    This will upcast a &dyn Bar where Foo is a supertrait of Bar.

    fn upcast<Dyn: ?Sized + Unsize<dyn Foo>>(bar: &Dyn) -> &dyn Foo { bar }

    Playground

    Would this mean the unsize feature should be incomplete as well?

  7. danielhenrymantilla commented on Feb 11, 2022

    @danielhenrymantilla
    danielhenrymantilla
    on Feb 11, 2022
    Contributor

    Came to comment the same observation that @dimpolo has made: I think the impl of Unsize<dyn '_ + Super> for dyn '_ + Sub ought, itself, to be feature-gated by trait_upcasting.

  8. nikomatsakis commented on Feb 14, 2022

    @nikomatsakis
    nikomatsakis
    on Feb 14, 2022
    Contributor

    That makes sense, although really I just want to stabilize this.

  9. joshtriplett
    added
    S-tracking-ready-to-stabilizeStatus: This is ready to stabilize; it may need a stabilization report and a PR
    S-waiting-on-team
    on Jun 22, 2022

  10. Mark-Simulacrum commented on Jul 27, 2022

    @Mark-Simulacrum
    Mark-Simulacrum
    on Jul 27, 2022
    Member

    cc #89460, which is tracking a future-compat lint that seems related to this feature.

  11. crlf0710
    changed the title [-]Tracking issue for trait upcasting coercion[/-] [+]Tracking issue for dyn upcasting coercion[/+] on Jul 28, 2022

  12. pnkfelix commented on Aug 4, 2022

    @pnkfelix
    pnkfelix
    on Aug 4, 2022
    Contributor

    @joshtriplett can you confirm that when you added the S-waiting-on-team label, that was a reference to T-lang, and not T-compiler?

  13. pnkfelix commented on Aug 18, 2022

    @pnkfelix
    pnkfelix
    on Aug 18, 2022
    Contributor

    @rustbot label: I-lang-nominated

    I am seeking an answer to the question, requoted here:

    @joshtriplett can you confirm that when you added the S-waiting-on-team label, that was a reference to T-lang, and not T-compiler?

  14. 124 remaining items

  15. hannobraun
    added a commit that references this issue on Feb 10, 2025

    Switch to nightly toolchain

    2dd604b
  16. github-actions
    added a commit that references this issue on Mar 11, 2025

    Rollup merge of rust-lang#134367 - WaffleLapkin:trait_upcasting_as_a_…

    faf1fa5
  17. cuviper
    added
    relnotesMarks issues that should be documented in the release notes of the next release.
    on Mar 21, 2025
  18. cosyntxd
    added a commit that references this issue on May 27, 2025

    trait_upcasting stabilized in 1.86

    580b66d
  19. sander-machado
    added a commit that references this issue on Jul 20, 2025

    🎨 zb: Remove unsafe type casting code

    7be5f89
  20. sander-machado
    added 2 commits that reference this issue on Dec 19, 2025

    🎨 zb: Remove unsafe type casting code

    d643b8f

    🎨 zb: Replace unsafe downcast API w/ std's

    1675ebb
  21. sander-machado
    added 2 commits that reference this issue on May 29, 2026

    🎨 zb: Replace unsafe downcast API w/ std's

    5c7f82f

    🎨 zb: Replace unsafe downcast API w/ std's

    489b0f1
  22. ogoffart
    added 3 commits that reference this issue on Aug 19, 2026

    Remove the AsAny helper trait

    0790833

    Remove the AsAny helper trait

    0a34210

    Remove the AsAny helper trait

    874a1e3
  23. foonerd
    added a commit that references this issue on Sep 1, 2026

    fix(clippy): four proper fixes verified under rustc 1.85 CI toolchain

    bf741e4
  24. kchibisov
    added a commit that references this issue on Sep 4, 2026

    Remove the AsAny helper trait

    898a7ed
  25. zeenix
    added a commit that references this issue on Oct 3, 2026

    🦺 zb: Replace unsafe downcast API w/ std's

    0e75b6e
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A-trait-systemArea: Trait systemC-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCF-trait_upcasting`#![feature(trait_upcasting)]`S-tracking-ready-to-stabilizeStatus: This is ready to stabilize; it may need a stabilization report and a PRT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-langRelevant to the language teamT-typesRelevant to the types team, which will review and decide on the PR/issue.disposition-mergeThis issue / PR is in PFCP or FCP with a disposition to merge it.finished-final-comment-periodThe final comment period is finished for this PR / Issue.relnotesMarks issues that should be documented in the release notes of the next release.

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