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Tracking issue for associated const equality #92827

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Tracking issue for associated const equality#92827
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fmease
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A-associated-itemsArea: Associated items (types, constants & functions)A-const-genericsArea: const generics (parameters and arguments)B-experimentalBlocker: In-tree experiment; RFC pending, not yet approved or unneeded (requires FCP to stabilize).C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCF-associated_const_equality`#![feature(associated_const_equality)]`F-gca_min_const_items`#![feature(gca_min_const_items)]` (previously: `min_generic_const_args`)S-tracking-impl-incompleteStatus: The implementation is incomplete.T-langRelevant to the language team

Description

@JulianKnodt
JulianKnodt
opened on Jan 12, 2022

This is a tracking issue for the experimental feature associated const equality brought up in #702561 (RFC pending).
The feature gate for the issue is #![feature(associated_const_equality)].

Important

This feature has now been integrated into the unstable feature min_generic_const_args (#132980).

About experimental features

An experimental feature is one that has not yet had an RFC. The idea is to allow implementation work to proceed to better inform an upcoming RFC. Experimental features cannot be stabilized without first having an RFC. The existence of an experimental feature does not indicate lang team consensus that the feature is desirable, only that there is a problem that is worthy of being solved and that the idea has enough merit to consider exploring. See the lang team process page for more details.

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

Unresolved Questions

None so far.

Implementation history

Footnotes

  1. Original proposal with support but no official RFC, but implementation delayed. ↩

  2. Initial effort to implement which includes parsing and use of term throughout the codebase, but still lacking a complete implementation. ↩

  3. More thorough implementation which works for basic cases. This allowed for consts to actually be bound, more than just parsing them. ↩

Activity

  1. JulianKnodt
    added
    C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFC
    on Jan 12, 2022

  2. pushkine commented on Feb 8, 2022

    @pushkine
    pushkine
    on Feb 8, 2022

    Hello!
    Is there a corresponding RFC, or any context whatsoever concerning those changes ?
    This is the first occurrence of language syntax change that is not backed by RFC and community feedback
    @Centril @oli-obk

  3. oli-obk commented on Feb 8, 2022

    @oli-obk
    oli-obk
    on Feb 8, 2022
    Contributor

    Well, we have #70256 which indeed has basically no discussion of the feature. It just seemed like an oversight to have associated types + associated type bounds, but only associated consts without any bounds for them.

  4. JulianKnodt commented on Feb 8, 2022

    @JulianKnodt
    JulianKnodt
    on Feb 8, 2022
    ContributorAuthor

    I probably should've tagged the original issue in this at the start (it was in the original PR if I remember correctly) but I'm bad at leaving a thorough paper trail. I'll add a reference to the original issue in the issue itself, and if there's more work that needs to be done please let know

  5. oli-obk commented on Feb 8, 2022

    @oli-obk
    oli-obk
    on Feb 8, 2022
    Contributor

    You did everything correctly. I found the issue because you did link it in the first sentence in this issue 😄

    We should probably do a lang team MCP for it along with some medium sized doc explaining what is going on and why. I'll open a hackmd and then we can collab on it

  6. github-actions
    added a commit that references this issue on Feb 12, 2022

    ices/93835.rs: fixed with errors

    ad3289e
  7. dtolnay
    added
    A-associated-itemsArea: Associated items (types, constants & functions)
    T-langRelevant to the language team
    on Mar 14, 2022
  8. github-actions
    added a commit that references this issue on Jul 31, 2022

    ices/99828.rs: fixed with errors

    104d1ac
  9. joshtriplett
    added
    S-tracking-needs-summaryStatus: It's hard to tell what's been done and what hasn't! Someone should do some investigation.
    on Aug 10, 2022

  10. joshtriplett commented on Aug 10, 2022

    @joshtriplett
    joshtriplett
    on Aug 10, 2022
    Member

    What's the current state of this? How much is implemented, and how functional is it?

  11. JulianKnodt commented on Aug 10, 2022

    @JulianKnodt
    JulianKnodt
    on Aug 10, 2022
    ContributorAuthor

    @joshtriplett I've started work on it, and the syntax/parsing is in place, but the implementation currently does not work. I started working on Chalk, but that has stalled since I wasn't really able to know whether it was necessary for implementing it in rustc. I've also gotten a bit sidetracked working on other things.

  12. urben1680 commented on Aug 12, 2022

    @urben1680
    urben1680
    on Aug 12, 2022

    Is it planned to recognize Associated Constant Equality as disjoint cases?

    trait Foo{
    const DECIDER: bool;
}
trait Bar{ /*stuff*/ }
impl<T: Foo<DECIDER = true>> Bar for T{ /*stuff*/ }
impl<T: Foo<DECIDER = false>> Bar for T{ /*stuff*/ }

  13. 17 remaining items

  14. apoelstra
    added a commit that references this issue on Sep 23, 2024

    decode: guard against panics when alloc is disabled

    ce802af
  15. fmease
    moved this to Fixed By in Associated Const Equality (ACE)on Jan 12, 2025
  16. fmease
    added this to Associated Const Equality (ACE)on Jan 12, 2025
  17. fmease
    changed the title [-]Tracking Issue for associated const equality[/-] [+]Tracking issue for associated const equality[/+] on Jan 12, 2025
  18. nikomatsakis
    added this to Lang team featureson Feb 21, 2025
  19. nikomatsakis
    moved this to Exploration in Lang team featureson Feb 21, 2025
  20. nikomatsakis
    moved this from Exploration to Needs champion in Lang team featureson Mar 3, 2025
  21. nikomatsakis
    moved this from Needs champion to Exploration in Lang team featureson Mar 5, 2025
  22. bors
    added a commit that references this issue on Jan 6, 2026

    Auto merge of #150640 - AprilNEA:mgca-merge-associated-const-equality…

    da476f1

  23. fmease commented on Jan 6, 2026

    @fmease
    fmease
    on Jan 6, 2026
    Member

    PSA: The feature (associated_const_equality) has now been integrated into the incomplete unstable feature min_generic_const_args (tracking issue: #132980). Please subscribe to said tracking issue if you'd like to continue watching this (now) subfeature! The plan is to include ACE in the upcoming mGCA RFC.

    This tracking issue will be closed soon once I've reorganized some things.

  24. rust-lang
    locked as resolved and limited conversation to collaborators on Jan 6, 2026
  25. fmease
    added
    F-gca_min_const_items`#![feature(gca_min_const_items)]` (previously: `min_generic_const_args`)
    on Jan 7, 2026
  26. fmease
    closed this as not plannedon Apr 7, 2026
  27. github-project-automation
    moved this from Fixed By to Done in Associated Const Equality (ACE)on Apr 7, 2026
  28. fmease
    moved this from Done to Abandoned in Associated Const Equality (ACE)on Apr 14, 2026
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A-associated-itemsArea: Associated items (types, constants & functions)A-const-genericsArea: const generics (parameters and arguments)B-experimentalBlocker: In-tree experiment; RFC pending, not yet approved or unneeded (requires FCP to stabilize).C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCF-associated_const_equality`#![feature(associated_const_equality)]`F-gca_min_const_items`#![feature(gca_min_const_items)]` (previously: `min_generic_const_args`)S-tracking-impl-incompleteStatus: The implementation is incomplete.T-langRelevant to the language team

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