Tracking Issue for associated const equality #92827

JulianKnodt · 2022-01-12T22:26:17Z

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)].

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

Fully implement the feature
- Address important open F-associated_const_equality `#![feature(associated_const_equality)]` issues
- Support const projections in the next trait solver
- Address all blocking FIXME(associated_const_equality)
Write an RFC
Adjust documentation (see instructions on rustc-dev-guide)
Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

None so far.

Implementation history

Original proposal with support but no official RFC, but implementation delayed. ↩
Initial effort to implement which includes parsing and use of term throughout the codebase, but still lacking a complete implementation. ↩
More thorough implementation which works for basic cases. This allowed for consts to actually be bound, more than just parsing them. ↩

The text was updated successfully, but these errors were encountered:

Continue work on associated const equality This actually implements some more complex logic for assigning associated consts to values. Inside of projection candidates, it now defers to a separate function for either consts or types. To reduce amount of code, projections are now generic over T, where T is either a Type or a Const. I can add some comments back later, but this was the fastest way to implement it. It also now finds the correct type of consts in type_of. --- The current main TODO is finding the const of the def id for the LeafDef. Right now it works if the function isn't called, but once you use the trait impl with the bound it fails inside projection. I was hoping to get some help in getting the `&'tcx ty::Const<'tcx>`, in addition to a bunch of other `todo!()`s which I think may not be hit. r? `@oli-obk` Updates rust-lang#92827

pushkine · 2022-02-08T17:13:42Z

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

oli-obk · 2022-02-08T17:22:48Z

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.

JulianKnodt · 2022-02-08T19:07:06Z

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

oli-obk · 2022-02-08T19:21:38Z

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

=== stdout === === stderr === error: comparison operators cannot be chained --> /home/runner/work/glacier/glacier/ices/93835.rs:2:8 | 1 | fn e() { | - while parsing this struct 2 | p:a<p:p<e=6>> | ^ ^ | = help: use `::<...>` instead of `<...>` to specify lifetime, type, or const arguments = help: or use `(...)` if you meant to specify fn arguments error[E0425]: cannot find value `p` in this scope --> /home/runner/work/glacier/glacier/ices/93835.rs:2:5 | 2 | p:a<p:p<e=6>> | ^ not found in this scope | help: you might have meant to write a `struct` literal | 1 ~ fn e() { SomeStruct { 2 | p:a<p:p<e=6>> 3 ~ }} | help: maybe you meant to write a path separator here | 2 | p::a<p:p<e=6>> | ~~ help: maybe you meant to write an assignment here | 2 | let p:a<p:p<e=6>> | ~~~~~ error[E0658]: associated const equality is incomplete --> /home/runner/work/glacier/glacier/ices/93835.rs:2:13 | 2 | p:a<p:p<e=6>> | ^^^ | = note: see issue #92827 <rust-lang/rust#92827> for more information = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable error[E0658]: associated type bounds are unstable --> /home/runner/work/glacier/glacier/ices/93835.rs:2:9 | 2 | p:a<p:p<e=6>> | ^^^^^^^^ | = note: see issue #52662 <rust-lang/rust#52662> for more information = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable error[E0601]: `main` function not found in crate `93835` --> /home/runner/work/glacier/glacier/ices/93835.rs:1:1 | 1 | / fn e() { 2 | | p:a<p:p<e=6>> 3 | | } | |_^ consider adding a `main` function to `/home/runner/work/glacier/glacier/ices/93835.rs` error: aborting due to 5 previous errors Some errors have detailed explanations: E0425, E0601, E0658. For more information about an error, try `rustc --explain E0425`. ==============

=== stdout === === stderr === error[E0658]: associated const equality is incomplete --> /home/runner/work/glacier/glacier/ices/99828.rs:1:43 | 1 | fn get_iter(vec: &[i32]) -> impl Iterator<Item = {}> + '_ { | ^^^^^^^^^ | = note: see issue #92827 <rust-lang/rust#92827> for more information = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable error: expected associated type bound, found constant --> /home/runner/work/glacier/glacier/ices/99828.rs:1:43 | 1 | fn get_iter(vec: &[i32]) -> impl Iterator<Item = {}> + '_ { | ^^^^^^^^^ | note: associated type defined here error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0658`. ==============

joshtriplett · 2022-08-10T17:19:07Z

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

JulianKnodt · 2022-08-10T20:25:35Z

@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.

urben1680 · 2022-08-12T18:23:15Z

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*/ }

JulianKnodt · 2022-09-28T07:15:05Z

@urben1680 I believe this would be the main point of this, but I have not got around to actually implementing it

Replace the hard-coded macro implementing `Frame` for `[impl Sample;0-32]`, with a generic impl based on const-generics. I discovered the need for this, when trying to simplify some macro- -implementations in the `ebur128` crate with const-generics. We cannot remove the need for `NumChannels`, since (see reference 1) "Associated Const Equality" is not yet stable. We can however adapt the `NumChannels` strategy from `N1..N32` to a `NChannels<N>`-generic type. Some opportunities like removing `unsafe` from `zip_map`, remain out of reach due to pending stabilization of the standard library. 1: rust-lang/rust#92827

cc rust-lang/rust#92827 - would be nice to be able to ensure a generic event is absolutely 32 bytes in ConstantX11Size::X11_SIZE, it could be very easy to wreak havoc by accidentally having an incorrectly sized event in SendEvent.

ifsheldon · 2023-03-21T06:25:15Z

Another use case would be checking tensor shapes. I hit this issue when compiling my code that constrains the shapes of input tensors to have the same dimensionality and/or rank.

For example, dumb code like the below

pub trait TensorShape1D {
    const SHAPE: usize;  // i.e. length
}

pub fn add<const L: usize, LEFT_TENSOR: TensorShape1D<SHAPE=L>, RIGHT_TENSOR: TensorShape1D<SHAPE=L>>(left: LEFT_TENSOR, right: RIGHT_TENSOR) {
    todo!()
}

Any two structs that implement TensorShape1D and have the same SHAPE can be the input of this function.

Right now, I have to use typenum crate and associated type to do the same checking, but it would be great to not have to type-dance like typenum. I think when this is implemented, typenum can be greatly simplified.

oli-obk · 2023-03-21T07:09:15Z

Tracking issues have a tendency to become unmanageable. Please open a dedicated new issue and label it with F-associated_const_equality `#![feature(associated_const_equality)]` for absolutely any topics you want to discuss or have questions about. See rust-lang/compiler-team#739 for details and discussions on this prospective policy.

JulianKnodt added the C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. label Jan 12, 2022

JulianKnodt mentioned this issue Jan 25, 2022

Continue work on associated const equality #93285

Merged

dtolnay mentioned this issue Mar 14, 2022

Parse trait bounds containing associated constant constraint dtolnay/syn#1138

Closed

dtolnay added A-associated-items Area: Associated items such as associated types and consts. T-lang Relevant to the language team, which will review and decide on the PR/issue. labels Mar 14, 2022

dtolnay mentioned this issue Mar 14, 2022

Equality constraints on associated constants #70256

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memoryruins mentioned this issue Apr 12, 2022

False syntax errors with associated_const_equality rust-lang/rust-analyzer#11965

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JulianKnodt mentioned this issue Apr 17, 2022

Add ProjectionTerm rust-lang/chalk#745

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joshtriplett added the S-tracking-needs-summary Status: It's hard to tell what's been done and what hasn't! Someone should do some investigation. label Aug 10, 2022

fmease mentioned this issue Sep 26, 2022

Compiler accepts const equality bound involving nonsensical nested equality constraint on associated const #102335

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rustbot added the F-associated_const_equality `#![feature(associated_const_equality)]` label Sep 27, 2022

Emoun mentioned this issue Oct 2, 2022

Blocking rust features Emoun/graphene#7

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joshlf mentioned this issue Oct 30, 2022

Automatically implement Unaligned without custom derive google/zerocopy#110

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joshlf mentioned this issue Nov 6, 2022

Add traits for size and alignment comparisons google/zerocopy#125

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rawler mentioned this issue Nov 15, 2022

dasp::Frame: Generic impl for all [impl Sample;const N: usize] RustAudio/dasp#180

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spki: make SubjectPublicKeyInfo own the public key RustCrypto/formats#790

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burrbull mentioned this issue Feb 18, 2023

ICE with associated_const_equality #108220

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workingjubilee added the A-const-generics Area: const generics (parameters and arguments) label Mar 13, 2023

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Tracking issue for "Lazy normalization" #60471

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Fragment Specifiers for Generic Arguments rust-lang/rfcs#3442

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Refactor: move rewrite inside hugr, Rewrite -> Replace implementing new 'Rewrite' trait CQCL/hugr#119

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Associated const equality conflicts with associated type equality #112560

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Validate transmutation size using const code google/zerocopy#239

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fmease changed the title ~~Tracking Issue for Associated Const Equality~~ Tracking Issue for associated const equality May 29, 2024

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Mark associated constants used in equality constraints as used #127505

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Permit trait object types where all (non-generic) associated constants are specified (via assoc item bindings) #130300

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Tracking Issue for associated const equality #92827

Tracking Issue for associated const equality #92827

JulianKnodt commented Jan 12, 2022 •

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Tracking Issue for associated const equality #92827

Tracking Issue for associated const equality #92827

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JulianKnodt commented Jan 12, 2022 • edited by fmease Loading

About experimental features

About tracking issues

Steps

Unresolved Questions

Implementation history

Footnotes

pushkine commented Feb 8, 2022

oli-obk commented Feb 8, 2022

JulianKnodt commented Feb 8, 2022

oli-obk commented Feb 8, 2022

joshtriplett commented Aug 10, 2022

JulianKnodt commented Aug 10, 2022

urben1680 commented Aug 12, 2022 • edited Loading

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JulianKnodt commented Sep 28, 2022

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oli-obk commented Mar 21, 2023 • edited Loading

JulianKnodt commented Jan 12, 2022 •

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oli-obk commented Mar 21, 2023 •

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